CHAPTER 1

SURFACE NAVIGATION SYSTEMS

INTRODUCTION

Today’s Navy uses various navigational systemsin the fleet. As an ET, you will be responsible formaintaining these systems.

In this volume, we will cover navigationfundamentals, the Ship’s Inertial Navigation System,Navy Satellite Navigation System, NAVSTAR GlobalPositioning System, fathometers, and TACAN. Let’sstart with navigation fundamentals.

NAVIGATION FUNDAMENTALS

In simple terms, navigation is a method of gettingfrom one known point to some distant point. Piloting,celestial navigation, and radio navigation are thecommonly used methods. In this chapter, we willdiscuss radio navigation and its components: deadreckoning, electronic navigation, and tacticalnavigation. The tactical use of NTDS data (tacticalnavigation) was covered in v o l u m e 3 ,Communications Systems. However, we will reviewit briefly here to help you see how it fits into radionavigation. We will then discuss dead reckoning andelectronic navigation in more detail.

TACTICAL NAVIGATION

You must understand the difference betweennavigation in the traditional sense and tacticalnavigation. Traditional navigation and piloting areconcerned primarily with safe maneuvering of theship with respect to hazards such as shoals, reefs, andso forth. Tactical navigation is not directly concernedwit h maneuvering the ship in navigable waters. Forthe purposes of tactical navigation, absolute positionis unimportant except to the extent that it supportsdetermining the relative position of hostile targets andfriendly cooperating platforms.

Remember, tactical navigation deals primarilywith fixing the location of the platform to (1) enableinstalled weapon systems to function against intendedtargets, (2) prevent ownship loss to or interferencewith friendly weapon systems, and (3) coordinateownship weapons systems with those of otherplatforms to achieve maximum effect.

In tactical navigation, navigation data is used bycombat systems, including NTDS, to ensure accuracyin target tracking. Ship’s movements areautomatically recorded by computer programs forapplications such as gun laying calculations and Link11 position reporting. Ship’s attitudes (pitch, roll, andheading) are transmitted to various display and userpoints, and electronic or mathematical computerstabilization is accomplished, depending on thesystem. For example, pitch and roll are used byNTDS, missile, sonar, gun, and TACAN systems forstabilization data and reference. Heading is used bythe EW direction finding, sonar, and radar systems fortrue and relative bearing display. Ship’s navigationand attitude data are provided by various equipment,depending on ship class.

DEAD RECKONING

Dead reckoning is the estimating of the ship’sposition between known navigational points or fixes.Radio navigation, consisting of terrestrial systemssuch as OMEGA and LORAN, and space-basedsystems, such as SATNAV, TRANSIT, andNAVSTAR GPS, provides accurate positions atspecific fixes. However, with the exception of somegunfire support systems that provide nearly constantpositional updates with respect to a fixed beacon orprominent landmark, there is a limit to how oftenfixes can be obtained. This requires us to dead reckon(DR) between the fixes. Dead reckoning can be asbasic as a DR line for course and speed on a plottingsheet or as sophisticated as an estimate made by an

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inertial navigation system that measures the ship’smotion in several planes and integrates the resultswith a high degree of accuracy. Although themethods of dead reckoning may vary, they all sharethe following characteristics: (1) the accuracy of theestimated position never exceeds the navigationmethod used to obtain the last fix, and (2) theaccuracy of the estimated position deteriorates overtime.

ELECTRONIC NAVIGATION

Simply put, electronic navigation is a form ofpiloting. Piloting is the branch of navigation in whicha ship’s position is determined by referring tolandmarks with known positions on the earth. Thesereference points may be bearing and distance to asingle object, cross bearings on two or more objects,or two bearings on the same object with a timeinterval in between.

Position in electronic navigation is determined inpractically the same way as piloting, though there isone important difference—the landmarks from whichthe ship’s position is determined do not have to bevisible from the ship. Instead, their bearings andranges are obtained by electronic means.

The advantages of electronic navigation areobvious. A ship’s position maybe fixed electronicallyin fog or heavy weather that makes it impossible totake visual fixes. Also, an electronic fix can be basedon stations far beyond the range of any local badweather.

Since electronic navigation is the primary form ofnavigation in today’s Navy, the rest of this chapterwill deal with electronic navigation and the rolesplayed by the following systems:

1. Long Range Aid to Navigation (LORAN)

2. VLF Radio Navigation (OMEGA)

3. Ship’s Inertial Navigation System (SINS)

4. Navy Navigation Satellite System (NNSS)

5. NAVSTAR Global Positioning System (GPS)

We will also briefly discuss navigation radar,surface search radar, and fathometers.

