Electronic Navigation

The idea that led to development of the satellite navigation systems dates back to 1957 and the first launch of an artificial satellite into orbit, Russia’s Sputnik I. Dr. William H. Guier and Dr. George C. Wieffenbach at the Applied Physics Laboratory of the Johns Hopkins University were monitoring the famous “beeps” transmitted by the passing satellite. They plotted the received signals at precise intervals, and noticed that a characteristic Doppler curve emerged. Since satellites generally follow fixed orbits, they reasoned that this curve could be used to describe the satellite’s orbit. They then demonstrated that they could determine all of the orbital parameters for a passing satellite by Doppler observation of a single pass from a single fixed station. The Doppler shift apparent while receiving a transmission from a passing satellite proved to be an effective measuring device for establishing the satellite orbit.

Dr. Frank T. McClure, also of the Applied Physics Laboratory, reasoned in reverse: If the satellite orbit was known, Doppler shift measurements could be used to determine one’s position on Earth. His studies in support of this hypothesis earned him the first National Aeronautics and Space Administration award for important contributions to space development.

In 1958, the Applied Physics Laboratory proposed exploring the possibility of an operational satellite Doppler navigation system. The Chief of Naval Operations then set forth requirements for such a system. The first successful launching of a prototype system satellite in April 1960 demonstrated the Doppler system’s operational feasibility.

The Navy Navigation Satellite System (NAVSAT, also known as TRANSIT) was the first operational satellite navigation system. The system’s accuracy was better than 0.1 nautical mile anywhere in the world, though its availability was somewhat limited. It was used primarily for the navigation of surface ships and submarines, but it also had some applications in air navigation. It was also used in hydrographic surveying and geodetic position determination.

The transit launch program ended in 1988 and the system was disestablished when the Global Positioning System became operational in 1996.

The Federal Radio navigation Plan has designated the Navigation System using Timing And Ranging (NAVSTAR) Global Positioning System (GPS) as the primary navigation system of the U.S. government. GPS is a spaced-based radio positioning system which provides suitably equipped users with highly accurate position, velocity, and time data. It consists of three major segments: a space segment, a control segment, and a user segment.

[Figure 2301 in Bowditch, Pub. No. 9: GPS Satellite Code by Broadcast Frequency.]

The space segment consists of 31 GPS satellites with at least 24 operational 95% of the time. Spacing of the satellites in their orbits is arranged so that at least four satellites are in view to a user at any time, anywhere on the Earth, including the North and South Poles. Each satellite transmits signals on three radio frequencies, superimposed on which are navigation and system data. Included in this data are predicted satellite ephemeris, atmospheric propagation correction data, satellite clock error information and satellite health data. The satellites orbit at an altitude of 20,200 km, in six separate orbital planes, each plane inclined 55° relative to the equator. The satellites complete an orbit approximately once every 12 hours.

GPS satellites transmit pseudorandom noise (PRN) sequence-modulated radio frequencies, designated L1 (1575.42 MHz), L2 (1227.60 MHz) and L5 (1176.45 MHz). Various transmissions are sent on these channels as shown in Table 2201.

Superimposed on both the legacy C/A and P(y) codes is the navigation message. This message contains the satellite ephemeris data, atmospheric propagation correction data, and satellite clock bias. In addition, four additional new messages have been introduced by the so called GPS modernization: L2-CNAV, CNAV-2, L5-CNAV and MNAV. The “legacy” message and the first three of the modernized GPS are civil messages, while the MNAV is a military message. In modernized GPS, the same type of contents as the legacy navigation message (NAV) is transmitted but at a higher rate and with improved robustness.

The messages L2-CNAV, L5-CNAV and MNAV have a similar structure and (modernized) data format. The new format allows more flexibility, better control and improved content. Furthermore, the MNAV includes new improvements for the security and robustness of the military message. The CNAV-2 is modulated onto L1CD, sharing the same band as the “legacy” navigation message.

GPS assigns a unique C/A code and a unique P code to each satellite. This practice, known as code division multiple access (CDMA), allows all satellites the use of a common carrier frequency while still allowing the receiver to determine which satellite is transmitting. CDMA also allows for easy user identification of each GPS satellite. Since each satellite broadcasts using its own unique C/A and P code combination, it can be assigned a unique PRN sequence number. This number is how a satellite is identified when the GPS control system communicates with users about a particular GPS satellite.

The control segment includes a master control station (MCS), a number of monitor stations, and ground antennas located throughout the world. The master control station, located in Colorado Springs, Colorado, consists of equipment and facilities required for satellite monitoring, telemetry, tracking, commanding, control, uploading, and navigation message generation. The monitor stations, located in Hawaii, Colorado Springs, Kwajalein, Diego Garcia, and Ascension Island, passively track the satellites, accumulating ranging data from the satellites’ signals and relaying them to the MCS. The MCS processes this information to determine satellite position and signal data accuracy, updates the navigation message of each satellite and relays this information to the ground antennas. The ground antennas then transmit this information to the satellites. The ground antennas, located at Ascension Island, Diego Garcia, and Kwajalein, are also used for transmitting and receiving satellite control information.

The user equipment is designed to receive and process signals from four or more orbiting satellites either simultaneously or sequentially. The processor in the receiver then converts these signals to navigation information. Since GPS is used in a wide variety of applications, from marine navigation to land surveying, these receivers can vary greatly in function and design.

GPS provides multiple users with accurate, continuous, worldwide, all-weather, common-grid, three-dimensional positioning and navigation information.

To obtain a navigation solution of position (latitude, longitude, and altitude) and time (four unknowns), four satellites must be used. The GPS user measures pseudorange and pseudorange rate by synchronizing and tracking the navigation signal from each of the four selected satellites. Pseudorange is the true distance between the satellite and the user plus an offset due to the user’s clock bias. Pseudorange rate is the true slant range rate plus an offset due to the frequency error of the user’s clock. By decoding the ephemeris data and system timing information on each satellite’s signal, the user’s receiver/processor can convert the pseudorange and pseudorange rate to three-dimensional position and velocity. Four measurements are necessary to solve for the three unknown components of position (or velocity) and the unknown user time (or frequency) bias.

The navigation accuracy that can be achieved by any user depends primarily on the variability of the errors in making pseudorange measurements, the instantaneous geometry of the satellites as seen from the user’s location on Earth, and the presence of Selective Availability (SA). Selective Availability is discussed further below.

GPS receivers (or user equipment) measure distances between satellites in orbit and a receiver on Earth, and computes spheres of position from those distances. The intersections of those spheres of position then determine the receiver’s position.

The distance measurements described above are done by comparing timing signals generated simultaneously by the satellites’ and receiver’s internal clocks. These signals, characterized by a special wave form known as the pseudo-random code, are generated in phase with each other. The

signal from the satellite arrives at the receiver following a time delay proportional to its distance traveled. This time delay is detected by the phase shift between the received pseudo-random code and the code generated by the receiver. Knowing the time required for the signal to reach the receiver from the satellite allows the receiver to calculate the distance from the satellite. The receiver, therefore, must be located on a sphere centered at the satellite with a radius equal to this distance measurement. The intersection of three spheres of position yields two possible points of receiver position. One of these points can be disregarded since it is hundreds of miles from the surface of the Earth. Theoretically, then, only three time measurements are required to obtain a fix from GPS.

In practice, however, a fourth measurement is required to obtain an accurate position from GPS. This is due to receiver clock error. Timing signals travel from the satellite to the receiver at the speed of light; even extremely slight timing errors between the clocks on the satellite and in the receiver will lead to tremendous range errors. The satellite’s atomic clock is accurate to 10-9 seconds; installing a clock that accurate on a receiver would make the receiver prohibitively expensive. Therefore, receiver clock accuracy is sacrificed, and an additional satellite timing measurement is made. The fix error caused by the inaccuracies in the receiver clock is reduced by simultaneously subtracting a constant timing error from four satellite timing measurements until a pinpoint fix is reached.

Assuming that the satellite clocks are perfectly synchronized and the receiver clock’s error is constant, the subtraction of that constant error from the resulting distance determinations will reduce the fix error until a “pinpoint” position is obtained. It is important to note here that the number of lines of position required to employ this technique is a function of the number of lines of position required to obtain a fix. GPS determines position in three dimensions; the presence of receiver clock error adds an additional unknown. Therefore, four timing measurements are required to solve for the resulting four unknowns.

The GPS L1 band (1575.42 MHz) has turned out to be the most important band for navigation purposes. Indeed most of the applications in the world today are based on the signals transmitted at this frequency. Three signals are transmitted at the moment by GPS in L1: C/A Code, P(Y) Code and M-Code. In the future, an additional new civil signal, known as L1C, will also be transmitted.

GPS is transmitting in the L2 band (1227.60 MHz). It is modernized civil signal known as L2C together with the P(Y) Code and the M-Code. The P(Y) Code and M-Code were already described shortly in the previous chapter and the properties and parameters are thus similar to those in the L1 band. In addition, for Block IIR-M, IIF, and subsequent blocks of SVs, two additional PRN ranging codes will be transmitted. They are the L2 Civil Moderate (L2 CM) code and the L2 Civil Long (L2 CL) code. These two signals are time multiplexed so that the resulting chipping rate is double as high as that of each individual signal.

The GPS L5 (1176.45 MHz) signal will be transmitted for the first time on board IIF satellites. The GPS carriers of the L5 band are modulated by two bit trains in phase quadrature: the L5 data channel and the L5 pilot channel. Moreover, two PRN ranging codes are transmitted on L5.

For a more detailed analysis of GPS signal coding see Appendix C in Volume I.

The correlation process compares the signal received from the satellites with the signal generated by the receiver by comparing the square wave function of the received signal with the square wave function generated by the receiver. The computer logic of the receiver recognizes the square wave signals as either a +1 or a 0 depending on whether the signal is “on” or “off.” The signals are processed and matched by using an autocorrelation function.

This process defines the necessity for a “pseudo-random code.” The code must be repeatable (i.e., non-random) because it is in comparing the two signals that the receiver makes its distance calculations. At the same time, the code must be random for the correlation process to work; the randomness of the signals must be such that the matching process excludes all possible combinations except the combination that occurs when the generated signal is shifted a distance proportional to the received signal’s time delay. These simultaneous requirements to be both repeatable (non-random) and random give rise to the description of “pseudo-random”; the signal has enough repeatability to enable the receiver to make the required measurement while simultaneously retaining enough randomness to ensure incorrect calculations are excluded.

Two levels of navigational accuracy are provided by the GPS: the Precise Positioning Service (PPS) and the Standard Positioning Service (SPS). GPS was designed, first and foremost, by the U.S. Department of Defense as a United States military asset; its extremely accurate positioning capability is an asset access to which the U.S. military may need to limit during time of war to prevent use by enemies. Therefore, the PPS is available only to authorized users, mainly the U.S. military and authorized allies. SPS, on the other hand, is available worldwide to anyone possessing a GPS receiver. The accuracy of the GPS signal in space is actually the same for both the civilian GPS service and the military GPS service. However, SPS broadcasts on one frequency, while PPS uses two. This means military users can perform ionospheric correction, a technique that

reduces radio degradation caused by the Earth's atmosphere. With less degradation, PPS provides better accuracy than the basic SPS.

The ongoing GPS modernization program is adding new civilian signals and frequencies to the GPS satellites, enabling ionospheric correction for all users. Eventually, the accuracy difference between military and civilian GPS will disappear. But military GPS will continue to provide important advantages in terms of enhanced security and jam resistance.

Anti-spoofing (A-S) is designed to negate any hostile imitation of GPS signals. The technique alters the P code into another code, designated the Y code. The C/A code remains unaffected. The U.S. employs this technique to the satellite signals at random times and without warning; therefore, civilian users are unaware when this P code transformation takes place. Since anti-spoofing is applied only to the P code, the C/A code is not protected and can be spoofed.

GPS PPS receivers can use either the P code or the C/A code, or both, in determining position. Maximum accuracy is obtained by using the P code on both L1 and L2. The difference in propagation delay is then used to calculate ionospheric corrections. The C/A code is normally used to acquire the satellite signal and determine the approximate P code phase. Some PPS receivers possess a clock accurate enough to track and lock on the P code signal without initially tracking the C/A code. Some PPS receivers can track only the C/A code and disregard the P code entirely. Since the C/A code is transmitted on only one frequency, the dual frequency ionosphere correction methodology is unavailable and an ionospheric modeling procedure is required to calculate the required corrections.

SPS receivers, as mentioned above, provide positions with a degraded accuracy. The A-S feature denies SPS users access to the P code when transformed to the Y code. Therefore, the SPS user cannot rely on access to the P code to measure propagation delays between L1 and L2 and compute ionospheric delay corrections. Consequently, the typical SPS receiver uses only the C/A code because it is unaffected by A-S. Like PPS, the C/A is transmitted only on L1, the dual frequency method of calculating ionospheric corrections is unavailable; an ionospheric modeling technique must be used. This is less accurate than the dual frequency method; this degradation in accuracy is accounted for in the 100-meter accuracy calculation.

In May 2000, President Bill Clinton directed the Department of Defense to turn off the GPS Selective Availability (SA) feature. In 2007, the U.S. government announced plans to permanently eliminate SA by building the GPS III satellites without it. SA was a method to degrade GPS accuracy to civilian users.

In order for the GPS receiver to navigate, it has to track satellite signals, make pseudorange measurements, and collect navigation data.

A typical satellite tracking sequence begins with the receiver determining which satellites are available for it to track. Satellite visibility is determined by user-entered predictions of position, velocity, and time, and by almanac information stored internal to the receiver. If no stored almanac information exists, then the receiver must attempt to locate and lock onto the signal from any satellite in view. When the receiver is locked onto a satellite, it can demodulate the navigation message and read the almanac information about all the other satellites in the constellation. A carrier tracking loop tracks the carrier frequency while a code tracking loop tracks the C/A and P code signals. The two tracking loops operate together in an iterative process to acquire and track satellite signals.

The receiver’s carrier tracking loop will locally generate an L1 carrier frequency which differs from the satellite produced L1 frequency due to a Doppler shift in the received frequency. This Doppler offset is proportional to the relative velocity along the line of sight between the satellite and the receiver, subject to a receiver frequency bias. The carrier tracking loop adjusts the frequency of the receiver-generated frequency until it matches the incoming frequency. This determines the relative velocity between the satellite and the receiver. The GPS receiver uses this relative velocity to calculate the velocity of the receiver. This velocity is then used to aid the code tracking loop.

The code tracking loop is used to make pseudorange measurements between the GPS receiver and the satellites. The receiver’s tracking loop will generate a replica of the targeted satellite’s C/A code with estimated ranging delay. In order to match the received signal with the internally generated replica, two things must be done: 1) the center frequency of the replica must be adjusted to be the same as the center frequency of the received signal; and 2) the phase of the replica code must be lined up with the phase of the received code. The center frequency of the replica is set by using the Doppler-estimated output of the carrier tracking loop. The receiver will then slew the code loop generated C/A code through a millisecond search window to correlate with the received C/A code and obtain C/A tracking.

Once the carrier tracking loop and the code tracking loop have locked onto the received signal and the C/A code has been stripped from the carrier, the navigation message is demodulated and read. This gives the receiver other information crucial to a pseudorange measurement. The navigation message also gives the receiver the handover word, the code that allows a GPS receiver to shift from C/A code tracking to P code tracking.

The handover word is required due to the long phase (seven days) of the P code signal. The C/A code repeats every millisecond, allowing for a relatively small search

window. The seven day repeat period of the P code requires that the receiver be given the approximate P code phase to narrow its search window to a manageable time. The handover word provides this P code phase information. The handover word is repeated every subframe in a 30 bit long block of data in the navigation message. It is repeated in the second 30 second data block of each subframe. For some receivers, this handover word is unnecessary; they can acquire the P code directly. This normally requires the receiver to have a clock whose accuracy approaches that of an atomic clock. Since this greatly increases the cost of the receiver, most receivers for non-military marine use do not have this capability.

Once the receiver has acquired the satellite signals from four GPS satellites, achieved carrier and code tracking, and has read the navigation message, the receiver is ready to begin making pseudorange measurements. Recall that these measurements are termed pseudorange because a receiver clock offset makes them inaccurate; that is, they do not represent the true range from the satellite, only a range biased by a receiver clock error. This clock bias introduces a fourth unknown into the system of equations for which the GPS receiver must solve (the other three being the x coordinate, y coordinate, and z coordinate of the receiver position). The receiver solves this clock bias problem by making a fourth pseudorange measurement, resulting in a fourth equation to allow solving for the fourth unknown. Once the four equations are solved, the receiver has an estimate of the receiver’s position in three dimensions and of GPS time. The receiver then converts this position into coordinates referenced to an Earth model based on the World Geodetic System (1984).

There are two formal position accuracy requirements for GPS:

1) The PPS spherical position accuracy shall be 16 meters SEP (spherical error probable) or better.

2) The SPS user two dimensional position accuracy shall be 100 meters 2 DRMS (distance root mean squared) or better.

Assume that a universal set of GPS pseudorange measurements results in a set of GPS position measurements. The accuracy of these measurements will conform to a normal (i.e. values symmetrically distributed around a mean of zero) probability function because the two most important factors affecting accuracy, the geometric dilution of precision (GDOP) and the user equivalent range error (UERE), are continuously variable.

The UERE is the error in the measurement of the pseudoranges from each satellite to the user. The UERE is the product of several factors, including the clock stability, the predictability of the satellite’s orbit, errors in the 50 Hz navigation message, the precision of the receiver’s correlation process, errors due to atmospheric distortion and the calculations to compensate for it, and the quality of the satellite’s signal. The UERE, therefore, is a random error which is the function of errors in both the satellites and the user’s receiver.

The GDOP depends on the geometry of the satellites in relation to the user’s receiver. It is independent of the quality of the broadcast signals and the user’s receiver. Generally speaking, the GDOP measures the “spread” of the satellites around the receiver. The optimum case would be to have one satellite directly overhead and the other three spaced 120° around the receiver on the horizon. The worst GDOP would occur if the satellites were spaced closely together or in a line overhead.

There are special types of DOP’s (dilution of precision) for each of the position and time solution dimensions; these particular DOP’s combine to determine the GDOP. For the vertical dimension, the vertical dilution of precision (VDOP) describes the effect of satellite geometry on altitude calculations. The horizontal dilution of precision (HDOP) describes satellite geometry’s effect on position (latitude and longitude) errors. These two DOP’s combine to determine the position dilution of precision (PDOP). The PDOP combined with the time dilution of precision (TDOP) results in the GDOP. See Figure 2309.

Section 2309 covered errors in GPS positions due to errors inherent in the satellite signal (UERE) and the geometry of the satellite constellation (GDOP). Another major cause of accuracy degradation is the effect of the ionosphere on the radio frequency signals that comprise the GPS signal.

A discussion of a model of the Earth’s atmosphere will be useful in understanding this concept. Consider the Earth as surrounded by three layers of atmosphere. The first layer, extending from the surface of the Earth to an altitude of approximately 10 km, is known as the troposphere. Above the troposphere and extending to an altitude of approximately 50 km is the stratosphere. Finally, above the stratosphere and extending to an altitude that varies as a function of the time of day is the ionosphere. Though radio signals are subjected to effects which degrade its accuracy in all three layers of this atmospheric model, the effects of the ionosphere are the most significant to GPS operation.

The ionosphere, as the name implies, is that region of the atmosphere which contains a large number of ionized molecules and a correspondingly high number of free electrons. These charged molecules have lost one or more electrons. No atom will lose an electron without an input of energy; the energy input that causes the ions to be formed in the ionosphere comes from the ultraviolet (U-V) radiation of the Sun. Therefore, the more intense the Sun’s rays,

[Figure 2309 in Bowditch, Pub. No. 9: Position and time error computations.]

the larger the number of free electrons which will exist in this region of the atmosphere.

The largest effect that this ionospheric effect has on GPS accuracy is a phenomenon known as group time delay. As the name implies, group time delay results in a delay in the time a signal takes to travel through a given distance. Obviously, since GPS relies on extremely accurate timing measurement of these signals between satellites and ground receivers, this group time delay can have a noticeable effect on the magnitude of GPS position error.

The group time delay is a function of several elements. It is inversely proportional to the square of the frequency at which the satellite transmits, and it is directly proportional to the atmosphere’s total electron content (TEC), a measure of the degree of the atmosphere’s ionization. The general form of the equation describing the delay effect is: K TEC ( × )

t

Δ = --------------------------- f2 where

Δt = group time delay

f = operating frequency

K = constant

Since the Sun’s U-V radiation ionizes the molecules in the upper atmosphere, it stands to reason that the time delay value will be highest when the Sun is shining and lowest at night. Experimental evidence has borne this out, showing that the value for TEC is highest around 1500 local time and lowest around 0500 local time. Therefore, the magnitude of the accuracy degradation caused by this effect will be highest during daylight operations. In addition to these daily variations, the magnitude of this time delay error also varies with the seasons; it is highest at the vernal equinox. Finally, this effect shows a solar cycle dependence. The greater the number of sunspots, the higher the TEC value and the greater the group time delay effect. The solar cycle typically follows an eleven year pattern. The current solar cycle began on January 4, 2008 with minimal activity until early 2010. The cycle is on track to have the lowest recorded sunspot activity since cycle 14 which reached maximum in 1906. See Figure 2210 Solar cycle 24 prediction.

Given that this ionospheric delay introduces a serious accuracy degradation into the system, how does GPS account for it? There are two methods used: (1) the dual frequency technique, and (2) the ionospheric delay method.

As the term implies, the dual frequency technique requires the ability to acquire and track both the L1 and L2 frequency signals. Recall from the discussion in Section 2304 that the C/A and P codes are transmitted on carrier frequency L1, but only the P code is transmitted on L2. Recall also that only authorized operators with access to DOD cryptographic material are able to copy the P code. It follows, then, that only those authorized users are able to copy the L2 carrier frequency. Therefore, only those authorized users are able to use the dual frequency correction method. The dual frequency method measures the distance between the satellite and the user based on both the L1 and L2 carrier signal. These ranges will be different because the group time delay for each signal will be different. This is because of the frequency dependence of the time delay error. The

[Figure 2310 in Bowditch, Pub. No. 9: Solar cycle prediction. Courtesy of NASA.]

range from the satellite to the user will be the true range combined with the range error caused by the time delay, as shown by the following equation:

R f( ) Ractual error term = + where R(f) is the range which differs from the actual range as a function of the carrier frequency. The dual frequency correction method takes two such range measurements, R(L1) and R(L2). Recall that the error term is a function of a constant divided by the square of the frequency. By combining the two range equations derived from the two frequency measurements, the constant term can be eliminated and one is left with an equation in which the true range is simply a function of the two carrier frequencies and the measured ranges R(L1) and R(L2). This method has two major advantages over the ionospheric model method: (1) it calculates corrections from real-time measured data, therefore, it is more accurate; (2) it alleviates the need to include ionospheric data on the navigation message. A significant portion of the data message is devoted to ionospheric correction data. If the receiver is dual frequency capable, then it does not need any of this data.

The vast majority of maritime users cannot copy dual frequency signals. For them, the ionospheric delay model provides the correction for the group time delay.

The ionospheric delay model mathematically models the diurnal ionospheric variation. The value for this time delay is determined from a cosinusoidal function into which coefficients representing the maximum value of the time delay (i.e., the amplitude of the cosine wave representing the delay function), the time of day, the period of the variation and a minimum value of delay are introduced. This model is designed to be most accurate at the diurnal maximum. This is obviously a reasonable design consideration because it is at the time of day when the maximum diurnal time delay occurs that the largest magnitude of error appears. The coefficients for use in this delay model are transmitted to the receiver in the navigation data message. As stated in Section 2311, this method of correction is not as accurate as the dual frequency method; however, for the non-military user, it is the only method of correction available.

Multipath reflection errors occur when the receiver detects parts of the same signal at two different times. The first reception is the direct path reception, the signal that is received directly from the satellite. The second reception is from a reflection of that same signal from the ground or any other reflective surface. The direct path signal arrives first, the reflected signal, having had to travel a longer distance to the receiver, arrives later. The GPS signal is designed to minimize this multipath error. The L1 and L2 frequencies used demonstrate a diffuse reflection pattern, lowering the signal strength of any reflection that arrives at the receiver. In addition, the receiver’s antenna can be designed to reject a signal that it recognizes as a reflection. In addition to the properties of the carrier frequencies, the high data frequency of both the P and C/A codes and their resulting good correlation properties minimize the effect of multipath propagation.

The design features mentioned above combine to reduce the maximum error expected from multipath propagation to less than 20 feet.

The discussions above make it clear that the Global Positioning System provides the most accurate positions available to navigators today. They should also make clear that the most accurate positioning information is available to only a small fraction of the using population: U.S. and allied military. For most open ocean navigation applications, the degraded accuracy inherent in selective availability and the inability to copy the precision code presents no serious hazard to navigation. A mariner seldom if ever needs greater than 100 meter accuracy in the middle of the ocean.

It is a different situation as the mariner approaches shore. Typically for harbor approaches and piloting, the mariner will shift to visual piloting. The increase in accuracy provided by this navigational method is required to ensure ship’s safety. The 100 meter accuracy of GPS in this situation is not sufficient. Any mariner who has groped his way through a restricted channel in a thick fog will certainly appreciate the fact that even a degraded GPS position is available for them to plot. However, 100 meter accuracy is not sufficient to ensure ship’s safety in most piloting situations. In this situation, the mariner needs P code accuracy. The problem then becomes how to obtain the accuracy of the Precise Positioning Service with due regard to the legitimate security concerns of the U.S. military. The answer to this seeming dilemma lies in the concept of Differential GPS (DGPS).

Differential GPS is a system in which a receiver at an accurately surveyed position utilizes GPS signals to calculate timing errors and then broadcasts a correction signal to account for these errors. This is an extremely powerful concept. The errors which contribute to GPS accuracy degradation, ionospheric time delay and selective availability, are experienced simultaneously by both the DGPS receiver and a relatively close user’s receiver. The extremely high altitude of the GPS satellites means that, as long as the DGPS receiver is within 100-200 km of the user’s receiver, the user’s receiver is close enough to take advantage of any DGPS correction signal.

The theory behind a DGPS system is straightforward. Located on an accurately surveyed site, the DGPS receiver already knows its location. It receives data which tell it where the satellite is. Knowing the two locations, it then calculates the theoretical time it should take for a satellite’s signal to reach it. It then compares the time that it actually takes for the signal to arrive. This difference in time between the theoretical and the actual is the basis for the DGPS receiver’s computation of a timing error signal; this difference in time is caused by all the errors to which the GPS signal is subjected; errors, except for receiver error and multipath error, to which both the DGPS and the user’s receivers are simultaneously subject. The DGPS system then broadcasts a timing correction signal, the effect of which is to correct for selective availability, ionospheric delay, and all the other error sources the two receivers share in common.

For suitably equipped users, DGPS results in positions at least as accurate as those obtainable by the Precise Positioning Service. This capability is not limited to simply displaying the correct position for the navigator to plot. The DGPS position can be used as the primary input to an electronic chart system, providing an electronic readout of position accurate enough to pilot safely in the most restricted channel.

The Wide Area Augmentation System (WAAS) program, which corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, provides vital integrity information regarding the health of each GPS satellite. The concept is similar to the DGPS concept, except that correctional signals are sent from geostationary satellites via HF signals directly to the user’s GPS receiver. This eliminates the need for a separate receiver and antenna, as is the case with DGPS. WAAS is intended

for en route air navigation, with 25 reference stations widely spaced across the United States that monitor GPS satellite data and two master stations on either coast, and creates a GPS correction message. WAAS provides coverage of the entire U.S. and parts of Mexico and Canada.

The Local Area Augmentation System (LAAS) is intended for precision airport approaches, with reference stations located at airports and broadcasting their correction message on VHF radio frequencies.

While many marine GPS receivers incorporate WAAS circuitry (but not the more accurate, shorter-range LAAS), WAAS is not optimized for surface navigation because the HF radio signals are line-of-sight and are transmitted from geostationary satellites. At low angles to the horizon, the WAAS signal may be blocked and the resulting GPS position accuracy significantly degraded with no warning. The DGPS signal, on the other hand, is a terrain-following signal that is unaffected by objects in its path. It simply flows around them and continues on unblocked.

The accuracy of WAAS and DGPS is comparable, on the order of a few meters. Any GPS receiver equipped to receive WAAS has its accuracy improved to less than 3 meters. Both systems have been found in actual use to provide accuracies somewhat better than designed. DGPS was designed to provide 10 meter accuracy 95% of the time, but in actual use one can expect about 1-3 meter accuracy when the user is within 100 miles of the DGPS transmitter. Over 100 miles, DGPS accuracy will commonly degrade by an additional 1 meter per 100 miles from the transmitter site.

The WAAS signal, while not certified for use in the marine environment as is DGPS, can be a very useful navigational tool if its limitations are understood. In open waters of the continental U.S., the WAAS signal can be expected to be available and useful, provided the receiver has WAAS circuitry and is programmed to use the WAAS data. Outside the U.S., or in any area where tall buildings, trees, or other obstructions rise above the horizon, the WAAS signal may be blocked, and the resulting GPS fix could be in error by many meters. Since the highest accuracy is necessary in the most confined waters, WAAS should be used with extreme caution in these areas.

WAAS can enhance the navigator’s situational awareness when available, but availability is not assured. Further, a marine receiver will provide no indication when WAAS data is not a part of the fix. [Aircraft GPS receivers may contain Receiver Autonomous Integrity Monitoring (RAIM) software, which provides warning of WAAS satellite signal failure, and removes the affected signal from the fix solution.]

LAAS data, broadcast on VHF, is less subject to blocking, but is only available in selected areas near airports. Its range is about 30 miles. It is therefore not suitable for general marine navigational use.

For more information on the Global Positioning System (GPS) and related topics see the link provided in Figure 2316.

[Figure 2316 in Bowditch, Pub. No. 9: GPS.gov at http://www.gps.gov]

SBAS systems are spreading out all over the world. More and more, it is believed that upon dual-frequency SBAS service provision, a seamless navigation will be possible from and to any two locations in the world.

Presently, three foreign SBAS systems are operational. These are Japan’s Multi-functional Transport Satellite based Augmentation System (MSAS), the European Geostationary Navigation Overlay Service (EGNOS) and India’s GPS and Geo-Augmented Navigation System (GAGAN),

Other foreign SBAS are under implementation such as SDCM (System of Differential Correction and Monitoring) in Russia and SNAS (Satellite Navigation Augmentation System) in China. Still others are under development or feasibility studies; SACCSA (Solucion de Aumentacion para Caribe, Centro y Sudamerica) would cover Central & South America including the Caribbean. Member States to SACCSA include Argentina Bolivia, Colombia, Costa Rica, Guatemala, Panama, Spain and Venezuela. Malaysia, much of Africa and South Korean SBAS are also studying SBAS particularly for aeronautical navigation.

Galileo is the global navigation satellite system (GNSS) that is currently being created by the European Union (EU) through the European Space Agency (ESA) and the European GNSS Agency (GSA), with two ground operations centers in Germany and Italy.

One of the aims of Galileo is to provide an indigenous alternative high-precision positioning system upon which European nations can rely, independently from other country systems, in case they were disabled by their operators.

The use of basic (low-precision) Galileo services will be free and open to everyone. The high-precision capabilities will be available for paying commercial users. Galileo

is intended to provide horizontal and vertical position measurements within one meter precision, and better positioning services at high latitudes than other positioning systems.

Galileo is to provide a new global search and rescue (SAR) function as part of the Medium-altitude Earth Orbit Search and Rescue (MEOSAR) system. Satellites will be equipped with a transponder which will relay distress signals from emergency beacons to a rescue coordination center, which will then initiate a rescue operation. At the same time, the system is projected to provide a signal, the Return Link Message (RLM), to the emergency beacon, informing victims that their situation has been detected and help is on the way. This latter feature is new and is considered a major upgrade compared to the existing international search and rescue system (Cospas-Sarsat), which does not provide feedback to the user.

Galileo will also provide an important feature for civilian use that GPS does not: integrity monitoring. Currently, a civilian GPS user receives no indication that his unit is not receiving proper satellite signals, there being no provision for such notification in the code. However, Galileo will provide such a signal, alerting the user that the system is operating improperly.

The first Galileo test satellite, the GIOVE-A, was launched 28 December 2005, while the first satellite to be part of the operational system was launched on 21 October 2011. As of May 2016 the system has 14 of 30 satellites in orbit. Galileo will start offering Early Operational Capability (EOC) from 2016, go to Initial Operational Capability (IOC) in 2017-18 and reach Full Operational Capability (FOC) in 2019. The complete 30-satellite Galileo system (24 operational and 6 active spares) is expected by 2020.

For detailed information on the Galileo signal structure see Appendix C in Volume I.

The Global Navigation Satellite System (GLONASS), under the control of the Russian military, has been in use since 1993, and is based on the same principles as GPS. The space segment consists of 24 satellites in three orbital planes, the planes separated by 120 degrees and the individual satellites by 45 degrees. The orbits are inclined to the equator at an angle of 64.8 degrees, and the orbital period is about 11hours, 15minutes at an altitude of 19,100 km (10,313 nm). The designed system fix accuracy for civilian use is 100 meters horizontal (95%), 150 meters vertical, and 15 cm/sec. in velocity. Military codes provide accuracies of some 10-20 meters horizontal.

The ground segment of GLONASS lies entirely within the former Soviet Union. Reliability has been an ongoing problem for the GLONASS system, but new satellite designs with longer life spans are addressing these concerns. The user segment consists of various types of receivers that provide position, time, and velocity information.

GLONASS signals are in the L-band, operating in 25 channels with 0.5625 MHz separation in 2 bands: from 1602.5625 MHz to 1615.5 MHz, and from 1240 to 1260 MHz.

For detailed information on the GLONASS signal structure see Appendix C in Volume I.

The BeiDou Navigation Satellite System (BDS), also known as BeiDou-2, is China's second-generation satellite navigation system that will be capable of providing positioning, navigation, and timing services to users on a continuous worldwide basis.

Although the evolution of its regional navigation system towards a global solution started in 1997, the formal approval by the Government of the development and deployment of BDS System was done in 2006 and it is expected to provide global navigation services by 2020, similarly to the GPS, GLONASS or Galileo systems.

As of December 2011, the BeiDou system was officially announced to provide Initial Operational Service providing initial passive positioning navigation and timing services for the whole Asia-Pacific region with a constellation of 10 satellites (5 GEO satellites and 5 Inclined Geosynchronous Satellite Orbit (IGSO) satellites). During 2012, 5 additional satellites (1 GEO satellite and 4 Medium-Earth Orbit (MEO) satellites) were launched increasing to 14 the number of satellites of the constellation. In 2020, the system is going to launch the remaining satellites and evolve towards global navigation capability.