We will cover TACAN in chapter 2.

LORAN/OMEGA—TRANSITION ANDBASIC OPERATION

LORAN and OMEGA have been the “workhorse”systems for many years. However, they are beingphased out. Based on the DOD policy statementreprinted below and because you may see a civilianversion aboard your ship from time to time, we willsimply give you an overview of the two systems. Inaccordance with the 1992 Federal Radio navigationPlan (FRP), NAVSTAR will become the primaryreference navigation system for surface ships,submarines, and aircraft. The DOD requirement forLORAN-C and OMEGA will end 31 December 1994and TRANSIT will be terminated in DECEMBER1996. Land-based TACAN and VOR/DME are to bephased out by the year 2000.

LORAN BASICS

LORAN is a long-distance radio navigationsystem used by ships at sea to obtain a position fix,The system is based on the difference in the transittime required for pulsed radio signals to arrive at theLORAN receiver from multiple, synchronized,omnidirectional ashore transmitters. LORAN alsotakes advantage of the constant velocity of radiosignals to use the time lapse between the arrival oftwo signals to measure the differences in distancefrom the transmitting stations to the point ofreception. The receiving set provides a direct reading,in microseconds, of the time difference in the arrivalof the signals. (Some sets automatically convert thereadings into latitude and longitude.) When the timedifference is measured between signals received fromany two LORAN transmitter stations, a ship’s line-of-position (LOP) can be determined.

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OMEGA BASICS ADVANTAGES

OMEGA is a hyperbolic phase-differencemeasurement system. Hyperbolic navigation involvescomparing the phase angles of two or more radiosignals that are synchronized to a common time base.By moving the OMEGA receiver (by ship’smovement) and keeping the transmitter stations onfrequency with a constant difference in time andphase, the system can measure the relative phaserelationship between two stations to determine a lineof position (LOP) for the ship. The relative phaseangle measured between paired transmitting stationsdepends upon the distance of the receiver from eacht r a n s m i t t e r .

It is important to understand that a minimum oftwo transmitters are required to obtain a basic positionfix. Three or four are necessary to obtain an accuratefix. Unfortunately, there are many times in whichonly two transmitters are available but three aredesired. One way around this problem is to use thereceiver oscillator as a third, or “phantom,”transmitter. By setting the receiver oscillator to thefrequency transmitted by each of the two OMEGAtransmitters, the operator can compare the actualtransmitted frequencies to the frequencies of the tworeceived signals. This comparison provides twophase angles. The operator can then compare the twophase angles to determine a third phase angle. Thethree phase angles will yield a fix as accurate as a fixdetermined from three actual transmitters.

SHIP’S INERTIALNAVIGATION SYSTEM

The Ship’s Inertial Navigation System (SINS) isa navigation system that (after initial latitude,longitude, heading, and orientation conditions are setinto the system) continuously computes the latitudeand longitude of the ship by sensing acceleration.This is in contrast to OMEGA and LORAN, which fixthe ship’s position by measuring position relative tosome known object. SINS is a highly accurate andsophisticated dead reckoning device. Let’s look atsome of the advantages of using the SINS.

SINS has a major security advantage over othertypes of navigation systems because it is completelyindependent of celestial, sight, and radio navigationaids. In addition, SINS has the following advantages:

1. It is self-contained.

2. It requires minimal outsideinformation.

3. It cannot be jammed.

4. It is not affected by adverse weatherconditions.

5. It does not radiate energy.

6. It is not detectable by enemy sensors.

Now that we have seen the advantages of thissystem, let’s look at its basic components.

BASIC COMPONENTS

Look at figure 1-1. The basic components of aninertial navigation system are accelerometers,gyroscopes, servo systems, and the computers (notshown). Accelerometers measure changes in speed ordirection along the axis in which they lie. Theiroutput is a voltage, or series of pulses (digital),proportional to whatever acceleration is experienced.

Figure 1-1.—Stable platform with inertial components.

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Figure 1-2 shows an E-transformer accelerometer,while figure 1-3 shows a pulse countingaccelerometer. Two accelerometers (orientated North-South and East-West, respectively) are mounted on agyro-stabilized platform to keep them in a horizontalposition despite changes in ship’s movement. Theaccelerometers are attached to the platform by anequatorial mount (gimbal) whose vertical axis ismisaligned parallel to the earth’s polar axis. Thispermits the N-S accelerometer to be aligned along alongitude meridian and the E-W accelerometer to bealigned along a latitude meridian.

Figure 1-2.—E-transformer accelerometer.