The BeiDou Space Segment consists of a constellation of 35 satellites, which include 5 geostationary earth orbit (GEO) satellites and 30 non-GSO satellites. The system is currently under development evolving from a regional system called BeiDou-1, and in the first phase will provide global navigation services by 2020, similarly to the GPS, GLONASS or Galileo systems.

For detailed information on the BeiDou signal plan see Appendix C in Volume I.

The Indian Regional Navigational Satellite System (IRNSS) is a regional satellite navigation system owned by the Indian government. The system is being developed by Indian Space Research Organization (ISRO).

In April 2016, with the last launch of the constellation's satellite, IRNSS was renamed Navigation Indian Constellation (NAVIC) by India's Prime Minister Narendra Modi.

IRNSS will be an independent and autonomous regional navigation system aiming a service area of about 1500 kilometers around India. The system will be under complete Indian control, with the space segment, ground segment and user receivers all being built in India. It will have a range of applications including personal navigation.

For detailed information on the IRNSS signal plan see Appendix C in Volume I.

The Quasi-Zenith Satellite System (QZSS) is a regional navigation satellite system commissioned by the Japanese Government as a National Space Development Program.

QZSS was authorized by the Japanese government in 2002. At the beginning the system was developed by the Advanced Space Business Corporation (ASBC) team, including Mitsubishi Electric Corp., Hitachi Ltd., and GNSS Technologies Inc. When in 2007 ASBC collapsed, the work was taken over by JAXA together with Satellite Positioning Research and Application Center (SPAC), established in February 2007 and approved by the Ministers associated with QZSS research and development.

The QZSS service area covers East Asia and Oceania region and its platform is multi-constellation GNSS. The QZSS system is not required to work in a stand-alone mode, but together with data from other GNSS satellites.

For detailed information on the QZSS signal plan see Appendix C in Volume I.

Ávila Rodríguez, José Ángel. (2008). On Generalized Signal Waveforms for Satellite Navigation. University FAF, Munich. Retrieved from: https://athene-forschung.unibw.de/doc/86167/86167.pdf

Radar determines distance to an object by measuring the time required for a radio signal (moving at the speed of light) to travel from a transmitting antenna to the object, reflect off that object, and return as a received echo.

Distance, or range, can be found by the simple formula:

range = 1/2 (C x t)

where range is in nautical miles,

C = the speed of light in nautical miles per second, and

t = the time in seconds from the time of pulse transmis-

sion to echo reception.

Because the value of C is very large (162,000 NM/sec), t is very small, 0.0001 sec for a target at a range of 10 miles for example.

Such measurements can be converted into lines of position (LOP's) comprised of circles with radius equal to the distance to the object. Since marine radars use directional antennae, they can also determine an object's bearing. However, due to its design, radar's bearing measurements are much less accurate than its distance measurements. Understanding this concept is crucial to ensuring the optimal employment of the radar for safe navigation.

In most marine navigation applications, the radar signal is pulse modulated. Signals are generated by a timing circuit so that energy leaves the antenna in very short pulses, usually less than one millionth of a second (or 1 µsec) in duration. When transmitting, the antenna is connected to the transmitter but not the receiver. As soon as the pulse leaves, an electronic switch disconnects the antenna from the transmitter and connects it to the receiver. Another pulse is not transmitted until after the preceding one has had time to travel to the most distant target within range and return. Since the interval between pulses is long compared with the length of a pulse, strong signals can be provided with low average power. The duration or length of a single pulse is called pulse length, pulse duration, or pulse width. This pulse emission sequence repeats a great many times, perhaps 1,000 per second. This rate defines the pulse repetition rate (PRR). The returned pulses are displayed on an indicator screen or display.

In traditional marine radar sets, those produced since the 1940s, the transmitter is a special electronic oscillator diode tube known as a magnetron. The magnetron produces very high power microwaves (25 KW and greater) for very short periods of time.

Recently, another type of radar has been introduced into the commercial marine industry know as solid state or coherent radar. In modern solid state radars, the pulses generated by special circuitry in the transmitter are of much less power, much longer in length, and of varying frequency. This type of radar does not use a magnetron and generates an entirely different waveform. Presently, solid state radar is only available in the S-Band and will be further discussed in the following sections.

The function of the receiver is to amplify the strength of the very weak return echoes. The enhanced signals can then be used to produce video signals which are presented as targets on the display. The amplifiers in a traditional magnetron radar have to deal with only one frequency, either 3000 MHz or 10000 MHz depending on the radar set.

A solid state radar receiver however, must process a much more complex signal with changing frequency. This variable frequency, or chirp, necessitates signal processing within the receiver known as pulse compression, which shortens the comparatively long, 5 - 18 microsecond transmitted pulse into a pulse of similar length to traditional radars (0.05 - 1.0 µsec), while at the same time increasing signal amplitude, thus yielding the same detection and range measuring capabilities. A very great advantage of solid state radars over magnetron radars is their superior ability to filter out rain and sea clutter effects and therefore assist the radar observer in identification of land targets used in radar navigation.

Nearly all modern commercial marine radars use a type of antenna known as a slotted waveguide. See Figure 2404

[Figure 2404 in Bowditch, Pub. No. 9: Slotted waveguide antenna.]

for a depiction of a slotted waveguide antenna. Both solid state and magnetron radar sets utilize this antenna configuration because it is simple, efficient, and produces a beam that minimizes unwanted side lobes (side lobes will be discussed later in this chapter).

The radar display is often referred to as the plan position indicator (PPI). On a PPI, the sweep appears as a radial line, centered at the center of the scope and rotating in synchronization with the antenna. Any returned echo causes a brightening of the display screen at the bearing and range of the object. The glow continues after the sweep rotates past the target.

On a PPI, a target’s actual range is proportional to its distance from the center of the scope. A movable cursor helps to measure ranges and bearings. In the “heading-upward” presentation, which indicates relative bearings, the top of the scope represents the direction of the ship’s head. In this destabilized presentation, the orientation changes as the ship changes heading. In the stabilized “north-upward” presentation, gyro north is always at the top of the scope.

The pulses of energy comprising the radar beam would form a single lobe-shaped pattern of radiation if emitted in free space. Figure 2406a shows this free space radiation pattern, including the undesirable minor lobes or side lobes associated with practical antenna design. This radiation pat-

tern, as well as the effects of diffraction, reflection and attenuation described below, are common to both magnetron and solid state generated radar signals. Although the radiated energy is concentrated into a relatively narrow main beam by the antenna, there is no clearly defined envelope of the energy radiated, although most of the energy is concentrated along the axis of the beam.

The radiation diagram shown in Figure 2406b depicts relative values of power in the same plane existing at the same distances from the antenna or the origin of the radar beam. Maximum power is in the direction of the axis of the beam. Power values diminish rapidly in directions away from the axis. The beam width is taken as the angle between the half-power points.

The beam width depends upon the frequency or wavelength of the transmitted energy, antenna design, and the dimensions of the antenna. For a given antenna size (antenna aperture), narrower beam widths result from using shorter wavelengths. For a given wavelength, narrower beam widths result from using larger antennas, or i.e., beam width is inversely proportional to antenna aperture. Because marine radar antennas are long in the horizontal dimension and narrow in the vertical dimension, they produce a beam that is narrow in the horizontal direction and somewhat wider in the vertical direction. The narrow horizontal beam is desirable for bearing accuracy while the wide vertical beam is needed to account for the pitching and rolling of a vessel in a seaway. If the vertical beam was as narrow as the horizontal beam, a vessel in rough weather would experience intermittent target response as the beam would not intersect the horizon at all times.

The main lobe of the radar beam is composed of a number of separate lobes in the vertical dimension, as opposed to the single lobe-shaped pattern of radiation as emitted in free space. This phenomenon is the result of interference between radar waves taking a direct line-of-sight path to a target, and those waves that are reflected from the surface of the sea before striking the target. There is a slight difference in distance between which the direct and indirect waves must travel. See Figure 2406c. These reflected (indirect) waves interfere either constructively or destructively with the direct waves depending upon the waves' phase relationship. This sets up the possibility of poor target response for objects at certain ranges from own ship.

[Figure 2406a in Bowditch, Pub. No. 9: Freespace radiation pattern.]

[Figure 2406b in Bowditch, Pub. No. 9: Radiation diagram.]

[Figure 2406c in Bowditch, Pub. No. 9: Direct and indirect waves.]

Just as a light source reflected in a mirror appears much dimmer than the direct image, radar echoes are much weaker than the transmitted pulses due to the general spreading out of the radar signal energy with distance. The strengths of these echoes are also dependent upon the amount of transmitted energy striking the targets and the size and reflecting properties of the targets known as radar cross section.

Attenuation is the scattering and absorption of the energy in the radar beam as it passes through the atmosphere. It causes a decrease in echo strength. Attenuation is greater in 3-cm rather than 10-cm radar. Atmospheric water particles (heavy fog, rain and snow) can significantly degrade the performance of a 3-cm radar system. During periods of heavy precipitation, the radar observer should switch to the 10-cm set if one is available.

Diffraction is the bending of a wave as it passes an obstruction. Because of diffraction there is some illumination of the region behind an obstruction or target by the radar beam. Diffraction effects are greater at the lower frequencies with longer wavelengths (S-Band). Thus, the radar beam of 10-cm radar tends to illuminate more of the shadow region behind an obstruction than the beam of X-Band radar of 3-cm wavelength.

If the radar waves traveled in straight lines, the distance to the radar horizon would be dependent only on the power output of the transmitter and the height of the antenna. In other words, the distance to the radar horizon would be the same as that of the geometrical horizon for the antenna height. However, atmospheric density gradients bend radar rays as they travel to and from a target. This bending is called refraction.

The distance to the radar horizon does not always limit the distance from which echoes may be received from targets. Assuming that adequate power is transmitted, echoes may be received from targets beyond the radar horizon if their reflecting surfaces extend above it. The distance to the radar horizon is the distance at which the radar rays pass tangent to the surface of the Earth.

The following formula, where h is the height of the antenna in feet, gives the theoretical distance to the radar horizon in nautical miles: 1.22 h

D =

D = the range in nautical miles

h = height of the antenna.

Radar’s value as a navigational aid depends on the navigator’s understanding its characteristics and limitations. Whether measuring the range to a single reflective object or trying to discern a shoreline lost amid severe clutter, knowledge of the characteristics of the individual radar used are crucial. Some of the factors to be considered in interpretation are discussed below:

Resolution in Range. In Part A of Figure 2409a, a •

transmitted pulse has arrived at the second of two tar-

gets of insufficient size or density to absorb or reflect

all of the energy of the pulse. While the pulse has trav-

eled from the first to the second target, the echo from

the first has traveled an equal distance in the opposite

direction. At B, the transmitted pulse has continued on

beyond the second target, and the two echoes are

returning toward the transmitter. The distance between

leading edges of the two echoes is twice the distance

between targets and so the display will indicate two

distinct targets. The correct distance between targets

will be shown on the display, which is calibrated to

show half the distance traveled out and back. At C the

targets are closer together and the pulse length has been

increased. The two echoes merge, and on the scope

they will appear as a single, large target. At D the pulse

length has been decreased, and the two echoes appear

separated. The ability of a radar to separate targets

close together on the same bearing is called resolution

in range. It is related primarily to pulse length. The

minimum distance between targets that can be distin-

guished as separate is one half the pulse length. This

(half the pulse length) is the apparent depth or thick-

ness of a target but in no way represents that actual size

of a small isolated target like a buoy or boat. Thus, sev-

eral ships close together on nearly the same bearing

may appear as an island. Echoes from a number of

small boats, piles, breakers, or even a single large ship

close to the shore may blend with echoes from the

shore, resulting in an incorrect indication of the posi-

tion and shape of the shoreline.

Resolution in Bearing. Echoes from two or more tar- •

gets close together at the same range may merge to

form a single, wider echo. The ability to separate tar-

gets close together at the same range is called resolu-

tion in bearing. Bearing resolution is a function of two

variables: horizontal beam width and range to the tar-

gets. A narrower horizontal beam and/or a shorter dis-

tance to the objects will allow for better bearing reso-

lution.

Height of Antenna and Target. If the radar horizon is •

between the transmitting vessel and the target, the

lower part of the target will not be visible. A large ves-

sel may appear as a small craft, or a shoreline may

appear at some distance inland.

[Figure 2409a in Bowditch, Pub. No. 9: Resolution in range.]

Reflecting Quality and Aspect of Target. Echoes •

from several targets of the same size may be quite dif-

ferent in appearance. A metal surface reflects radio

waves more strongly than a wooden surface. A surface

perpendicular to the beam returns a stronger echo than a non-perpendicular one. A vessel seen broadside returns a stronger echo than one heading directly toward or away. Some surfaces absorb most radar energy rather that reflecting it.

Frequency. A 3-cm radar has the ability to discern •

smaller targets than a 10-cm set. For example, a very

small boat or a submarine periscope might be invisible

in S-Band but detectable in X-Band. In a calm sea, a 3-

cm radar, properly tuned, can detect a single bird or

even a soda can.

Atmospheric noise, sea return, and precipitation complicate radar interpretation by producing clutter. Clutter is usually strongest near the vessel. Strong echoes from targets of interest can sometimes be discerned by reducing receiver gain to eliminate weaker signals. By watching the display during several rotations of the antenna, the operator can often discriminate between clutter and a target even when the signal strengths from clutter and the target are equal. The echoes from real targets will remain relatively stationary on the display while those caused by clutter will appear to move around randomly with each sweep.

Another major problem lies in determining which features in the vicinity of the shoreline are actually represented by echoes shown on the display. Particularly in cases where a low lying shore remains below the radar horizon, there may be considerable uncertainty.

A related problem is that certain features on the shore will not return echoes because they are blocked or shadowed from the radar beam by other physical features or obstructions. This shadowing effect in turn causes the image painted on the display to differ from the charted image of the area.

If the navigator is to be able to interpret the presentation on the radar display, he or she must understand the characteristics of radar propagation, the capabilities of his radar set, the reflecting properties of different types of radar targets, and the ability to analyze his chart to determine which charted features are most likely to reflect the transmitted pulses or to be shadowed. Experience gained during clear weather comparison between radar and visual images is invaluable.

Land masses are generally recognizable because of the steady brilliance of the relatively large areas painted on the PPI. Also, land should be at positions expected from the ship's navigational position. Although land masses are readily recognizable, the primary problem is the identification of specific land features. Identification of specific features can be quite difficult because of various factors in addition to shadowing, including distortion resulting from beam width and pulse length, and uncertainty as to just which charted features are reflecting the echoes

Sand spits and smooth, clear beaches normally do not appear on the PPI at ranges beyond 1 or 2 miles because these targets have almost no area that can reflect energy back to the radar. Such a smooth horizontal surface will reflect all radar signals away from the antenna and so are essentially invisible. If waves are breaking over a sandbar, echoes may be returned from the surf. Waves may, however, break well out from the actual shoreline, so that ranging on the surf may be misleading.

Mud flats and marshes normally reflect radar pulses only a little better than a sand spit. The weak echoes received at low tide disappear at high tide. Mangroves and other thick growth may produce a strong echo. Areas that are indicated as swamps on a chart, therefore, may return either strong or weak echoes, depending on the density type, and size of the vegetation growing in the area.

Sand dunes covered with vegetation are usually well back from a low, smooth beach, and the apparent shoreline determined by radar appears at the line of the dunes rather than the true shoreline. This can lead navigators to believe they are farther away from the beach than they really are, a potentially hazardous situation.

Lagoons and inland lakes usually appear as blank areas on a PPI because the smooth water surface returns no energy to the radar antenna. In some instances, even the sandbar or reef surrounding the lagoon may not appear on the PPI because it lies too close to the water.

Coral atolls and long chains of islands may produce long lines of echoes when the radar beam is directed perpendicular to the line of the islands. This indication is especially true when the islands are closely spaced. The reason is that the spreading resulting from the width of the radar beam exceeds the radar's resolution in bearing and causes the echoes to blend into continuous lines. When the same chain of islands is viewed lengthwise, or obliquely, however, each island may produce a separate return if the distance between the islands does not exceed the radar's resolution in range.

Surf breaking on a reef around an atoll produces a ragged, variable line of echoes. Even the smallest of rocks projecting above the surface of the water may be discerned depending on their shape and distance from own ship.

If the land rises in a gradual, regular manner from the shoreline, no part of the terrain produces an echo that is stronger than the echo from any other part. As a result, a general haze of echoes appears on the PPI, and it is difficult to ascertain the range to any particular part of the land.

Blotchy echoes are returned from hilly ground, because the crest of each hill returns a good echo though the area beyond is in a radar shadow. If high receiver gain is used, the pattern may become solid except for very deep depressions.

Low islands ordinarily produce small echoes. When thick palm trees or other foliage grow on the island, strong echoes often are produced because the horizontal surface of the water around the island forms a sort of corner reflector with the vertical surfaces of the trees. As a result, wooded islands give good echoes and can be detected at a much greater range than barren islands.