A three-gyro stabilized platform is maintained inthe horizontal position regardless of the pitch, roll, oryaw of the ship. Figure 1-4 shows a gimbal-mountedgyro. Ship’s heading changes cause the gyro signalsto operate servo system motors, which in turn keepthe platform stabilized. High-performance servosystems keep the platform stabilized to the desiredaccuracy. (You will find in-depth information onaccelerometers, gyros, and servo systems in NEETSModule 15, Principles of Synchros, Servos, andGyros.).

Maintaining this accuracy over long periods oftime requires that the system be updated periodically.This is done by resetting the system using informationfrom some other navigation means; i.e., electronic,celestial, or dead reckoning.

Figure 1-3.—Pulse counting accelerometer.

Several models of SINS are in use. In general,AN/WSN-2 systems are installed on auxiliary ships,AN/WSN-2A systems are installed on submarines,and AN/WSN-5 systems are installed or beinginstalled on surface combatants. In the followingparagraphs, you will be introduced to the AN/WSN-5SINS and its advantages over these earlier systems.

Figure 1-4.— Gimbal-mounted rate gyro.

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AN/WSN-5 SINS Ship’s north, east, and vertical velocitycomponents

The AN/WSN-5 is a stand-alone set that replacesthe MK 19 MOD 3 gyrocompass in the followingclass ships: CG 16, CG 26, CGN 9, CGN 25, CGN35, CGN 36, CGN 38 (except for CGN 41), DDG 37,DD 963, and LHA 1. It also replaces the AN/WSN-2stabilized gyrocompass set in DDG 993, DD 997, andCGN 41 class ships.

Functional Description

The AN/WSN-5 has the same output capabilitiesas the AN/WSN-2. It uses an accelerometer-controlled, three axis, gyro-stabilized platform toprovide precise output of ship’s heading, roll, andpitch data in analog, dual-speed synchro format tosupport ship’s navigation and fire control systems.Ship’s heading and attitude data are continually andautomatically derived while the equipment senses andprocesses physical and electrical inputs of sensedmotion (inertial), gravity, earth’s rotation, and ship’sspeed. The equipment has an uninterruptible backuppower supply for use during power losses, and built-in test equipment (BITE) to provide fault isolation tothe module/assembly level.

Characteristics

In addition to the common functions describedabove, the AN/WSN-5 adds an increased level ofperformance to serve as an inertial navigator andprovides additional analog and digital outputs.Additional data provided includes position, velocity,attitude, attitude rates, and time data in both serial andparallel digital formats, providing a variety ofinterfaces. The AN/WSN-5 commonly exists in adual-system configuration on surface combatants.Some examples of AN/WSN-5 digital data outputsare :

1. Two Naval Tactical Data System (NTDS)serial channels transmitting:

Ship’s heading, roll, and pitch

Ship’s heading rate, roll rate, and pitch rate

Ship’s latitude, longitude, and GMT

2. Two MIL-STD-1397 NTDS type D high-levelchannels to an external computer

3. One MIL-STD-1397 NTDS type A slow, 16-bit, parallel input/output channel to a NavigationSatellite (NAVSAT) receiver AN/WRN-5A, GlobalPositioning System (GPS) receiver AN/WRN-6, orI/O console.

4. One serial AN/WSN-5 to AN/WSN-5 digitallink that provides alignment data, Navigation Satellite(NAVSAT) fix data, calibration constant data, andother navigation data to the remote AN/WSN-5.

5, An additional variety of input/output NTDSchannels, depending on which field changes areinstalled.

SATELLITE NAVIGATION SYSTEMS

Scientists realized that navigation based onsatellite signals was possible after listening to thebeep generated by Russia’s first artificial satellite,Sputnik I. They noticed a shift in the received radiofrequency signals as the satellite passed by. Thisshift, known as the Doppler effect, is an apparentchange in a received frequency caused by relativemotion between a transmitter and a receiver. As thedistance between the transmitter and the receiverdecreases, the received frequency appears to increase.As the distance increases, the received frequencyappears to decrease.

With this discovery, scientists were able to showthat by accurately measuring a satellite’s Doppler shiftpattern, they could determine the satellite’s orbit.They then determined that by using a known satellite’sorbit, a listener could determine his own position onthe earth’s surface by observing the satellite’s Dopplerpattern.

Following the first successful satellite launch inApril 1960, the U.S. Navy Navigation Satellite

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System (NNSS) became operational. This system is anall-weather, highly accurate navigation aid, enablingnavigators to obtain accurate navigation fixes from thedata collected during a single pass of an orbitingsatellite.

The following paragraphs describe the NNSS, itssatellites, Doppler principles, system accuracy , and twocommon shipboard equipments—the AN-WRN-5( V) andthe AN/SRN-19(V)2.

NAVY NAVIGATION SATELLITE SYSTEM

This highly accurate, world-wide, all weather systemenables navigators to obtain fixes approximately every 2hours, day or night. Looking at figure 1-5, you can seethat it consists of earth-orbiting satellites, trackingstations, injection stations, the U.S. Naval Observatory,a computing center, and shipboard navigationequipment.

System Satellites

Satellites are placed in a circular polar orbit, asillustrated in figure 1-6, at an altitude of 500 to 700(nominally 600) nautical miles. Each satellite orbits inapproximately 107 minutes, continually transmittingphase-modulated data every 2 minutes on two rfcarriers. This data includes time synchronizationsignals, a 400-Hz tone, and fixed and variableparameters that describe the satellite’s orbit.

The fixed parameters describe the nominal orbit ofthe satellite. Variable parameters (small corrections tothe fixed parameters) are transmitted at two-minuteintervals and describe the fine structure of the satelliteorbit. The satellite memory stores sufficient variableparameters to provide the two-minute orbit correctionsfor 16 hours following injection of fresh data into thememory. Since data injections occur about every 12hours, the satellite memory will not

Figure 1-5.—Navy Navigation Satellite System.

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Figure 1-6.—Satellite orbits.

run out. Each two-minute long satellite message istimed so that the end of the 78th bit, which is the lastbit of the second synchronization signal, coincideswith even 2 minutes of Greenwich mean time (GMT).Thus the satellites can also be used as an accuratetime reference by all navigators equipped with asatellite navigation set.

Each satellite is designed to receive, sort, andstore data transmitted from the ground and toretransmit this data at scheduled intervals as it circlesthe earth. Each satellite tells users which satellite itis, the time according to the satellite clock, and itspresent location. With this information, the user’snavigation set can determine exactly where thesatellite is, one of the necessary steps towarddetermining a precise navigational position.

Tracking Stations

Tracking stations are located in Maine,Minnesota, California, and Hawaii. As each satellitepasses within radio line-of-sight (los) of each of thesetracking stations, it is tracked to accurately determineits present and future orbits. Just before predictedsatellite acquisition, the tracking station’s antenna ispointed toward the satellite to acquire its signals. Asthe satellite rises above the horizon, the trackingantenna continues to follow the satellite’s predicted

path until the radio receiver in the tracking stationlocks on to the satellite’s transmitted signal. Thereceiver processor and data processing equipmentdecode and record the satellite message. The Dopplertracking signal is digitized and sent with the satellitetime measurements to the computing center, via acontrol center, where a refined orbit is calculated.

The tracking stations maintain highly stable oscil-lators that are continually compared against a WWVtransmitted frequency standard. In addition, theNaval Observatory sends daily messages that give theerror in the transmitted standard. The Navalobservatory error is then added to the data obtainedfrom the frequency standard, and corrections aremade to the station oscillators. The station oscillatorsare used to drive station clocks, which are comparedwith the time marks received from the satellite. Thistime data is transmitted by the tracking stations to thecontrol center, where the satellite clock error iscalculated and the necessary time correction bits areadded or deleted in the next injection message to thesatellite.

Computing Center

The central computing center continually acceptssatellite data inputs from the tracking stations and theNaval Observatory. Periodically, to obtain fixedorbital parameters for a satellite, the centralcomputing center computes an orbit for each satellitethat best fits the Doppler curves obtained from alltracking stations. Using this computed orbital shape,the central computing center extrapolates the positionof the satellite at each even 2-minutes in universaltime for the 12 to 16 hours subsequent to datainjection. These various data inputs are supplied tothe injection stations via the control center, as is dataon the nominal space of the orbits of the othersatellites, commands and time correction data for thesatellite, and antenna pointing orders for the injectionstation antennas.

Injection Station

The injection stations, after receiving andverifying the incoming message from the controlcenter, store the message until it is needed for

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transmission to the satellite. Just before satellite time-of-rise, the injection station’s antenna is pointed toacquire, lock on, and track the satellite through thepass. The receive equipment receives and locks on tothe satellite signals and the injection station transmitsthe orbital data and appropriate commands to thesatellite. Transmission to the satellite is at a high bitrate, so injection is completed in about 15 seconds.

The message transmitted by the satelliteimmediately after an injection contains a mix of oldand new data. The injection station compares areadback of the newly injected data with data thesatellite should be transmitting as a check for errors.If no errors are detected, injection is complete, If oneor more errors are detected, injection is repeated attwo-minute intervals (updating the variableparameters as necessary) until satellite transmission isverified as being correct.