Sizable land masses may be missing from the radar display because of shadowing. A shoreline which is continuous on the PPI display when the ship is at one position, may not appear continuous when the ship is at another position and scanning the same shoreline. The radar beam may be blocked from a segment of this shoreline by an obstruction

[Figure 2409b in Bowditch, Pub. No. 9: Effects of ship’s position, beam width, and pulse length on radar shoreline. Figure 2409c, Figure 2409d and Figure 2409e correspond to position A, B and C in the image above.]

such as a promontory. An indentation in the shoreline, such as a cove or bay, appearing on the PPI when the ship is at one position, may not appear when the ship is at another position nearby. Radar shadowing alone can cause considerable differences between the PPI display and the chart presentation. This effect in conjunction with beam width and pulse length distortion of the PPI display can cause even greater differences, possibly leading to confusion and navigational error.

The returns of objects close to shore may merge with the shoreline image on the PPI, because of distortion effects of horizontal beam width and pulse length. Target images on the PPI are distorted angularly by an amount equal to the effective horizontal beam width. Also, the target images always are distorted radially by an amount at least equal to one-half the pulse length (150 meters per microsecond of pulse length).

See Figure 2409b. It illustrates the effects of own ship position, horizontal beam width, and pulse length on the radar image of a coastline. Because of beam width distortion, a straight, or nearly straight shoreline often appears crescent-shaped on the PPI. This effect is greater with the wider beam widths. Note that this distortion increases as the angle between the beam axis and the shoreline decreases. Figure 2409c, Figure 2409d and Figure 2409e correspond to positions A, B and C in Figure 2409b.

See Figure 2409f. View A shows the actual shape of the shoreline and the land behind it. Note the steel tower on the low sand beach and the two ships at anchor close to shore. The heavy line in View B represents the shoreline on the PPI. The dotted lines represent the actual position and shape of all targets. Note in particular: 1. The low sand beach is not detected by the radar. 2. The tower on the low beach is detected, but it looks like

a ship in a cove. At closer range the land would be

detected and the cove-shaped area would begin to fill in;

then the tower could not be seen without reducing the

receiver gain. 3. The radar shadow behind both mountains. Distortion

owing to radar shadows is responsible for more confu-

sion than any other cause. The small island does not

appear because it is in the radar shadow. 4. The spreading of the land in bearing caused by beam

width distortion. Look at the upper shore of the penin-

sula. The shoreline distortion is greater to the west

because the angle between the radar beam and the shore

is smaller as the beam seeks out the more westerly

shore.

[Figure 2409c in Bowditch, Pub. No. 9: 12 mile scale (off-center display) at 0700 position. See position A in Figure 2409b.]

[Figure 2409d in Bowditch, Pub. No. 9: 12 mile scale (display centered) at 0730 position. See position B in Figure 2409b.]

5. Ship No. 1 appears as a small peninsula. Its return has

merged with the land because of the beam width distor-

tion. 6. Ship No. 2 also merges with the shoreline and forms a

bump. This bump is caused by pulse length and beam

width distortion. Reducing receiver gain might cause

the ship to separate from land, provided the ship is not

too close to the shore. The rain clutter control could also

[Figure 2409e in Bowditch, Pub. No. 9: 6 mile scale (display center) at 0800 position. See position C in Figure 2409b.]

be used to attempt to separate the ship from land by

effectively reducing the pulse lengths within the

receiver.

Indirect or false echoes are caused by reflection of the main lobe of the radar beam off own ship's structures such as masts, stacks, kingposts or deck cargo, especially containers. When such reflection from obstructions does occur, the echo will return from a legitimate radar contact to the antenna by the same indirect path. Consequently, the echo will appear on the PPI at the bearing of the reflecting surface. As shown in Figure 2410a, the indirect echo will appear on the PPI at the same range as the direct echo received, assuming that the additional distance by the indirect path is negligible.

Characteristics by which indirect echoes may be recognized are summarized as follows:

1. Indirect echoes will often occur in shadow sectors.

2. They are received on substantially constant relative

bearings (the direction of the obstruction), although

the true bearing of the radar contact may change

appreciably.

3. They appear at the same ranges as the correspond-

ing direct echoes.

4. When plotted, their movements are usually abnor-

mal.

5. Their distorted or fuzzy shapes may indicate that

they are not direct echoes.

[Figure 2409f in Bowditch, Pub. No. 9: Distortion effects of radar shadow, beam width, and pulse length.]

Side-lobe effects are readily recognized in that they produce a series of echoes (See Figure 2410b) on each side of the main lobe echo at the same range as the latter. Semicircles, or even complete circles, may be produced. Because of the low energy of the side-lobes, these effects will normally occur only at the shorter ranges. The effects may be minimized or eliminated, through use of the gain and anti-clutter controls, but always at the risk of failing to detect weaker targets like buoys or small boats. The introduction of slotted wave guide antennas has drastically reduced the side-lobe problem. Nevertheless, when strong reflecting targets are present at close range, side lobe effects will still be encountered and may be difficult to eliminate entirely without severely reducing gain.

[Figure 2410a in Bowditch, Pub. No. 9: Indirect echo.]

[Figure 2410b in Bowditch, Pub. No. 9: Side lobe effects.]

Multiple echoes may occur when a strong echo is received from another ship at close range. A second or third or more echoes may be observed on the radarscope at double, triple, or other multiples of the actual range of the radar contact (Figure 2410c).

Second-trace echoes (multiple-trace echoes) are echoes received from a contact at an actual range greater than the radar range setting. If an echo from a distant target is received after the next pulse has been transmitted, the echo will appear on the display at the correct bearing but not at the true range. Second-trace echoes are unusual, except under abnormal atmospheric conditions, or conditions under which super-refraction or ducting is present.

[Figure 2410c in Bowditch, Pub. No. 9: Multiple echoes.]

[Figure 2410d in Bowditch, Pub. No. 9: Second-trace echo.]

Second-trace echoes may be recognized through changes in their positions on the display when changing range scales with different pulse repetition rates (PRR), their hazy, streaky, or distorted shapes (especially noticeable with large land targets), and their erratic movements on plotting.

As illustrated in Figure 2410d, a target echo is detected on a true bearing of 090° at a distance of 7.5 miles. On changing the PRR from 2,000 to 1,800 pulses per second in Figure 2410e, the same target is detected on a bearing of 090° at a distance of 3 miles. The change in the position of the target indicates that the echo is a second-trace echo. The actual distance of the target is the distance as indicated on the PPI plus half the distance the radar waves travel between pulses. In this case, (162,000 NM/sec ÷ 2000 PPS

[Figure 2410e in Bowditch, Pub. No. 9: Second-trace echo after altering PRR.]

÷ 2) + 7.5 = 48 nautical miles.

Naturally, since we are on the 12-mile scale, the target should not be visible and so must be a second-trace echo.

Electronic interference effects, which may occur when near another radar operating in the same frequency band as that of own ship, are usually seen on the radar as a large number of small bright dots either scattered at random or in the form of curving dotted lines extending from the center to the edge of the PPI.

Interference effects are greater at the longer radar range scale settings. Interference effects can be distinguished easily from normal echoes because they do not appear in the same places on successive rotations of the antenna. Most radar systems have interference rejection controls (IR) that eliminate most of the unwanted interference effects.

Stacks, masts, containers, and other structures, may cause a reduction in the intensity of the radar beam beyond these obstructions, especially if they are close to the radar antenna. If the angle at the antenna subtended by the obstruction is more than a few degrees, the reduction of the intensity of the radar beam beyond the obstruction may produce a blind sector. Less reduction in the intensity of the beam beyond the obstructions may produce shadow sectors. Within a shadow sector, small targets at close range may not be detected, while larger targets at much greater ranges will appear.

The echo from an overhead power cable can be wrongly identified as the echo from a ship on a steady bearing and decreasing range. Course changes to avoid the contact are ineffective; the contact remains on a steady bearing, decreasing range. This phenomenon is particularly apparent for the power cable spanning the Straits of Messina.

Radar navigation aids help identify radar targets and increase echo signal strength from otherwise poor radar targets.

Buoys are particularly poor radar targets. Weak, fluctuating echoes received from these targets are easily lost in the sea clutter. To aid in the detection of these targets, radar reflectors, designated corner reflectors, may be used. These reflectors may be mounted on the tops of buoys or designed into the structure.

Each corner reflector, as shown in Figure 2411a, consists of three mutually perpendicular flat metal surfaces. A radar wave striking any of the metal surfaces or plates will be reflected back in the direction of its source. Maximum energy will be reflected back to the antenna if the axis of the radar beam makes equal angles with all the metal surfaces. Frequently, corner reflectors are assembled in clusters to maximize the reflected signal.

Although radar reflectors are used to obtain stronger echoes from radar targets, other means are required for more positive identification of radar targets. Radar beacons are transmitters operating in the marine radar frequency band, which produce distinctive indications on the radar displays of ships within range of these beacons. There are two general classes of these beacons: racons, which provide both bearing and range information to the target, and ramarks which provide bearing information only. However, if the ramark installation is detected as an echo on the display, the range will be available also.

[Figure 2411a in Bowditch, Pub. No. 9: Corner reflectors.]

A racon is a radar transponder which emits a characteristic signal when triggered by a ship's radar. The signal is emitted on the same frequency as that of the triggering radar, in which case it is superimposed on the ship's radar display automatically. However, the only racons in service are "in band" beacons which transmit in one of the marine radar bands, usually only the 3-centimeter band.

[Figure 2411b in Bowditch, Pub. No. 9: Coded racon signal.]

The racon signal appears on the PPI as a radial line originating at a point just beyond the position of the radar beacon, or as a Morse Code signal as shown in Figure 2411b, emanating from the beacon in a direction radially outward from the center of the display. The Morse Code symbol of the racon signal helps to identify important navigational aids on the navigator’s chart.

A ramark is a radar beacon which transmits either continuously or at intervals. The latter method of transmission is used so that the PPI can be inspected without any clutter introduced by the ramark signal on the scope. The ramark signal as it appears on the PPI is a radial line from the center. The radial line may be a continuous narrow line, a broken line, a series of dots, or a series of dots and dashes (See Figure 2411c). Ramarks are not as common as racons and are not as useful for navigational purposes as they do not indicate the range to the transmitting beacon.

[Figure 2411c in Bowditch, Pub. No. 9: Ramark appears a broken radial line.]

When navigating in restricted waters, a mariner most often relies on visual piloting to provide the accuracy required to ensure ship safety. Visual piloting, however, requires clear weather; often, mariners must navigate through fog or other conditions of restricted visibility. When weather conditions render visual piloting impossible on a vessel not equipped with ECDIS, radar navigation provides a method of fixing a vessel's position with sufficient accuracy to allow safe passage. See Chapter 10 Piloting for a detailed discussion of integrating radar into a piloting procedure on a vessel using paper charts. However, even on ECDIS equipped vessels, radar provides a vital positional cross-checking capability that is paramount to the practice of safe and prudent navigation.

Since radar can more accurately determine ranges than bearings, the most accurate radar fixes result from measuring and plotting a series of ranges to two or more objects. If one measures the range to objects directly ahead or astern first and objects closest to the beam last, the time of the fix will be the time the ranges were measured to objects ahead or astern. In other words, the fix time is the time that distances were measured to objects with the greatest rate of change of range (range rate) due to own ship's motion. This minimizes measurement time delay errors without resorting to the use of running fixes. Record the ranges to the navigation aids used and lay the resulting range arcs down on the chart. Theoretically, these lines of position should intersect at a point coincident with the ship's position at the time of the fix. Where possible, use objects widely separated in bearing (60o-90o) for the greatest accuracy. See Figure 2413.

Though verifying soundings is always a good practice in all navigation scenarios, its importance increases when piloting using only radar. One of the most common and serious errors in radar navigation involves object misiden-

[Figure 2413 in Bowditch, Pub. No. 9: Fix by radar ranges.]

tification. These errors can be discovered through correlation of fathometer readings with expected charted depths. Assuming proper operation of the fathometer, soundings give the navigator invaluable confirmation on the reliability of radar fixes.

When determining a fix by radar bearings (or visual bearings) take bearings of objects on the beam first and those ahead or astern last. The time of the fix will be the time that the objects abeam were measured. This is because the rate of change of bearing is highest for objects on the beam and lowest for those ahead and astern. Again, this procedure minimizes the fix error due to the time delay in taking a round of bearings.

But the inherent inaccuracy of fixes composed solely of radar bearings as discussed above makes this method less accurate than fixing position by radar ranges. Use this method to plot a position quickly on the chart when approaching restricted waters to obtain an approximate ship's position for evaluating radar targets to use for range measurements. This method is not suitable while piloting in restricted waters and should only be used if no more accurate method (combining visual bearings with radar ranges for example) is available.

Visual piloting requires bearings from at least two objects; radar, with its ability to determine both bearing and range from one object, allows the navigator to obtain a fix where only a single navigation aid is available. An example of using radar in this fashion occurs in approaching a harbor whose entrance is marked with a single, prominent object such as Chesapeake Light at the entrance of the Chesapeake Bay. Well beyond the range of any land-based visual navigation aid, and beyond the visual range of the light itself, a shipboard radar can detect the light and provide bearings and ranges for the ship's piloting party. But care should be taken. Navigators must ensure they take fixes on the navigation aid and not some nearby stationary vessel.

This methodology is limited by the inherent inaccuracy associated with radar bearings; typically, a radar bearing is accurate to within about 5° of the true bearing due to factors such as beam width distortion. Therefore, the navigator must carefully evaluate the resulting position, possibly checking it with a sounding. If a visual bearing is available from the object, use that bearing instead of the radar bearing when laying down the fix. This illustrates the basic concept discussed above: radar ranges are inherently more accurate than radar bearings. One must also be aware that even though the radar is gyro stabilized, there may be a gyro error of more than a degree or so. Radar and visual bearings will be in error by that amount.

Prior to using this method, navigators must ensure they have correctly identified the object from which the bearing and range are to be taken. Using only one navigation aid for both lines of position can lead to disaster if the navigation aid is not properly identified.

This method combines bearings tangent to an object with a range measurement from some point on that object. The object must be large enough to provide sufficient bearing spread between the tangent bearings; often an island or peninsula works well. Identify some prominent feature of the object that is displayed on both the chart and the radar display. Take a range measurement from that feature and plot it on the chart. Then determine the tangent bearings to the feature and plot them on the chart. The range LOP should not intersect where the tangent bearing LOPs intersect but somewhat farther out. The fix position will be the point midway between the tangent bearing lines along the range LOP (see Figure 2416).

Steep-sided features work the best. Tangents to low, sloping shorelines will seriously reduce accuracy, as will tangent bearings in areas of excessively high tides, which can change the location of the apparent shoreline by many meters.

Whenever a vessel is being navigated in confined waters, traditional position fixing methods become inadequate. The time lag inherent in taking a visual bearing, radar bearing or radar range, plotting positions on a nautical chart, obtaining a fix, and then acting on the information with a possible course change may be as much as five minutes or more, even for experienced navigators. If sea room is severely restricted and there are hazards to navigation in the area, such delays could lead to disaster. What we must do in this unforgiving situation is to monitor the vessel's

[Figure 2416 in Bowditch, Pub. No. 9: Fix using tangent bearings and range.]

position constantly through continuous position fixes. ECDIS is of course greatly preferable to paper chart navigation in these circumstances but suffers from complete reliance on GPS position fixes. Radar can provide similar real-time navigation capability not reliant on GPS utilizing a technique known as parallel indexing.

A properly prepared parallel indexing plot will quickly show the navigator when the vessel begins to deviate from the desired track. This will enable corrective measures to be taken immediately without resorting to time-consuming standard fixing methods. Parallel indexing can be indispensable when a vessel must be navigated through confined waters during restricted visibility or when executing a critical turn. Also, in areas with few or unreliable navigational aids, parallel indexing can prove decisive to safe navigation.

The first step in setting up a parallel indexing plot is to examine the nautical chart where the piloting will take place. Imagine that we wish to follow a track line that leaves a small island or rock to starboard at a distance of 2 miles off when abeam. The track line course is 045º (see Figure 2417). If we are able to place an electronic line on the radar screen bearing 045º-225º at a range of 2 miles to starboard, all we will have to do when the island comes onto the radar display is to maneuver the ship to keep the island on that line which in turn locates (indexes) the vessel on the track line.

One way to conduct parallel indexing on a modern radar display is to utilize the Electronic Bearing Line (EBL) feature. Most radars have the ability to offset the EBL from the center of the display. This allows it to be used as a single parallel index line. Once the EBL bearing is set to that of the vessel's track line, the origin can be floated out to the desired distance tangent to a Variable Range Marker (VRM) set to that distance.

[Figure 2417 in Bowditch, Pub. No. 9: Parallel indexing setup.]

Modern radar sets are usually fitted with a dedicated parallel indexing (PI) feature that may take many forms depending on the radar manufacturer, and are easier to use than the floating EBL. While the details of these PI features

may be quite different, they all have the following in common:

1. 1. The display of an electronic PI line, wholly or

partially across the radar screen.

2. The PI line is adjustable in direction (bearing) and

distance (range) from own ship.

3. Once set at desired bearing and range, the PI line is

fixed relative to own ship.

It is vital that when placing a single PI line on the radar display, the bearing of the line is set first, then the range. If done in reverse order, the distance of the PI line from own ship to target will be less than desired.

The method described above is very basic and utilizes only a single index line and a single index target. But the level of sophistication of indexing required varies with the situation. A passage may call for many lines on different scales, multiple index targets, margin lines, danger zones and wheel over points. The more complicated the setup, of course, the more time and effort on the part of the navigator is demanded. More complex indexing schemes, however elegant, also carry a greater risk of error in construction. A point will be reached where there is little to be gained by an excessively elaborate setup because it may also lead to a more cluttered and confusing radar display. A vessel that routinely makes passages through navigationally challenging waters would be better advised to rely more on the ECDIS and use a simpler parallel indexing setup on the radar as a backup and for cross checking.