DOPPLER PRINCIPLES

Look at figure 1-7. Stable oscillator frequenciesradiating from a satellite coming toward the receiverare first received (T1) at a higher frequency thantransmitted, because of the velocity of theapproaching satellite. The satellite’s velocity producesaccordion-like compression effects that squeeze theradio signals as the intervening distance shortens. Asthe satellite nears its closest point of approach, thesecompression effects lessen rapidly, until, at themoment of closest approach (T2), the cycle count ofthe received frequencies exactly matches those whichare generated. As the satellite passes beyond thispoint and travels away from the receiver (T3),expansion effects cause the received frequencies todrop below the generated frequencies proportionallyto the widening distance and the speed of the recedingsatellite.

FACTORS AFFECTING ACCURACY

Measurement of Doppler shift is complicated bythe fact that satellite transmissions must pass throughthe earth’s upper atmosphere on their way from spaceto the receiver. Electrically charged particals in theionospheric layer cause refraction of thesetransmissions. To solve this problem, the satellites are

designed to broadcast on two frequencies (150 and400 MHz). The receiver measures the difference inrefraction between the two signals and supplies thismeasurement to the computer. The computer uses thisrefraction measurement as part of its computation toobtain accurate fixes. The most serious problemaffecting accuracy is the effect of uncertainty in thevessel’s velocity on the determination of position.Velocity computation problems are inherent in thesystem. Position error resulting from an error invelocity measurement is somewhat dependent on thegeometry of the satellite pass. You can expect abouta 0.2 mile error for every one-knot error in thevessel’s velocity. Knowing this, you can see thatprecision navigation of a moving vessel requires anaccurate measurement of the velocity of the movingvessel, such as is provided by a good inertialnavigation system (See the section on Ship’s InertialNavigation System.). In general, intermittentprecision navigation fixes would not be of extremevalue for a moving vessel unless it had some means ofinterpolating between these precision fixes. A goodinertial navigation system provides such a means, andsimultaneously provides the accurate velocitymeasurements required to permit position fixes withthe NNSS.

In summary, precision navigation for movingvessels can’t be provided by the Navy NavigationSatellite System alone, but can be provided by the useof this system in conjunction with a good inertialsystem. Given the orbital parameters of a satellite, theDoppler shift of the signal transmitted from thatsatellite, and the velocity of the vessel, it is possibleto obtain a navigational fix if the satellite is within losof the navigation set and has a maximum elevation atthe time of closest approach (TCA) of between 10and 70 degrees. Satellite passes suitable for use inobtaining a navigational fix will usually occur at nomore than 2-hour intervals (depending on userlatitude and configuration of the satellite cons-tellation). It is a matter of your viewpoint whether youconsider the inertial system as a means ofinterpolating between the satellite navigation fixes orconsider the satellite fixes as a means for correctingthe inevitable long term drills (see the paragraphs onbasic components of an inertial navigation system) ofeven the best inertial navigation systems.

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Figure 1-7.—Doppler shift relative to satellite transmitted frequency.

The two most common satellite navigation AN/WRN-5(V) RADIO NAVIGATION SET

systems used by the Navy are the AN/WRN-5 and theAN/SRN-19. The following paragraphs provide The AN/WRN-5 Radio Navigation Set, shown in

descriptions of these navigation sets. figure 1-8, is a receiver-data processor-display set

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designed to recieve and phase track signalstransmitted by satellites of the NNSS. These signalsare processed to obtain navigation information that ismonitored on video displays and used elsewhere forship navigation.

The AN/WRN-5 is designed to be used in variousconfigurations as described below. Each of theseconfigurations is defined by options in externalequipment used or variations in inputs and outputs.The options available for alternative configurationsare:

1.

2.

3.

4.

5.

6.

Teleprinter, ASR-33

Additional remote video displays, IP-1154(U)

Frequency standard, AN/URQ-10/23(external reference)

Dual antennas (separate 400-MHz and150-MHz antennas)

Input/output bus

External lock indicator

7. 100-KHz output

The functional elements of the AN/WRN-5include the following components:

1.

2.

3.

4.

5.

6.

7.

8.

Preamplifier unit

Built-in two channel receiver

Built-in expanded data processor unit(XPDU) with 16K word memory

Front panel keyboard for operator-to-system interface

Front panel magnetic tape cassette transportwith read/write capability for OPNAVprogram loading or data recording

Front panel video display for system tooperator input/output

Remote video monitor

Built-in synchro-to-digital convertor forinterface with the ship’s speed and headingsensors to provide dead reckoning capability

Figure 1-8.—AN/WRN-5 front panel.

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and accurate satellite position fixes duringship maneuvers

6. Displays inputted speed and heading.

7. Displays inputted set and drift.9. Optional addition of a teleprinter

8. Displays data on a tracked satellite.The combination of fictional elements in the

AN/WRN-5 provides many capabilities includingautomatic storage of satellite information, time-ordered alerts for up to eight satellites, and built-inself test. The front panel video display providescurrent time, latitude/longitude, dead reckoningposition (automatically updated by satellite fixes), andsatellite tracking information such as fix merit andsatellite alerts. You will find specific information onthe capabilities of this navigation set in the AN/WRN-5 operation and maintenance technical manual.

9. Performs a self-test of computer functions[limited to verification of the digital circuitry).

The AN/SRN-19(V)2 consists of the majorcomponents shown in figure 1-9.

Figure 1-10 shows a simplified block diagram ofthis system. The following paragraphs describe thesecomponents.

ANTENNA GROUP OE-284/SRN-19(V)AN/SRN-19(V)2 RADIO NAVIGATION SET

The AN/SRN-19(V)2 is an automatic shipboardnavigation set that provides a continuous display ofthe ship’s position. The ship’s position, which isobtained by dead reckoning on true speed andheading, is periodically corrected by satellite fixes.Specifically, the navigation set can perform thefollowing functions:

1. After each successful satellite pass, computesand displays the present location of the ship to anominal at-sea accuracy of 0.25 nautical mile.

The antenna group consists of the AS-3330/SRN-19(V) antenna and AM-7010/SRN-19(V) rf amplifier

Antenna

The antenna is a linear, vertically-polarized typethat receives rf signals transmitted by the satellite. Itshorizontal pattern is omnidirectional; its verticalpattern varies approximately 11 dB from 10 to 70degrees above the horizontal plane.

Rf Amplifier

Note: Accuracy of the fix is affected by high The rf amplifier provides initial amplification ofsunspot activity. During these periods, nominal the 400-MHz satellite signals from the antenna andat-sea accuracy may degrade to approximately then sends them, via rf coaxial cable, to the receiver0.5 nautical mile. for further amplification and processing. The rf

amplifier consists of a bandpass filter module, a 400-2. Dead reckons between satellite fixes MHz amplifier, and a dc block module.

3. Computes and displays the range and bearing RECEIVER-PROCESSOR R-2135/SRN-19(V)from the present position to any destination using thegreat circle program. The receiver-processor consists of a single

channel (400-MHz) receiver, a 5-MHz reference4. Computes and displays the next expected rise oscillator, a data processor with a programmable read-

time and elevation at closest approach of the only memory (PROM) program, a keyboard, display,previously tracked satellite, cassette recorder, two synchro-to-digital (S/D)

converters, and a power supply. It processes inputs5. Displays GMT accurate to 1 second. from the rf amplifier, ship’s EM log, gyrocompass,

and receiver-processor keyboard.

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Figure 1-9.—AN/SRN-19(V)2 major components.

Receiver

The receiver extracts, amplifies, and formatsmessage information from the rf signal transmittedby the satellite, and measures the Doppler shift of thesignal. The message data obtained by demodulation ofthe rf carrier describes the satellite’s position at thetime of transmission.

Data Processor

This unit processes inputs from the receiver, ship’sEM log, and gyrocompass through the S/D convertorsand the keyboard. It then performs computations andprovides the desired outputs to the front paneldisplay, readout indicator, teleprinter, and cassetterecorder.

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READOUT INDICATORAND TELEPRINTER

The readout indicator provides an identical visualreadout of the data displayed on the front panel of thereceiver-processor. The readout indicator is usuallylocated at a site some distance from the receiver-processor.

The teleprinter provides a permanent record ofdisplayed data. The printouts for modes 01 and 03occur every 15 minutes or as selected by the operator.A printout also occurs each time a display mode iselected and when satellite fix data is received.

One final note on the AN/SRN-19 system. Youmust “tell” the equipment where it is when it is

initialized. You must also enter information onantenna height before the system can provide anaccurate fix.

You can find specific information on theAN/SRN-19(V)2 in the shipboard operations andmaintenance manual for this navigation set.

NAVSTAR GLOBALPOSITIONING SYSTEM

NAVSTAR GPS is a space-based, radionavigation system that provides continuous,extremely accurate three-dimensional position,velocity, and timing signals to users world-wide. Itconsists basically of ground control, satellites, anduser equipment, as shown in figure 1-11.

Figure 1-10.—AN/SRN-19(V)2 simplified block diagram.

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NOTEGPS will become the primary referencenavigation system for surface ships, submarines,and aircraft. Refer to the DOD policy statementunder the LORAN and OMEGA section of thischapter for specific details on this importanttransition.