Pecota, S., (2006). Radar Observer Manual, 6th. Marine Education Textbooks. Section 2317 reprinted with permission.

Until the arrival of this age of electronic technology, mariners relied solely on celestial navigation, paper charts and mechanical soundings techniques to navigate the world’s oceans, Now, however, satellite technology, GNSS, and electronic positioning systems are capable of achieving sub-meter positioning accuracy, and vessels can even navigate using automated means alone. Satellite navigation has become so reliable that some maritime academic institutions have removed celestial navigation from their curriculum. Hydrographic offices, too, put the bulk of their efforts on producing electronic navigational charts in response to increasing industry demand for digital products and decreasing need for paper charts.

However, all things electronic are subject to the potential for failure, and as technology advances it is possible to become over-reliant on a single set of tools. As the maritime sector gradually acknowledges this vulnerability, there is renewed interest in traditional navigation techniques. For example, bathymetric navigation, which utilizes charted seafloor features and contours to help determine the position of a vessel, is once again being actively used in combination with celestial navigation or dead reckoning to provide a position solution in the absence of satellite navigation.

Bathymetry is the science of mapping seafloor relief. Accurate bathymetric surveys help hydrographers identify submerged hazards to navigation, and allow oceanographers and geologists to better understand seafloor morphology and its impact on the ocean environment.

The principle behind bathymetric navigation is simple. When a mariner knows a vessel’s last position with reasonable confidence, and has nautical charts that depict soundings, seafloor features and depth curves, then the mariner can use those charted bathymetric features to refine their assumed position.

For example, if a mariner were to be navigating in the vicinity of a charted seamount, and there exists a measure of uncertainty regarding the accuracy of their positioning fix, the mariner can validate the vessel’s position by comparing echo sounder readings with the assumed position while sailing over the submerged seamount.

The usefulness of this technique is dependent upon several factors: the accuracy of the chart, the reliability of the last position fix, and the capabilities of the vessel's echo sounder.

The National Oceanic and Atmospheric Administration (NOAA) produces a series of bathymetric maps of the waters adjacent to portions of the coast of the United States. These maps extend seaward somewhat beyond the 100- fathom curve and show the contour of the bottom in considerable detail. Such maps can be of great assistance in fixing position by means of the depth finder. The maps are available online and can be accessed via the link provided in Figure 2701.

[Figure 2701 in Bowditch, Pub. No. 9: NOAA - U.S. Bathymetric Maps https://www.ngdc.noaa.gov/mgg/bathymetry/maps/nos_int ro.html]

Nautical charts are compiled from a combination of bathymetric surveys, soundings collected using a variety of historical techniques, and depths reported by mariners. Although it may be tempting to assume that where there are soundings on a chart, the area has been thoroughly surveyed, this can be a dangerous presumption. It is not an over-generalization to say that most of the world's oceans are still unsurveyed.

The ocean is vast, and although technology is always improving, modern hydrographic surveys are still expensive and time-consuming. In many areas, charted soundings are compiled from pre-1900 lead line surveys, or from 20th century singlebeam echo sounder surveys (see chapter on Hydrography for more information about survey techniques). These survey methods do not provide full sea floor coverage, and could miss significant seafloor features.

In addition, much of the depth information in nautical

charts was collected before modern satellite positioning techniques were available. This introduces a degree of uncertainty in the location of some charted depths. Mariners should always consult the chart's source diagram to determine the type and age of data that was used to compile soundings for any specific region of the chart, and always use the best-scale, most current product available for bathymetric navigation.

When vessels navigate using GNSS, mariners can usually be confident in the accuracy of their position fix. There may be times when the quality of the satellite signal degrades due to poor geometry overhead, or steep terrain that blocks signals or causes multipath (such as in narrow fjords), but generally, satellite navigation is reliable.

If, however, a navigator is unable to use satellite positioning systems, they will need to rely on other methods, such as celestial navigation or dead reckoning (or a combination of the two). With accurate celestial navigation measurements, obtained through practice and skill, mariners can determine their position with a reasonable degree of accuracy (see Part 3 on Celestial Navigation for more information).

When persistent inclement weather or overcast skies prevent mariners from taking star sights or sun fixes, then mariners must resort to dead reckoning. Dead reckoning measures the amount of time elapsed since the last known position fix, the speed of the vessel, its ordered course, and known set and drift to derive an estimate of the vessel's location. The reliability of dead reckoning degrades with time, as the compilation of slight errors compound.

Because there is almost always some uncertainty in position fixes when using celestial navigation and dead reckoning, mariners find it useful to better determine their location using bathymetric navigation.

Most modern vessels, from small pleasure boats to large cargo ships, have some type of electronic echo sounder mounted on the keel. These echo sounders measure the time it takes for a pulse of sound to travel to the seafloor and return to the transducer. This measurement of time is then electronically translated into a depth measurement, and the navigator of the vessel uses the depth measurement, in tandem with a nautical chart, to determine a safe course.

Echo sounders vary widely in design and capability. Many models collect depth information about only a narrow cone of water beneath the vessel. These may be referred to as singlebeam SONAR, fathometers, or depth finders. Some higher-end models emit many beams of sound, in a wide swath below, or in front of the vessel. These designs are called multibeam systems, and are capable of generating a very high-resolution three-dimensional SONAR image of the seafloor or approaching obstacles (see chapter on Hydrography for more information about echo sounder designs).

All echo sounders are limited to a certain depth operating range, which is constrained by the power and frequency of the system. In general, shallow-water echo sounders will be higher-frequency, and require less power. Deep-water echo sounders will be lower-frequency, and require more power. Echo sounders are capable of detecting smaller features in shallow water, and their resolution degrades with depth pulses.

Before attempting bathymetric navigation, mariners should determine what kind of echo sounder they have on board. This will help them identify the capabilities and limitations of their system. For instance, with a singlebeam echo sounder, the mariner would be able to compare charted soundings to depth measurements, and follow patterns in depth trends that correspond to charted contours. With a multibeam echo sounder, mariners might be able to generate very high-resolution SONAR maps of the seafloor. While this could help the mariner identify charted features, it is also possible to collect higher-resolution data than depicted on the chart!

Regardless the type of echo sounder used, it is likely that, at some point, vessels will collect depth information in transit where there is no charted data. This information can be valuable for oceanographic studies, hydrographic purposes, and the greater public good. For those who will donate their data, the International Hydrographic Organization (IHO) supports a crowdsourced bathymetry initiative that encourages mariners to connect data loggers to their echo sounders, and submit the collected information to a public database. For more information, See the link provided in Figure 2704 or visit www.iho.int.

It is also possible to collect depth information without an echo sounder in shallow water, using a lead line or sounding pole. These techniques are not widely used today, as they are time-intensive, and require stopping the vessel and manually deploying a weighted line or long pole over the side. However, they are reliable methods of obtaining depth information, and in theory can be used to compare charted depths with measured soundings.

[Figure 2704 in Bowditch, Pub. No. 9: IHO World Bathymetry https://iho.int/en/data-centre-for-digital-bathymetry]

Mariners should be aware that the depth measurements collected by echo sounders will vary based on the temperature, salinity, and depth of the local water column. Each of these factors has an impact on the speed and path of sound waves through water. In general, sound travels faster through warm water, saltier water, and deeper (denser) water.

Because echo sounders generate a depth measurement based on the two-way travel time of a sonar beam, differences in water column composition can generate variations in depth measurements. For example, if a vessel travels through a coastal area where a freshwater river runs into the sea, the speed of sound will slow, and the depth measurement may be slightly incorrect.

Some echo sounders have a surface sound velocimeter installed on the hull to at least partially correct for these local water column variations, others do not. Before attempting bathymetric navigation, one should determine whether or not the echo sounder is equipped with integrated sound velocity corrections.

Sound waves from an echo sounder will reflect off of anything in the water column, and may even penetrate the surface layer of soft or muddy bottom sediment, and reflect off of the underlying bedrock. Fish, bubbles (from marine life or another ship's wake), dense layers of plankton or marine life, vegetation, variations in salinity, and marine mammals can all cause 'false bottom' readings, or obscure the true bottom. Mariners should be aware of these exceptions when using fathometers to identify seafloor features.

Once the mariner has obtained an initial position fix, determined a rough assessment of the age and accuracy of the information used to compile their chart, and identified the type of echo sounder that is mounted in their vessel, they are ready to use bathymetric information to verify their position.

For the purposes of this chapter, we will assume that the mariner is using hardcopy charts and non-satellite positioning methods, and is navigating out of sight of land. Note that if the mariner is within sight of land, it may be easier and more accurate to verify position by simply taking bearings on features on shore, rather than by comparing bathymetric features to echo sounder readings.

If the mariner is beyond the sight of land, they should choose a prominent charted seafloor feature or set of features that fall near their estimated position, and are at a depth that permits the vessel to safely transit across it. The features should be unique enough to be readily detectable (such as seamounts, ridges or canyons), but should not be complex or 'clumped' features, as they will be more difficult to distinguish and use for positioning. The feature should have enough relief to be easily distinguishable from the surrounding seafloor. If the seafloor is flat and featureless bathymetric navigation will not work, as all depths will appear relatively uniform.

Ideally, the mariner should select a feature that falls within an area recently surveyed for hydrographic purposes (as indicated on the source diagram); those features are likely to be more accurately and fully represented than features from other sources. For example, if an area was fully surveyed using multibeam sonar, a cartographer knows exactly where the 50m contour is located. If an area was surveyed with isolated lead line soundings, the cartographer has to make an educated guess about where to draw the 50m contour between soundings.

If there are no recent hydrographic surveys in the area, the mariner should simply choose a feature that is prominent, and is not listed as 'reported,' 'position doubtful,' etc. Some features that may be useful for bathymetric navigation are:

• Seamounts (isolated or in small groups)

• Ridges

• Canyons

• Plateaus

Once the mariner has selected the feature or set of features, they should plot a course across the feature (or features), reduce vessel speed so that the echo sounder return provides a clear and easily readable bottom trace, and then transit over the feature, attempting to intersect it in a direction that provides the clearest delineation of its location.

If the mariner is using a ridge or a canyon to verify position, the vessel should cut across the feature in a direction that is perpendicular to its main (long) axis. The echo sounder will provide a clear profile of the sides of the feature, and the positions and depths can then be compared to the contours or depths on the chart.

If the mariner is using a plateau or seamount to verify a position fix, the vessel may need to make one or more passes over the feature, which each line offset at a different angle from the last, to ensure that they have located the feature, and not just clipped the edge. If the area is poorly surveyed, caution should be exercised when doing this, to prevent the vessel from encountering a portion of the seamount that is shoaler than charted.

Whenever possible, it is best to transit more than one feature in a row, as locating multiple features that area aligned at a single known bearing provides the most accurate position verification

[Figure 2708 in Bowditch, Pub. No. 9: Bottom features (top row) compared with measured echogram (bottom row).]

When using bathymetric features to validate a position fix, it is important to note that the vessel's last known position should be reasonably reliable. If there are gross errors in dead reckoning measurements or celestial navigation calculations, bathymetric positioning will be of little value, as the mariner will be searching in the wrong area to begin with. Bathymetric navigation should be used as a refinement of last known position, not the sole positioning determination method.

In addition, it should be noted that the sonic footprint of some singlebeam echo sounders can make the sides of submerged features appear more rounded than they actually are (Figure 2708). This should be taken into account when comparing charted contours to echo sounder traces.

Profile-matching. If a vessel is operating in an area where there are no significant features, but the charted contours are varied enough to assist with position identification, a vessel could transit back and forth across the area in a grid pattern, recording the depth profiles with each pass, and then correlate those sequential profiles to the charted contours.

Profile-matching is more time-consuming than the feature recognition method, but if executed properly, and where good comparison contour data is available, this could yield very accurate positioning information. If the vessel has a multibeam echosounder on board, this process would produce a fairly high-resolution map of the seafloor.

Contour Advancement. As with profile-matching, contour advancement does not require that significant features be present, but it is desirable to transit across a gently sloping area with slopes that are greater than one degree, but no more than four or five degrees. The area should also be well-charted, with moderately reliable contours.

To use the contour advancement technique to verify a position fix, a vessel must transit across an area at a constant bearing and speed, in line with the direction of a known slope. When the (singlebeam) echo sounder depth matches a charted contour depth, the navigator knows that the ship is somewhere on that charted depth contour - but precisely where is unknown. This first contour becomes the 'reference' contour, and is traced onto a transparent overlay that will be shifted (or advanced) on the chart as new contour depths are collected.

When the echo sounder indicates that the vessel has reached the next charted contour depth, the navigator moves the reference contour overlay forward to the new estimated position on the chart. The distance that the contour is advanced is determined by measuring the time it took to travel between observed contour soundings, and multiplying that time by the vessel's constant speed.

Advancing the reference contour has the effect of moving every possibility of the ship's starting location on that first contour visually into the vicinity of the next contour (basically, offsetting every point on the first contour by the distance covered, without having to manually draw all of those infinite offset points). If executed accurately, the intersection of the advanced contour (i.e. the first contour, offset by the distance covered) and the next charted contour will note the true location of the ship, provided that there is only one intersection.

Multiple intersections of the reference and charted contours means that there is more than one possible position. The mariner must then continue the contour advancement process to determine which candidate is the true track.

In the Figure 2709, the solid lines are charted contours, and the dashed lines are the advancement of the initial reference contour. The ship's estimated track (which the navigator plots on the overlay, and could have started anywhere on the reference contour) is the dashed line perpendicular to the contours. The time between echo sounder observations of charted contour depths is annotated on each track. The ship's true position is shown on the right; it identifies the areas where the vessel crossed each of the contours, after the advanced contour indicated the points of intersection.

Typical accuracy for contour advancement is approximately one-quarter of the contour spacing of the chart.

[Figure 2709 in Bowditch, Pub. No. 9: The technique of contour advancing is illustrated in this figure. The isobaths shown in Figure 4 as solid lines are the contour lines on the chart; all dashed isobaths are results of contour advancement of the chart isobaths. Shown here for convenience is the true ship's position track, which is unknown to the navigator. The time of passage of the charted isobaths is also shown on the true track for convenience, more importantly, they are shown on the ship's estimated position track, which the navigator is plotting on the chart. Initially, the navigator traces the 140 isobath onto the overlay at 9:30 and establishes a reference point anywhere on the traced 140 isobath. Also note that this procedure can be performed after the fact if the times of passage, their charted depth, and the ship's velocity have been recorded for later use. When the next charted isobath, 150, is passed at 9:45, the navigator advances the overlay to the new estimated position. The advanced 140 isobath must intersect the charted 150 isobath at the ship's true location. If this is the only place of intersection of these two isobaths, the fix can be established at this time; the location of the intersection of the two isobath's is the ship's true position at 9:45. But there may be multiple locations of intersection, one of these is hinted at on the far right side of the displayed contours and perhaps one slightly to the left of the ship's estimated position. The advancement can then continue to the next charted isobath, 160, to help resolve the true location. In that case, the advanced 140 and 150 isobaths hint at possibly intersecting again at the far right, but the charted 160 isobath diverges from them there, and the location slightly to the left of the estimated ship's track doesn't show a strong three-way intersection like at the point on the true track at the point labeled 10:20, showing that the point labeled 10:20 on the true track would be the ship's true position at that time. The intersections may form a triangle instead of a point due to errors, the smaller the triangle, the better the confidence in the fix; or perhaps additional contours can be advanced to help resolve the fix if the error triangle is too large. Typical accuracy is on the order of ±100 yards, or about one-quarter of the line spacing of the chart.]

Basic Rules of Thumb for Contour Advancing are as follows:

• An accurate bathymetric chart of the region being

traversed is required.

• Slopes should be between 1° to 4° (no more than 5°), and they should be varying; use of areas with constant slopes could result in intersections which are along lines, not at points, and so would not reveal precise location.

[Figure 2710 in Bowditch, Pub. No. 9: Line of soundings technique. Image courtesy of Johns Hopkins University - Applied Physics Laboratory.]

• Contour advancing is made easier by using the larg-

est scale chart available, assuming the area is not

absolutely flat. A given chart may not show any

slope for a given area, but the area may show some

relief on a larger chart.

Recovering a position fix using the line-of-soundings technique is similar to contour advancement, but usually requires collecting more observed depths to obtain an accurate fix. A vessel using a singlebeam echo sounder runs a single straight line over a charted area at a constant bearing and speed, and the sounding values that correspond to charted contours are plotted onto a trackline on a clear chart overlay. The line is then moved across the charted contours, until the plotted soundings match up with the charted contour interval. This provides a position fix. If a vessel is using a multibeam echo sounder, the continuous swath of seafloor data can also be compared to the charted contours, to identify matching patterns with higher fidelity.

In Figure 2710, the red lines represent the singlebeam echo sounder trackline of plotted overlay soundings. The navigator moved this trackline across the charted contours, using the 500m contour as a central reference guide, until they found a match (the trackline in the center of the chart). Like other forms of bathymetric navigation, this method would not work in a very flat area, since the trackline would appear to match the depths in many directions.