GROUND CONTROL

The ground control segment tracks the satellites,monitors and controls satellite orbits, and updates thesatellite navigation data message. The ground controlsystem consists of unmanned monitor stations and amanned control center. Monitor stations, locatedthroughout the world, use GPS receivers to track eachsatellite. Tracking information gathered by themonitor stations is sent to the control center, where a

precise position and a clock error for each satellite arecalculated. The control center also calculates satellitepositioning for the group of satellites. Positioningdata for a single satellite is called ephemeris data;data for a group of satellites is called almanac data.Once each 24 hours, the control center transmits theephemeris and almanac data to each satellite to updatethe navigation data message.

SATELLITES

There are 21 active operational and 3 active sparesatellites in circular orbits, with a 55-degreeinclination to the earth. These satellites providenavigation data to the navigation sets. The satellitesare arranged in six concentric rings that allow them toorbit the earth twice a day and provide world-widecontinuous coverage. Each satellite broadcasts two

Figure 1-11.—NAVSTAR GPS major elements.

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spread-spectrum rf signals, 1575.42 MHz (LI-RF)and 1227.60 MHz (L2-RF). Each signal is modulatedwith a unique code sequence and a navigation datamessage. The code sequence allows the navigationsets to identify the satellite, and the data messageprovides the navigation sets information about theoperation of the satellite.

An observer on the ground will observe the samesatellite ground track twice each day, but the satellitewill become visible 4 minutes earlier each daybecause of a 4 minute per day difference between therotation of the earth and the satellite orbit time. Thesatellites are positioned so a minimum of foursatellites are always observable to a user anywhere onearth.

Satellite Signal Structure

The satellites transmit their signals using spreadspectrum techniques. Two types of techniques areused: course acquisition (C/A) code and precise (P)code. The C/A code is available to military andcivilian GPS users. The P code is available only toU.S. military, NATO military and other users asdetermined by the DOD.

Since only the P code is on both frequencies, themilitary users can make a dual-frequency comparisonto compensate for ionospheric propagation delay.The C/A code-only users must use an ionosphericmodel, which results in lesser navigation accuracy.Superimposed on both codes is the NAVIGATION-message (NAV-msg), containing satellite ephemerisdata, atmospheric propagation correction data, andsatellite clock-bias information.

Satellite Ranging

GPS navigation is based on the principle ofsatellite ranging. Satellite ranging involves measuringthe time it takes the satellite signal to travel from thesatellite to the navigation set. By dividing the traveltime by the speed of light, the distance between thesatellite and the navigation set is known. By rangingthree satellites, a three-dimensional picture, such asthe one shown in figure 1-12, can be developed. Thedistance measurement to each satellite results in a

sphere representing the distance from the navigationset to the satellite. The point where the three spheresintersect (X) is the position of the navigation set,This explanation does not account for errors. Forsatellite ranging to provide accurate position data, thefollowing three sources of error must be compensatedfor:

Satellite position and clock error

Atmospheric delay of satellite signals

Navigation set clock error

With these errors compensated for, the GPS candetermine position fixes within 50 feet or less and isaccurate to within a tenth of a meter-per-second forvelocity and 100 nanoseconds for time. This accuracy,however, requires inputs from four satellites.

USER EQUIPMENT

User equipment is installed in ships, aircraft, andmotorized vehicles. The vehicle version can also becarried by personnel (particularly SEAL teams andother special forces units) as a manpack. The mostcommon manpack version is the AN/PSN-8( ). Themost common shipboard GPS receiver is theAN/WRN-6. These GPS receivers will be describedlater in this chapter.

Signal Acquisition

During operation, navigation sets collect andstore satellite almanac data in critical memory. Thealmanac data is normally available when thenavigation set is first turned on and providesinformation on satellite locations. Operators mayinput information about the navigation set position,time, and velocity to enhance the information incritical memory. With this information, the navigationset determines which satellites are available andsearches for the code sequences that identify thoseparticular satellites. When the C/A code of anavailable satellite is identified, the navigation setswitches to the more accurate P code, collects thenavigation data message, and updates criticalmemory.

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Figure 1-12.—Satellite ranging.

Navigation Set Clock Error

GPS navigation sets determine distance to asatellite by accurately measuring the time differencebetween satellite signal transmission and when thenavigation set receives this signal. This difference intime is directly proportional to the distance betweenthe satellite and the receiver. Therefore, the sametime reference must be used by both the receiver andthe satellite.

The clock in the GPS receiver in not nearly asaccurate as the atomic clock in the satellite. Thiscauses the receiver and satellite clocks to be slightlyout of sync, which in turn causes the timemeasurements to be inaccurate. The error is furthercompounded by the distance calculation, so theposition of the navigation set cannot be accuratelydetermined.

The navigation set compensates for these errorsby using the distance measurement from a fourthsatellite to calculate the clock error common to allfour satellites. The navigation set then removes theclock error from the distance measurements, and then

determines the correct navigation set position.

Signal Delay and Multipath Reception

Two types of atmospheric delay can affect theaccuracy of navigation set signal measurements. Thefirst is tropospheric delay. Tropospheric delay can beaccurately predicted; the prediction is included in thealmanac data.

The second type of delay is caused when thesatellite signal passes through the ionosphere. Thistype of signal delay is caused by the ionosphere beingthicker in some areas and by satellite signals receivedfrom nearer the horizon having to pass through moreof the ionosphere than those received from directlyoverhead. Ionospheric delay will phase shift thelower satellite transmission frequency, L2-RF, morethan the higher frequency, L1-RF. The navigation setmeasures ionospheric delay by measuring the phaseshift between these two signals and then uses thiscomputation to compensate for the ionospheric delay.

Multipath reception is caused by a satellite signalreflecting off of one or more objects. This causes the

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reflected signals to reach the navigation set at differenttimes than the original signal. The reception of multipathsignals may cause errors in the navigation setcalculations. The AN/WRN-6 navigation set makesoperators aware of multipath errors by a “fail” or “warn”message and/or fluctuations in the carrier-to-noise ratio.Multipath reception may be corrected by changing theship’s position.

AN/WRN-6(V) Satellite Signals Navigation Set

The Satellite Signals Navigation Set AN/WRN-6(V)computes accurate position coordinates, elevation,speed, and time information from signals transmitted byNAVSTAR Global Positioning System (GPS) satellites. Inthe P mode, it has an accuracy of 16 meters. In the C/A

C/A mode. it has an accuracy of 100 meters, thoughbetter results have been obtained by individual users.

The AN/WRN-6(V), shown in figure 1-13, operates inthree modes.

The “Initialization” mode is part of the set start-up.During initialization, the operator tests current position,date, and time data, either manually or from otherequipment. The data entered is used to speed up satelliteacquisition.

“Navigation” is the normal operating mode. Duringthe navigation mode, the set receives satellite data,calculates

Figure 1-13.—Satellite Navigation Set AN/WRN-6(V).

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navigation data, exchanges data with other interconnectedsystems, and monitors the set’s performance. Thenavigation mode allows the operator to enter mission data;view position, velocity, and time data; and control the set’sconfiguration.

The “self-test’’ mode allows the operator to perform acomplete test of the navigation set at any time. When theset is in “test,” it will not track satellites.

The two major components of the AN/WRN-6(V) arethe R-2331/URN receiver and the indicator control C-11702/UR. The other units (antenna, antenna amplifier,and mounting base) perform functions similar to those ofsimilar units in other systems. For more detailed

operation and maintenance technical manual.

AN/PSN-8( ) Manpack Navigation Set

The AN/PSN-8( ) operates similarly to the AN/WRN-6(V), though obviously it is not interfaced with otherequipment. Shown in figure 1-14, each manpack containsa receiver section and a computer section. The receiverprocesses the rf signals from the satellites and sends thesatellite’s positions and times to the computer. Thecomputer uses the positions and times to find the satelliteset’s position coordinates, elevation, and changes in theposition of the manpack set. The time it takes for the setto change position is used to compute speed. For moredetailed information on this navigation set. refer to the

information on this system, refer to the AN/WRN-6(V) operator’s manual for the AN/PSN-8( ) Manpack

Figure 1-14.—Manpack Navigation Set AN/PSN-8( ).

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Navigation Set. The AN/VSN-8( ) VehicularNavigation Set is also included in this manual.

NAVIGATIONAL AIDS

Other equipment used for navigation that ETs areresponsible for includes: navigation radars, surfacesearch radars (sometimes used as navigation radars)and fathometers. Information on surface search andnavigation radars is contained in NAVEDTA 12414,Radar Systems.

The following paragraphs will discuss

fathometers.

FATHOMETERS

Fathometers are used for taking depth soundings.They are particularly useful when the vessel istransitioning shallow, unfamiliar waters. A blockdiagram of the Sonar Sounding Set AN/UQN-4A isshown in figure 1-15,

On many ships the Sonar Technicians will beresponsible for this equipment, but there are ships

(mostly noncombatants) on which ETs are responsiblefor the fathometers. For more detailed information onfathometers, refer to the appropriate equipmenttechnical manual.

Figure 1-15.—AN/UQN-4A functional diagram.

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