The side-echo technique is useful for determining position when traversing seamounts. For this method, the vessel must conduct at least two transit lines across the seamount, each of constant bearing and speed, offset from each other at right angles. The depth trend on each line indicates the quadrant location of the shoalest (shallowest) point of the seamount, relative to the intersection of the transit lines.

The navigator should plan the initial transit line so that it approaches the seamount from a distance of at least 20 or 30 nautical miles. In deep water, this distance will help the navigator identify changes in seafloor relief and will help prevent missing the feature (the track should capture at least the base of a large feature, even if the shoalest point is missed). The vessel should maintain a constant course

while approaching the seamount. If the vessel is using a singlebeam echosounder, the navigator should plot the depths at regular intervals (e.g. once per minute) while crossing the feature. The minimum depth should be noted and marked on the trackline.

Once the initial transit has crossed the feature, the vessel should run another transit line, exactly perpendicular to the first. The navigator should again plot the depths periodically, and annotate the point of least depth. If a line is then drawn between the shallowest point on each line, the point where they intersect indicates the quadrant of the shallowest point on the seamount. If the vessel is using a multibeam sonar, the overlapping swaths from each line should show a clear depth trend towards one quadrant, and could even capture the shoalest point of the feature.

Once the approximate location of the shallowest point of the seamount is determined, that location can be compared to the charted minimum depth, to provide a position location for the vessel. Additional lines, offset to the first two, could help to more precisely locate the shoalest points. However, it is important to note that the charted minimum depth of the seamount could be wrong; previous surveys or reports may have only crossed over one side of the seamount, instead of directly over the shallowest part, or the horizontal positioning methods used at the time may have been inaccurate, and the seamount's location could be slightly incorrect (see Figure 2711b). Whenever possible, the mariner should try to select a feature that comes from a reliable source, such as a hydrographic survey.

If a mariner finds that the actual depth of the seamount is shallower than the charted least depth, that information should be reported to NGA's Maritime Safety Office as soon as possible (see link in Figure 2711a), so that the

[Figure 2711a in Bowditch, Pub. No. 9: Link to NGA’s Maritime Safety Office - Contact Information. https://msi.nga.mil/NavWarnings]

Automated programs exist that can incorporate single-beam or multibeam SONAR data, along with the speed and course of the vessel, and compare that data directly to digital features and chart contours. These programs can provide an approximate position fix without the need to manually overlay and plot soundings.

Cohen, P M., (1970). Bathymetric Navigation and Charting. U.S. Naval Institute Press, Annapolis, MD. Sections reprinted with permission.

Cutler, T J., (2003). Dutton’s Nautical Navigation, 15th Edition. U.S. Naval Institute Press, Annapolis, MD. Sections reprinted with permission.

[Figure 2711b in Bowditch, Pub. No. 9: Side-echo bathymetric navigation. This image is provided courtesy of Johns Hopkins University- Applied Physics Laboratory.]

The navigator’s chief responsibility is the safety of the vessel and its crew. Fulfilling this duty consists mostly of ascertaining the ship’s position and directing its course to avoid dangers. But accidents can happen to the most cautious, and the most prudent of navigators may experience an emergency which requires outside assistance. Distress incidents at sea are more likely to be resolved without loss of vessel and life if they are reported immediately. The more information that rescue authorities have, and the sooner they have it, the more likely it is that the outcome of a distress at sea will be favorable.

Global distress communication systems, ship reporting systems, emergency radiobeacons, commercial ship tracking and other technologies have greatly enhanced mariners’ safety. Therefore, it is critical that mariners understand the purpose, functions, and limitations of maritime safety systems as well as threats to maritime security.

The mariner’s direct high-seas link to shoreside rescue authorities is the Global Maritime Distress and Safety System (GMDSS), which was developed to both simplify and improve the dependability of communications for all ships at sea. GMDSS nicely compliments the operation of the U.S. Coast Guard’s AMVER system, which tracks participating ships worldwide and directs them as needed to distress incidents. GMDSS and AMVER rely on radiotelephone or satellite communications for passing information. But even with normal communications disabled, a properly equipped vessel has every prospect of rapid rescue or aid if it carries a SOLAS-required Emergency Position Indicating Radiobeacon (EPIRB) and a Search and Rescue radar Transponder (SART). These systems are the subject of this chapter.

The Global Maritime Distress and Safety System (GMDSS) represents a significant improvement in maritime safety over the previous system of short range and high seas radio transmissions. Its many parts include satellite as well as advanced terrestrial communications systems. Operational service of the GMDSS began on February 1, 1992, with full implementation accomplished by February 1, 1999.

GMDSS was adopted in 1988 by amendments to the Conference of Contracting Governments to the International Convention for the Safety of Life at Sea (SOLAS), 1974. This was the culmination of more than a decade of work by the International Maritime Organization (IMO) in conjunction with the International Telecommunications Union (ITU), International Hydrographic Organization (IHO), World Meteorological Organization (WMO), Inmarsat (International Maritime Satellite Organization), and others.

GMDSS offers the greatest advancement in maritime safety since the enactment of regulations following the Titanic disaster in 1912. It is an automated ship-to-ship, shore-to-ship and ship-to-shore communications system covering distress alerting and relay, the provision of maritime safety information (MSI), and routine communications. Satellite and advanced terrestrial systems are incorporated into a communications network to promote and improve safety of life and property at sea throughout the world. The equipment required on board ships depends on their tonnage and the area in which the vessel operates. This is fundamentally different from the previous system, which based requirements on vessel size alone. The greatest benefit of the GMDSS is that it vastly reduces the chances of ships sinking without a trace, and enables search and rescue (SAR) operations to be launched without delay and directed to the exact site of a maritime disaster.

By the terms of the SOLAS Convention, the GMDSS provisions apply to cargo ships of 300 gross tons and over and ships carrying passengers on international voyages. Unlike previous shipboard carriage regulations that specified equipment according to size of vessel, the GMDSS carriage requirements stipulate equipment according to the area in which the vessel operates (and vessel size in some cases). These sea areas are designated as follows: Sea Area A1. An area within the radiotelephone coverage

of at least one VHF coast station in which continuous

Digital Selective Calling is available, as may be

defined by a Contracting Government to the 1974

SOLAS Convention. This area extends from the coast

to about 20 miles offshore. Sea Area A2. An area, excluding sea area A1, within the

radiotelephone coverage of at least one MF coast sta-

tion in which continuous DSC alerting is available, as

may be defined by a Contracting Government. The

general area is from the A1 limit out to about 100 miles

offshore. Sea Area A3. An area, excluding sea areas A1 and A2,

within the coverage of an Inmarsat geostationary satel-

lite in which continuous alerting is available. This area

is from about 70°N to 70°S. Sea Area A4. All areas outside of sea areas A1, A2 and A3.

This area includes the polar regions, where geostation-

ary satellite coverage is not available.

Ships at sea must be capable of the following functional GMDSS requirements:

1. Ship-to-shore distress alerting, by at least two sep-

arate and independent means, each using a different

radio communication service

2. Shore-to-ship distress alerting

3. Ship-to-ship distress alerting

4. SAR coordination

5. On-scene communications

6. Transmission and receipt of emergency locating

signals

7. Transmission and receipt of MSI

8. General radio communications

9. Bridge-to-bridge communications

To meet the requirements of the functional areas above the following is a list of the minimum communications equipment needed for all ships:

1. VHF radio capable of transmitting and receiving

DSC on channel 70, and radio telephony on chan-

nels 6, 13 and 16

2. Radio receiver capable of maintaining a continuous

Digital Selective Calling (DSC) watch on channel

70 VHF

3. Search and rescue transponders (SART). Only one

SART is required if the vessel is under 500 gross

tons. Two SARTs are required if the vessel is over

500 tons and must be capable of operating in the 9

GHz band (AIS SART meets carriage require-

ments).

4. Receiver capable of receiving NAVTEX broad-

casts anywhere within NAVTEX range

5. Receiver capable of receiving SafetyNET any-

where NAVTEX is not available

6. Satellite emergency position indicating radiobea-

con (EPIRB), manually activated and float-free

self-activated

7. Two-way handheld VHF radios (two sets minimum

on 300-500 gross tons cargo vessels and three sets

minimum on cargo vessels of 500 gross tons and

upward and on all passenger ships)

Additionally, each sea area has its own requirements under GMDSS which are as follows: Sea Area A1

1. General VHF radio telephone capability

2. Free-floating satellite EPIRB

3. Capability of initiating a distress alert from a navi-

gational position using DSC on either VHF, HF or

MF; manually activated EPIRB; or Ship Earth Sta-

tion (SES) Sea Areas A1 and A2

1. Radio telephone MF radiotelephony or direct print-

ing 2182 kHz, and DSC on 2187.5 kHz

2. Equipment capable of maintaining a continuous

DSC watch on 2187.5 kHz

3. General working radio communications in the MF

band (1605-4000 kHz), or Inmarsat SES

4. Capability of initiating a distress alert by HF (using

DSC), manual activation of an EPIRB, or Inmarsat

SES Sea Areas A1, A2 and A3

1. Radio telephone MF 2182 kHz and DSC 2187.5

kHz.

2. Equipment capable of maintaining a continuous

DSC watch on 2187.5 kHz

3. Inmarsat-C (class 2) or Fleet 77 SES Enhanced

Group Call (EGC), or HF as required for sea area

A4

4. Capability of initiating a distress alert by two of the

following:

a. Inmarsat-C (class 2) or Fleet 77 SES

b. Manually activated EPIRB

c. HF/DSC radio communication Sea Area A4

1. HF/MF receiving and transmitting equipment for

band 1605-27500 kHz using DSC, radiotelephone

and direct printing

2. Equipment capable of selecting any safety and dis-

tress DSC frequency for band 4000-27500 kHz,

maintaining DSC watch on 2187.5, 8414.5 kHz and

at least one additional safety and distress DSC fre-

quency in the band

3. Capability of initiating a distress alert from a navi-

gational position via the Polar Orbiting System on

406 MHz (manual activation of 406 MHz satellite

EPIRB)

Inmarsat (International Maritime Satellite Organization), a key player within GMDSS, is an international corporation comprising over 75 international partners providing maritime safety communications for ships at sea. Inmarsat provides the space segment necessary for improving distress communications, efficiency and management of ships, as well as public correspondence services.

The basic components of the Inmarsat system include the Inmarsat space segment, Land Earth Stations (LES), also referred to as Coast Earth Stations (CES), and mobile Ship Earth Stations (SES).

The Inmarsat space segment consists of 11 geostationary satellites. Four operational Inmarsat satellites provide primary coverage, four additional satellites (including satellites leased from the European Space Agency (ESA) and the International Telecommunications Satellite Organization (INTELSAT) serve as spares and three remaining leased satellites serve as back-ups.

The polar regions are not visible to the operational satellites but coverage is available from about 75°N to 75°S. Satellite coverage (Figure 3103) is divided into four overlapping regions:

1. Atlantic Ocean - East (AOR-E)

2. Atlantic Ocean - West (AOR-W)

3. Pacific Ocean (POR)

4. Indian Ocean (IOR)

The LES’s provide the link between the Space Segment and the land-based national/international fixed communications networks. These communications networks are funded and operated by the authorized communications authorities of a participating nation. This network links registered information providers to the LES. The data then travels from the LES to the Inmarsat Network Coordination Station (NCS) and then down to the SES’s on ships at sea. The SES’s provide two-way communications between ship and shore. Fleet 77 service is digital and operates at up to 64kbps.

Inmarsat-C provides a two-way store and forward data messaging capability (but no voice) at 600 bits per second and was designed specifically to meet the GMDSS requirements for receiving MSI data on board ship. These units are small, lightweight and use an omni-directional antenna.

[Figure 3103 in Bowditch, Pub. No. 9: The four regions of Inmarsat coverage.]

Major categories of MSI for both NAVTEX and SafetyNET are:

1. Navigational warnings

2. Meteorological warnings

3. Ice reports

4. Search and rescue information

5. Meteorological forecasts

6. Pilot service messages (not in the U.S.)

7. Electronic navigation system messages (i.e., GPS,

DGPS, etc.)

Broadcasts of MSI in NAVTEX international service are in English, but may be in languages other than English to meet requirements of the host government.

SafetyNET is a broadcast service of Inmarsat-C’s Enhanced Group Call (EGC) system. The EGC system (Figure 3105a) is a method used to specifically address particular regions or groups of ships. Its unique addressing capabilities allow messages to be sent to all vessels in both fixed geographical areas or to predetermined groups of ships. SafetyNET is a service designated by the IMO through which ships receive maritime safety information. The other service under the EGC system, called FleetNET, is used by commercial companies to communicate directly and privately with their individual fleets.

SafetyNET is an international shore to ship satellite-based service for the promulgation of distress alerts, navigational warnings, meteorological warnings and forecasts, and other safety messages. It fulfills an integral role in GMDSS as developed by the IMO. The ability to receive SafetyNET messages is required for all SOLAS ships that sail beyond coverage of NAVTEX (approximately 200 miles from shore).

SafetyNET can direct a message to a given geographic area based on EGC addressing. The area may be fixed, as in the case of a NAVAREA or weather forecast area, or it may be uniquely defined by the originator. This is particularly useful for messages such as local storm warnings or focused shore to ship distress alerts.

SafetyNET messages can be originated by a Registered Information Provider anywhere in the world and broadcast to the appropriate ocean area through an Inmarsat-C LES. Messages are broadcast according to their priority (i.e. Distress, Urgent, Safety, and Routine).

Virtually all navigable waters of the world are covered by the operational satellites in the Inmarsat system. Each satellite broadcasts EGC traffic on a designated channel. Any ship sailing within the coverage area of an Inmarsat satellite will be able to receive all the SafetyNET messages broadcast over this channel. The EGC channel is optimized to enable the signal to be monitored by SES’s dedicated to the reception of EGC messages. This capability can be built into other standard SES’s. It is a feature of satellite communications that reception is not generally affected by the position of the ship within the ocean region, atmospheric conditions, or time of day.

Messages can be transmitted either to geographic areas (area calls) or to groups of ships (group calls):

1. Area calls can be to a fixed area such as one of the

16 NAVAREA’s or to a temporary geographic area

selected by the originator (circular or rectangular).

Area calls will be received automatically by any

ship whose receiver has been set to one or more

fixed areas.

2. Group calls will be received automatically by any

ship whose receiver acknowledges the unique

group identity associated with a particular mes-

sage.

Reliable delivery of messages is ensured by forward error correction techniques. Experience has demonstrated that the transmission link is generally error-free and low error reception is achieved under normal circumstances.

Given the vast ocean coverage by satellite, some form of discrimination and selectivity in printing the various messages is required. Area calls are received by all ships within the ocean region coverage of the satellite; however, they will be printed only by those receivers that recognize the fixed area or the geographic position in the message. The message format includes a preamble that enables the microprocessor in a ship’s receiver to decide to print those MSI messages that relate to the present position, intended route or a fixed area programmed by the operator. This preamble also allows suppression of certain types of MSI that are not relevant to a particular ship. As each message will also have a unique identity, the reprinting of messages already received correctly is automatically suppressed.

MSI is promulgated by various information providers around the world. Messages for transmission through the SafetyNET service will, in many cases, be the result of coordination between authorities. Information providers will be authorized by IMO to broadcast via SafetyNET. Authorized information providers are:

1. National hydrographic offices for navigational

warnings

2. National weather services for meteorological warn-

ings and forecasts

3. Rescue Coordination Centers (RCC’s) for ship-to-

shore distress alerts and other urgent information

4. In the U.S., the International Ice Patrol (IIP) for

North Atlantic ice hazards

Each information provider prepares their SafetyNET messages with certain characteristics recognized by the EGC service. These characteristics, known as “C” codes are

[Figure 3105a in Bowditch, Pub. No. 9: SafetyNET EGC concept.]

combined into a generalized message header format as follows: C1:C2:C3:C4:C5. Each “C” code controls a different broadcast criterion and is assigned a numerical value according to available options. A sixth “C” code, “C0” may be used to indicate the ocean region (i.e., AOR-E, AOR-W, POR, IOR) when sending a message to an LES which operates in more than one ocean region. Because errors in the header format of a message may prevent its being broadcast, MSI providers must install an Inmarsat SafetyNET receiver to monitor the broadcasts it originates. This also ensures quality control.

The “C” codes are transparent to the mariner, but are used by information providers to identify various transmitting parameters. C1 designates the message priority, either distress to urgent, safety, or routine. MSI messages will always be at least at the safety level. C2 is the service code or type of message (for example, long range NAVAREA warning or coastal NAVTEX warning). It also tells the receiver the length of the address (the C3 code) it will need to decode. C3 is the address code. It can be the two-digit code for the NAVAREA number for instance, or a ten-digit number to indicate a circular area for a meteorological warning. C4 is the repetition code which instructs the LES when to send the message to the NCS for actual broadcast. A six minute echo (repeat) may also be used to ensure that an urgent (unscheduled) message has been received by all ships affected. C5 is a constant and represents a presentation code, International Alphabet number 5, “00”.

Broadcasts of MSI in the international SafetyNET service must be in English, but may be supplemented by other languages to meet requirements of the host government.

The International SafetyNET Manual can be found online via the link provided in Figure 3105b.

[Figure 3105b in Bowditch, Pub. No. 9: International SafetyNET Manual. https://iho.int/en/wwnws-publications-documents]

NAVTEX is a maritime radio warning system consisting of a series of coast stations transmitting radio teletype (standard narrow-band direct printing, called Sitor for Simplex Telex Over Radio) safety messages on the internationally standard medium frequency of 518 kHz (490kHz local language). It is a GMDSS requirement for the reception of MSI in coastal and local waters. Coast stations transmit during previously arranged time slots to minimize mutual interference. Routine messages are normally broadcast four times daily. Urgent messages are broadcast upon receipt,

provided that an adjacent station is not transmitting. Since the broadcast uses the medium frequency band, a typical station service radius ranges from 100 to 500 NM day and night (although a 200 mile rule of thumb is applied in the U.S.). Interference from or receipt of stations further away occasionally occurs at night.

Each NAVTEX message broadcast contains a four-character header describing: identification of station (first character), message content or type (second character), and message serial number (third and fourth characters). This header allows the microprocessor in the shipboard receiver to screen messages from only those stations relevant to the user, messages of subject categories needed by the user and messages not previously received by the user. Messages so screened are printed as they are received, to be read by the mariner when convenient. All other messages are suppressed. Suppression of unwanted messages is becoming more and more a necessity to the mariner as the number of messages, including rebroadcast messages, increases yearly. With NAVTEX, a mariner will not find it necessary to listen to, or sift through, a large number of non-relevant data to obtain the information necessary for safe navigation.

The NAVTEX receiver is a small unit with an internal printer, which takes a minimum of room on the bridge. Its antenna is also of modest size, needing only a receive capability.

Valuable information regarding NAXTEX and navigational warnings can be found in Pub No. 117 Radio Navigation Aids via the link provided in Figure 3106.

[Figure 3106 in Bowditch, Pub. No. 9: NGA- Radio Navigational Aids (Pub. No. 117). https://msi.nga.mil/Publications/RNA]

Digital Selective Calling (DSC) is a system of digitized radio communications which allows messages to be targeted to all stations or to specific stations, allows for unattended and automated receipt and storage of messages for later retrieval, and permits the printing of messages in hardcopy form. All DCS calls automatically include error-checking signals and the identity of the calling unit. Digital codes allow DSC stations to transmit and receive distress messages, transmit and receive acknowledgments of distress messages, relay distress messages, make urgent and safety calls, and initiate routine message traffic.

Each unit has a MAYDAY button which allows the instant transmittal of a distress message to all nearby ships and shore stations. The location of the distress will be automatically indicated if the unit is connected to a GPS receiver. Each unit must be registered with the Coast Guard and have unique identifier programmed into it. Distress alerts can be sent on only one or as many as six channels consecutively on some units.

Listening watch on 2182 kHz ended with implementation of GMDSS in 1999. When DSC has been implemented worldwide, the traditional listening watch on Channel 16 VHF will no longer be necessary. The introduction of DSC throughout the world is expected to take a number of years.

There are four basic types of DSC calls:

• Distress

• Urgent

• Safety

• Routine

Distress calls are immediately received by rescue authorities for action, and all vessels receiving a distress call are alerted by an audible signal.

Each DSC unit has a unique Maritime Mobile Service Identity (MMSI) code number, which is attached to all outgoing messages. The MMSI number is a nine-digit number to identify individual vessels, groups of vessels, and coast stations. Ship stations will have a leading number consisting of 3 digits which identify the country in which the ship is registered, followed by a unique identifying number for the vessel. A group of vessels will have a leading zero, followed by a unique number for that group. A coast station will have 2 leading zeros followed by a code number. Other codes may identify all stations, or all stations in a particular geographic area.

DSC frequencies are found in the VHF, MF and HF bands. Within each band except VHF, one frequency is allocated for distress, urgent, and safety messages. Other frequencies are reserved for routine calls. In the VHF band, only one channel is available, Channel 70 (156.525 MHz), which is used for all calls. In the MF band, 2187.5 kHz and 2189.5 kHz are reserved for distress/safety, and 2177 kHz for ship-to-ship. 2189.5 kHz (in conjunction with 2177 kHz) is for routine ship-to-shore calls.

A distress call consists of a Format Specifier--Distress; the MMSI code; the nature of the distress (selected from a list: fire/explosion, flooding, collision, grounding, listing, sinking, disabled/adrift, or abandoning ship; defaults to Undesignated); the time of the call, and the format for subsequent communications (radiotelephone or NDBP). Once activated, a distress signal is repeated automatically every few minutes until an acknowledgment is received or the function is switched off. As soon as an acknowledgment is received by the vessel in distress, it must commence communications with an appropriate message by radiotelephone or NDBP according to the format:

“MAYDAY”

MMSI CODE NUMBER AND CALL SIGN

NAME OF VESSEL

POSITION

NATURE OF DISTRESS

TYPE OF ASSISTANCE NEEDED

OTHER INFORMATION

Routine calls should be made on a channel reserved for non-distress traffic. Once made, a call should not be repeated, since the receiving station either received the call and stored it, or did not receive it because it was not in service. At least 5 minutes should elapse between calls by vessels on the first attempt, then at 15 minute minimum intervals.

To initiate a routine ship to shore or ship to ship call to a specific station, the following procedures are typical (consult the operator’s manual for the equipment for specific directions):

• Select the appropriate frequency

• Select or enter the MMSI number of the station to be

called

• Select the category of the call

• Select subsequent communications method (R/T,

NDBP)

• Select proposed working channel (coast stations will

indicate vacant channel in acknowledgment)

• Select end-of-message signal (RQ for acknowledg-

ment required)

• Press <CALL>

The digital code is broadcast. The receiving station may acknowledge receipt either manually or automatically, at which point the working channel can be agreed on and communications begin.

Watchkeeping using DSC consists of keeping the unit ON while in the appropriate Sea Area. DSC watch frequencies are VHF Channel 70, 2187.5 kHz, 8414.5 kHz, and one HF frequency selected according to the time of day and season. Coast stations maintaining a watch on DCS channels are listed in NGA Pub. 117 Radio Navigational Aids and other lists of radio stations.

AMVER is an international maritime mutual assistance program that coordinates search and rescue efforts around the world. It is voluntary, free of charge, and endorsed by the IMO. The AMVER system is discussed in detail in Chapter 32. The AMVER website can be accessed through the link provided in Figure 3109.

[Figure 3109 in Bowditch, Pub. No. 9: AMVER website. https://www.amver.com]

Emergency Position Indicating Radiobeacons (EPIRBs) are designed to save lives by automatically alerting rescue authorities and indicating the distress location. EPIRB types are described below (Table 3110):

121.5/243 MHz EPIRBs (Class A, B, S): As of 1 January, 2007 the operation of 121.5 MHz EPIRBs has been prohibited in the United States. Satellite monitoring of the 121.5 MHz and 243.0 MHz frequencies was ceased 1 February, 2009.

All mariners using emergency beacons on either of these frequencies will need to upgrade to beacons operating on the newer, more reliable, 406 MHz digital EPIRBs in order to be detected by satellites.

406 MHz EPIRBs (Category I, II): The 406 MHz EPIRB was designed to operate with satellites. Its signal allows authorities to locate the EPIRB much more accurately than 121.5/243 MHz devices and identify the individual vessel anywhere in the world. There is no range limitation. These devices also include a 121.5 MHz homing signal, allowing aircraft and rescue vessels to quickly locate the vessel in distress once underway. These are the only type of EPIRB which must be tested by Coast Guard-approved independent laboratories before they can be sold for use in the United States.

An automatically activated, float-free version of this EPIRB has been required on SOLAS vessels (cargo ships over 300 tons and passenger ships on international voyages) since August 1, 1993. The Coast Guard requires U.S. commercial fishing vessels to carry this device, and requires the same for other U.S. commercial uninspected vessels which travel more than 3 miles offshore.

Owners of 406 MHz EPIRBs furnish registration information about their vessel, type of survival gear, and emergency points of contact ashore, all of which greatly enhance the quality of the response. The database for U.S. vessels is maintained by the National Oceanic and Atmospheric Administration, and is accessed worldwide by SAR authorities to facilitate SAR response.

EPIRB Registration data provides search and rescue authorities with contact and vessel information which they use solely to locate the user in an emergency. The data can cut down the time needed to confirm an EPIRB distress location or allow authorities to locate a vessel even in rare instances where the EPIRB location cannot be determined.

When registering ensure the EPIRBs 15-digit Unique Identification Number (UIN) is entered properly and validated with the EPIRBs checksum (if provided). The UIN is what links registration data to a specific EPIRB.

In the U.S. EPIRB registration is required by the Code of Federal Regulations in the US (Title 47, Part 80, Section 80.1061, Paragraph (f)). Failure to register can, in some instances, result in penalties and/or fines issued by the FCC.

EPIRBs can be registered with NOAA through one of the following methods:

• Register online at:

https://beaconregistration.noaa.gov/rgdb/

• Mail the original, signed registration form, available

on the website or with your beacon literature, to

NOAA at: NOAA SARSAT BEACON REGISTRATION NSOF, E/SPO53 1315 East West Hwy Silver Spring, MD 20910

• Or, fax the signed form to NOAA at 301-817-4565.

If you have any questions or comments pertaining to beacon registration, please call 301-817-4515 or toll-free at 1-888-212-SAVE (7283), or you may email your question to the Beacon Registration Staff at:

beacon.registration@noaa.gov.

False alerts, transmission of an alert signal by an activated COSPAS-SARSAT EPIRB in situations other than distress, can cause delays in the responses of rescue agencies and can potentially overwrite actual distress alerts in the satellite memory.

To prevent false alerts follow your manufacturer directions for mounting and testing the EPIRB.

If your EPIRB is accidentally activated turn it off and contact the U.S. Coast Guard to report the activation with your 15-digit UIN available.

Intentionally transmitting a false alert can result in fines and jail time.

When disposing of an old or unneeded EPIRB precautions must be taken to prevent accidental transmission from the disposal site. Before disposal, consult with the EPIRB manufacturer's instructions for specific guidance on procedures and recommendations. Contacts for EPIRB manufacturers can be found at:

https://www.cospas-sarsat.int/en/contacts-pro/contacts-details-all.

At a minimum the EPIRB battery should be removed, the EPIRB should be clearly labeled as inactive, and the EPIRB registration should be updated to reflect disposal of the unit.

When possible the components of the old EPIRB and the EPIRB batteries should be recycled at an appropriate facility.

EPIRB owners should periodically check for water tightness, battery expiration date, and signal presence. 406 MHz EPIRBs have a self-test function which should be used in accordance with manufacturers' instructions at least monthly.

COSPAS is a Russian acronym for “Space System for Search of Distressed Vessels”; SARSAT signifies “Search And Rescue Satellite-Aided Tracking.” COSPAS-SARSAT is an international satellite-based search and rescue system established by the U.S., Russia, Canada, and France to locate emergency radiobeacons transmitting on the 406 MHz frequency. Since its inception in 1982, the COSPAS-SARSAT system (SARSAT satellite only) has contributed to saving over 39,000 lives.

The USCG receives data from Maritime Rescue Coordination Center (MRCC) stations and SAR Points of Contact (SPOC). See Table 3115.

When an EPIRB is activated, COSPAS/SARSAT

Country Location Designator Algeria Algiers ALMCC Argentina El Palomar ARMCC Australia Canberra AUMCC Brazil San Paulo BBMCC Canada Trenton CMCC Chile Santiago CHMCC China Beijing CNMCC France Toulouse FMCC Greece Athens GRMCC Hong Kong Hong Kong HKMCC India Bangalore INMCC Italy Bari ITMCC Indonesia Jakarta IONCC ITDC Taipei TAMCC Japan Tokyo JAMCC Korea (Rep. of) Incheon KOMCC New Zealand* - - Nigeria Abuja NIMCC Norway Bodo NMCC Pakistan Karachi PAMCC Peru Calloa PEMCC Russian Federation Moscow CMC Saudi Arabia Jiddah SAMCC Singapore Singapore SIMCC South Africa Cape Town ASMCC Spain Maspalomas SPMCC Thailand Bangkok THMCC Turkey Ankara TRMCC UAE Abu Dhabi UKMCC United Kingdom Kinloss UKMCC United States Suitland USMCC * New Zealand’s ground stations connect directly to Australia’s AUMCC. picks up the signal, locates the source and passes the information to a land station. From there, the information is relayed to Rescue Coordination Centers, rescue vessels and nearby ships. This constitutes a one-way only communications system, from the EPIRB via the satellite to the rescuers. COSPAS/SARSAT instruments are carried by two satellite constellations which provide for global detection and location of emergency beacons. The Low Earth Orbit (LEO) constellation consists of low-altitude, near-polar orbiting satellites. These satellites exploit the Doppler principle to locate the 406 MHz EPIRB within approximately 5km.

[Table 3115 in Bowditch, Pub. No. 9: Participants in Cospas-Sarsat system.]

As a LEO satellite approaches a transmitting EPIRB, the frequency of the signals it receives is higher than that being transmitted; when the satellite has passed the EPIRB, the received frequency is lower. This creates a notable Doppler shift. When the satellite approaches a ground station, known as a Local User Terminal (LUT), the LUT receives the recorded beacon frequency data from the satellite and then calculates the position of the EPIRB taking into account the Earth's rotation and other factors.

Because of the low orbit and small footprint of LEO satellites a satellite will pass overheard roughly every 45 minutes and delays are possible.

The Geo-stationary Earth Orbit (GEO) constellation consists of high-altitude satellites in orbits which keep them in a fixed location over the equator. The large footprint of GEO satellites complements the LEO constellation by providing for instantaneous detection of an active beacon anywhere in the world. However, as GEO satellites are stationary relative to the ground, they cannot independently locate a beacon.

Newer EPIRBs incorporate an additional GPS chip and use a protocol which encodes GPS coordinates into the beacon's digital transmission. The Geostationary segment of the SARSAT constellation retransmits the encoded message to ground stations, providing a near-instantaneous position which can minimize the delay in identifying the distress location.

Each 406 MHz EPIRB incorporates a unique identification code. Once the satellite receives the beacon's signals, the beacon's digital data is recovered from the signal, time-tagged, and transferred to the repeater downlink for real time transmission along with any Doppler frequency or position data to a LUT. The digital data coded into each 406 MHz EPIRBs memory indicates the identity of the vessel to SAR authorities. They can then refer to the EPIRB registration database for information about the type of vessel, survival gear carried aboard, whom to contact in an emergency, etc. The data includes a maritime identification digit (MID, a three digit number identifying the administrative country) and either a ship station identifier (SSI, a 6 digit number assigned to specific ships), a ship radio call sign or a serial number to identify the ship in distress.

See Figure 3116a for a graphical overview of the COSPAS-SARSAT system.

For MF (i.e. 2182 kHz), the signal consists of either (1) a keyed emission modulated by a tone of 1280 Hz to 1320 Hz with alternating periods of emission and silence of 1 to 1.2 seconds each; or (2) the radiotelephone alarm signal followed by Morse code B (— • • •) and/or the call sign of the transmitting ship, sent by keying a carrier modulated by a tone of 1300 Hz or 2200 Hz. For VHF (i.e. 121.5 MHz and 243 MHz), the signal characteristics are in accordance with the specifications of Appendix 37A of the ITU Radio Regulations. For 156.525 MHz and UHF (i.e. 406 MHz to 406.1 MHz and 1645.5 MHz to 1646.5 MHz), the signal characteristics are in accordance with CCIR recommendations.

The purpose of these signals is to help determine the position of survivors for SAR operations. They indicate that one or more persons are in distress, may no longer be aboard a ship or aircraft, and may not have a receiver available.

[Figure 3116a in Bowditch, Pub. No. 9: COSPAS-SARSAT System Overview. (1) Emergency locater transmitters (ELTs), EPIRBs, and personal locater beacons (PLBs) operate on the 406 MHz frequency. Each 406 MHz beacon transmits a unique digital code that identifies the type of beacon and that allows registration data to be associated with the beacon. The registration data provides information such as the beacon owner; the type of platform the beacon is associated with; emergency points of contact; and much more. (2) After the satellite receives a beacon signal, it relays the signal to ground stations referred to as local user terminals (LUTs). There are two types of LUTs: Low-Earth Orbiting LUTs (LEOLUTs) which receive and process alert data from the polar-orbiting satellites; and Geostationary LUTs (GEOLUTs) which receive and process alert data from geostationary satellites.(3) The LUT processes the data and transmits an alert message to its respective Mission Control Center (MCC) via a data communication network. (4) The MCC performs matching and merging of alert messages with other received messages, geographically sorts the data, and transmits a distress message to another MCC, an appropriate SAR authority such as a national Rescue Coordination Center (RCC) or a foreign SAR Point of Contact (SPOC). (5) The RCC investigates the beacon alert and launches assets to find the parties in distress when necessary.]

Automatic Identification System (AIS) is a navigation-communication protocol used in the maritime VHF-FM band, to autonomously exchange real-time navigation information amongst other AIS users or stations. Given that AIS is digital protocol, it facilitates that its data can -but not currently required to-be used or portrayed on other systems, such as ECDIS, radar, VTS monitors, personal computers, shore-side web services, etc.

Upon proliferation of regional (and disparate) tracking systems in the early 1990’s (i.e. Dover Straits, Panama Canal, Sweden, Prince William Sound, AK), various member entities exhorted IMO to consider the development of a universal tracking system, which in 1998 led to the IMO Marine Safety Committee to formally agree to and adopt Performance Standards for a Universal Shipborne Automatic Identification System (later to be solely known as AIS), which use was mandated in 2000 on all tankers, seagoing passenger and cargo ships (those over 300 GT), via

an amendment to the Safety of Life at Sea Convention (SOLAS Regulation V/19.2.4). Since then, AIS carriage requirements have expanded domestically and on smaller vessels; particularly in the United States which requires AIS on all commercial self-propelled vessels of sixty-five feet or greater, most commercial towboats, and any vessel moving certain dangerous cargoes or flammable in bulk.

The IMO (Resolution MSC74(69)) defines the primary functions or purposes of AIS as:

1. in ship-to-ship mode, for collision avoidance;

2. in a ship-to-shore mode, as a tool for vessel traffic

management, and,

3. a means to obtain specific data about ships, and

their cargo, operating in coastal waters.

AIS devices are designed to operate autonomously, without user intervention or external infrastructure or signals, as transponders require; albeit they can be interrogated (polled) or tele-commanded to report faster or not all. AIS rely upon time-division multiple access (TDMA) procedures. The VHF data link (VDL) is divided into 2,500 equal slots of time per channel to reserve or schedule its transmissions, which it does randomly accessing a free slot, when available. What makes AIS unique to cellular telephones that use TDMA, is that certain AIS devices (i.e. Class A, Class B-SO) self-organize themselves on the VDL. So rather the 'dropping a call' as cell-phone users may experience when they go beyond a cell tower range or when the cell therein is beyond capacity, each AIS acts as its own cell tower and coordinates its reception so to favor AIS transmissions that are closest to themselves, and which pose the greater collision risk.

The range of AIS-as with all VHF (line-of-sight) systems-is mostly affected by antenna height; however, since VHF-FM wavelengths are slightly longer than radars, AIS signals tend to cross land and other obstructions moderately well. At sea, an AIS on the water can usually be seen at 3-4 miles, from a lifeboat at 6-8 miles, from fishing boats and pleasure craft at 8-12 miles, and, from large or higher ships at 15-30 miles. Given that its transmissions are line of sight, AIS can also be received from far ashore (25-50 miles out) and from satellite (1,000 miles).

Each IMO required AIS device consists of a: VHF transmitter; three receivers, two dedicated to AIS transmission, and another backwards compatible to VHF DSC (Ch. 70); a VHF and GPS antenna; an internal GPS for timing and positioning; a built in integrity test (BIIT) processor; a minimal keyboard display (MKD); two input-output interfaces, and, at least one output interface.

There are two classes of shipborne AIS transceivers: Class A devices which meet all IMO standards and Class B devices which are intended for non-compulsory use. Each AIS transmission (message) denotes the time of transmission and its source ID; a unique 9-digit Maritime Mobile Station Identity (MMSI) number. In addition, Class A devices transmit the following data, autonomously and continuously every 2-10 seconds (dependent on speed and changing course) and every three minutes when at anchor or moored (if its navigation status has been updated to reflect so), at a power of 12.5 watts:

• Navigation status: underway, anchored, not under

command, etc. (manually selected)

• Lat. and long. to 1/10,000 minute

• Course over ground

• Speed over ground Position accuracy; source, i.e.

GPS, GLONAS, INS, manually entered, etc.; and,

whether Receiver Autonomous Integrity Monitoring

(RAIM) is used True heading, to 1/10 degree (via

external gyro or transmitting heading device (THD),

if connected)

• Rate of turn indication (if connected)

In addition, the Class A AIS will transmit, static and voyage related data, every six minutes:

• IMO number, a unique identifier related to ship's

hull

• Radio call sign.

• Name of ship, up to 20 characters

• Type of ship, from predefined list of types

• Dimensions of ship, to nearest meter (derived from

the positioning system antenna location)

• Source of positioning system, i.e. GPS, GLONAS,

Integrated Navigation System (INS), manually

inputted, etc.

• Static Draft, to 0.1 meter; air draft is not defined

• Destination, to 20 characters

• ETA: month, day, hour, and minute in UTC.

There are two variants of Class B AIS devices: Self-Organizing (same as Class A) or Carrier-Sense Mode (only transmit if they 'sense' a free slot is available) devices. Both are interoperable with other AIS devices, but, dissimilar to Class A devices in that they operate at a lower power (Class B-SO @ 5 Watts; Class B0-CS @ 2 Watts) and either every 5-15 seconds (Class B-SO) or at a 30 second fixed reporting rate; and, do not support external sensors (i.e. gyro, rate of turn indicator, etc.); or report their IMO number, destination, static draft, and navigational status; or have the facilities to transmit safety text messages.

Since the advent of the IMO AIS mandate, AIS technology has expanded to other devices such as:

• onboard Search and Rescue (SAR) aircraft;

• as shore stations that perform as a network base sta-

tion;

• co-located on aids to navigation (Real AIS ATON);

• remotely used to transmit ATON information to

coincide with an existing physical aid to navigation

(Synthetic AIS ATON);

• or where an ATON is electronically charted but does

not physically exist (Virtual AIS ATON);

• and, as AIS locating devices, such as AIS EPIRB,

AIS Man-overboard devices, and AIS Search and

Rescue Transmitter (AIS-SART).

Each unique type of AIS device can be identified by its MMSI format: 111YYYXXX for SAR aircraft AIS; 00YYYXXXX for AIS Base Stations; 99YYYXXXX for AIS ATON; 970YYXXXX for AIS-SART; 9702YXXXX for MOB-AIS; 974YYXXXX for EPIRB-AIS.

AIS locating devices operate differently than most other AIS devices, using 'burst behavior'. To facilitate their locating, these devices transmit a pre-formatted safety text message which states whether the locating device is 'ACTIVE' or under 'TEST'. If the former, they will also transmit 7 position reports per minute, increasing the likelihood that at least one is transmitted on the crest of swell or wave. The great benefit of AIS locating devices is that each transmission includes the device's location. The SAR response unit does not need to continuously hone in or tediously adjust direction finding antennas as is required to locate other beacons or radar transponders; the latter is discussed in the following section.

There are two variants of Search and Rescue Transmitters: one that operates on AIS channels (see Section 3123), and the other that operates as a radar transponder, hereinafter SART. Operating much like a RACON, the Search and Rescue Radar Transponder (SART) is a passive rescue device which, when it senses the pulse from a radar operating in the 9 gHz frequency band, emits a series of pulses in response, which alerts the radar operator that some sort of maritime distress is in progress. Further, the SART signal allows the radar operator to home in on the exact location of the SART. The SART can be activated manually, or will activate automatically when placed in water.

The SART signal appears on the radar screen as a series of 12 blips, each 0.64 nautical miles apart. As the vessel or aircraft operating the radar approaches the SART location, the blips change to concentric arcs, and within about a mile of the SART become concentric circles, centered on the SART.

Because the SART actively responds to radar pulses, it also informs its user, with an audible or visual signal, that it is being triggered. This alerts the user in distress that there is an operating radar in the vicinity, whereupon they may send up flares or initiate other actions to indicate their position.

Approved SARTs operate in standby mode for at least 96 hours and actively for at least 8 hours. Because the SART signal is stronger than any surrounding radar returns, it will be easily sensed by any nearby radar. But because it is much weaker than the radar, its own range is the limiting factor in detection.

SART range is affected by three main factors. First, the type of radar and how it is operated is most important. Larger vessels with powerful, high-gain antennae, set higher above sea level, will trigger and detect the SART signal sooner than low-powered radars set closer to sea level. The radar should be set to a range of 12 or 6 miles for best indication of a SARTs signal, and should not have too narrow a receive bandwidth, which might reduce the strength of the received signal.

Second, weather is a factor in SART range. A flat calm might cause multipath propagation and distort the SARTs signal. Heavy seas may cause the SART signal to be received intermittently as the transponder falls into the troughs of the seas. Careful adjustment of the sea and rain clutter controls will maximize the SARTs received signal strength.

Third, the height of the SART will greatly affect the range, because the signal obeys the normal rules for radio waves in its spectrum and does not follow the curvature of the earth, except for a small amount of refraction. Tests indicate that a SART floating in the sea will have a range of about 2 nautical miles when triggered by a radar mounted 15 meters above sea level. At a height of 1 meter, range increases to about 5 miles. To an aircraft actively searching for a SART at an altitude of 3,000 feet, the range increases to about 40 miles.

Only an X-band (3 cm) radar can trigger and sense a SART. An S-Band (10 cm) radar will neither trigger nor detect a SART. Normally, an X-band radar will sense a SART at about 8 nm. When triggered by an incoming radar signal, the SART will transmit a return signal across the entire 3 cm radar frequency band. The first signal is a rapid 0.4 microsecond sweep, followed by a 7.5 microsecond sweep, repeated 12 times. This will cause a series of 12 blips on the radar, spaced 0.64 nm apart. See Figure 3122a.

For best reception, the radar should be set to medium bandwidth and to the 12 or 6 mile range. Too narrow a bandwidth will cause the SART signal to be weakened, as the radar is not sensing the entire SART pulse. The radar operator’s manual should be consulted for these settings. Less expensive radars may not be able to change settings.

As the range to the SART decreases to about 1 nm, the initial 0.4 microsecond sweeps may become visible as weaker and smaller dots on the radar screen. When first

[Figure 3122a in Bowditch, Pub. No. 9: SART 12-dot blip code.]

[Figure 3122b in Bowditch, Pub. No. 9: SART arcs.]

sensed, the first blip will appear about 0.6 miles beyond the actual location of the SART. As range decreases, the blips will become centered on the SART.

As the SART is approached more closely, the blips appearing on the radar become concentric arcs centered on the SART itself. The arcs are actually caused by the radar return of side lobes associated with the radar signal. While use of the sea return or clutter control may decrease or eliminate these arcs, it is often best to retain them, as they indicate the proximity of the SART. See Figure 3122b. Eventually the arcs become rings centered on the SART, as in Figure 3122c.

[Figure 3122c in Bowditch, Pub. No. 9: SART rings.]

On some radars it may be possible to detune the radar signal in situations where heavy clutter or sea return obscures the SART signal. With the Automatic Frequency Control (AFC) on, the SART signal may become more visible, but the radar should be returned to normal operation as soon as possible. The gain control should usually be set to normal level for best detection, with the sea clutter control at its minimum and rain clutter control in normal position for the ambient conditions.

January 1, 2010 the AIS-SART was added to GMDSS regulations as an alternative to the Radar SART. With the approval from IMO SOLAS Amendment in Resolution MSC 256(84) ship owners may choose either Radar SART or AIS SART to be carried on the vessel. AIS-SARTs have a built in GPS and transmit an alert message including the vessel ID and GPS position from the AIS tracking system. This information will appear on an AIS equipped vessel's chart plotter or ECDIS which differs from the traditional SART which displays on the Radar. The much lower operating frequency (160 MHz vs 9000MHz) from the AIS SART significantly increases the range of the signal and because VHF can propagate around land, the signal may be seen “around corners”. This is an improvement over Radar SART, particularly in areas of heavily incised coastlines and/or island archipelagos.

AIS ATON stations broadcast their presence, identity (9-digit Marine Mobile Service Identity (MMSI) number), position, and status at least every three minutes or as needed. These broadcasts can originate from an AIS station located on an existing physical aid to navigation (Real AIS ATON) or from another location (i.e., AIS Base Station). An AIS Base Station signal broadcasted to coincide with an existing physical aid to navigation is known as a Synthetic AIS ATON. An electronically charted, but non-existent as a physical aid to navigation, is identified as a Virtual AIS ATON. The latter two can be used to depict an existing aid to navigation that is off station or not watching properly or to convey an aid to navigation that has yet to be charted. All three variants can be received by any existing AIS mobile device, but they would require an external system for their portrayal (i.e., AIS message 21 capable ECDIS, ECS, radar, PC). How they are portrayed currently varies by manufacturer, but the future intention is for the portrayal to be in accordance with forthcoming International Standards (i.e., IEC 62288 (Ed. 2), IHO S-4 (Ed. 4.4.0)).

Maritime authorities can quickly use Synthetic and Virtual AIS (SAIS/VAIS) ATON, sometimes referred to as eATON, to temporarily reconstitute port ATON constellations in response to storm or hurricane damage. This grants recovery assets more time to address missing and/or off station aids.

The Long-Range Identification and Tracking (LRIT) system, designated by the International Maritime Organization (IMO), provides for the global identification and tracking of ships. See Figure 3125 for more information.

The obligations of ships to transmit LRIT information and the rights and obligations of SOLAS Contracting Governments and of Search and rescue services to receive LRIT information are established in regulation V/19-1 of the 1974 SOLAS Convention.

The LRIT system consists of the shipborne LRIT information transmitting equipment, the Communication Service Provider(s), the Application Service Provider(s), the LRIT Data Center(s), including any related Vessel Monitoring System(s), the LRIT Data Distribution Plan and the International LRIT Data Exchange. Certain aspects of the performance of the LRIT system are reviewed or audited by the LRIT Coordinator acting on behalf of all SOLAS Contracting Governments.

LRIT information is provided to Contracting Governments to the 1974 SOLAS Convention and Search and rescue services entitled to receive the information, upon request, through a system of National, Regional and Cooperative LRIT Data Centers using the International LRIT Data Exchange.

Each Administration should provide to the LRIT Data Centre it has selected, a list of the ships entitled to fly its flag, which are required to transmit LRIT information, together with other salient details and should update, without undue delay, such lists as and when changes occur. Ships should only transmit the LRIT information to the LRIT Data Centre selected by their Administration.

Additional information concerning LRIT is available at the IMO website via the link in Figure 3125.

The USCG maintains a National Data Center (NDC). The NDC monitors IMO member state ships that are 300 gross tons or greater on international voyages and either bound for a U.S. port or traveling within 1000 nm of the U.S. coast. LRIT complements existing classified and unclassified systems to improve Maritime Domain Awareness.

[Figure 3125 in Bowditch, Pub. No. 9: Long-Range Identification and Tracking IMO website. https://www.imo.org/en/OurWork/Safety/Pages/LRIT.aspx]

LRIT is a satellite-based, real-time reporting mechanism that allows unique visibility to position reports of vessels that would otherwise be invisible and potentially a threat to the United States.

The user interface for the US NDC is located at the Navigation Center (NAVCEN) in Alexandria, Virginia. NAVCEN operates the US LRIT interface called the Business Help Desk (BHD). BHD operators can perform a multitude of operations with a web-based user interface. Within this web-based application, the BHD watchstanders can view and request vessel status, see vessel information, request vessel positions, and increase and decrease vessel reporting rates.

The US NDC stores all of the positions from any LRIT ship, foreign or domestic, that enters our coastal water polygons. This information is available in real time to the BHD watchstander after performing a basic search for a vessel using the vessel name, IMO number, or MMSI (Maritime Mobile Service Identity) number. Per the LRIT international guidelines, the default ship reporting rate is every six hours. However, functionality is built in to allow end users to request a onetime poll that gives an on-demand current

position. Watchstanders can also increase the reporting rate to every 3 hours, 1 hour, 30 minutes, or 15 minutes for a specified period of time.

AIS data is viewable publicly, on the internet, without the need for an AIS receiver. Global AIS transceiver data collected from both satellite and internet-connected shore-based stations are aggregated and made available on the internet through a number of service providers. Data aggregated this way can be viewed on any internet-capable device to provide near global, real-time position data from anywhere in the world. Typical data includes vessel name, details, location, speed and heading on a map, is searchable, has potentially unlimited, global range and the history is archived. Most of this data is free of charge but satellite data and special services such as searching the archives are usually supplied at a cost. The data is a read-only view and the users will not be seen on the AIS network itself.

For an example of a commercial ship tracking website, providing AIS data on merchant vessels to the public, follow the link in Figure 3126a. Figure 3126b depicts a moment in time for ships transiting the Indian Ocean while transmitted AIS data.

[Figure 3126a in Bowditch, Pub. No. 9: https://shipfinder.co/]

[Figure 3126b in Bowditch, Pub. No. 9: Typical AIS ship data for Indian Ocean.]

In late 2016, MARAD launched the new U.S. Maritime Advisory System, which represents the most significant update since 1939 to the U.S. government process for issuing maritime security alerts and advisories. The new system establishes a single federal process to expeditiously provide maritime threat information to maritime industry stakeholders including vessels at sea. In response to valuable feedback from stakeholders, the Maritime Advisory System was developed to streamline, consolidate, and replace maritime threat information previously disseminated in three separate government agency instruments: Special Warnings, MARAD Advisories, and global maritime security related Marine Safety Information Bulletins.

The U.S. Maritime Advisory System includes two types of notifications: A U.S. Maritime Alert and a U.S. Maritime Advisory. Maritime Alerts quickly provide basic threat information to the maritime industry. When amplifying information is available, a more detailed U.S. Maritime Advisory may be issued on a threat and could include recommendations and identify available resources. U.S. Maritime Alerts and U.S. Maritime Advisories will be broadcast by the National Geospatial-Intelligence Agency, emailed to maritime industry stakeholders, and posted to the Maritime Security Communications with Industry (MSCI) web portal. A link to the web portal is provided in Figure 3127.

The U.S. Maritime Advisory System is a whole-of-government notification mechanism. The Departments of State, Defense, Justice, Transportation, and Homeland Security, and the intelligence community, supported the development of this new system in coordination with representatives from the U.S. maritime industry through the Alerts, Warnings and Notifications Working Group.

Questions regarding the U.S. Maritime Advisory System may be emailed to MARADSecurity@dot.gov. Additional contact information is available on the MSCI web portal.

[Figure 3127 in Bowditch, Pub. No. 9: MARAD’s Maritime Security Communications with Industry website. http://www.marad.dot.gov/MSCI]