Electronics Material Officer Course

MODULE NUMBER FOUR

LESSON TOPIC FOUR

NAVIGATION SYSTEMS/EQUIPMENT

LESSON TOPIC OVERVIEW

LESSON TOPIC FOUR

NAVIGATION SYSTEMS/EQUIPMENT

This lesson topic presents information common to U.S. Navy navigation systems and equipment. The Navy uses various navigational systems in today's fleet. Some are permanently mounted in various locations on the earth, others use satellites, and some track the recorded movement of the ship. As EMO, you will be responsible for the maintenance of this navigation equipment.

The LEARNING OBJECTIVES of this LESSON TOPIC are as follows:

The student should review the "LIST OF STUDY RESOURCES" and read the Lesson Topic LEARNING OBJECTIVES before beginning the lesson topic.

LIST OF STUDY RESOURCES

NAVIGATION SYSTEMS/EQUIPMENT

To learn the material in this LESSON TOPIC, you will use the following study resources:

Written Lesson Topic presentations in the Module Booklet:

Additional Materials:

References:

LESSON TOPIC SUMMARY

NAVIGATION SYSTEMS/EQUIPMENT

This lesson topic will introduce you to the purpose, basic theory, and operation of navigation systems, system designations and characteristics, and flight certification requirements. The lesson narrative is organized as follows:

NARRATIVE FORM

OF

NAVIGATION SYSTEMS/EQUIPMENT

LESSON TOPIC 4.4

Radio navigation can be subdivided into terrestrial systems, such as Omega or LORAN, and space-based systems, such as SATNAV, TRANSIT, and NAVSTAR GPS. In all cases, except some naval gunfire support systems that provide near constant positional updates with respect to a fixed beacon or prominent landmark, there is a limit to the frequency with which fixes can be obtained. As a result, dead reckoning is used between fixes, making assumptions about the ship's movements and estimating instantaneous positions based on those assumptions. Dead reckoning may be as basic as a DR line for course and speed on a plotting sheet or as sophisticated as inertial navigation systems that measures ship motion in several planes and integrates the result to estimate location with very good accuracy. The common characteristic of all dead reckoning methods is that the accuracy of the estimated position can never exceed that of the navigation method used to obtain the last fix, and that the accuracy of the estimated position deteriorates over time.

Navigation data is required by the combat systems, including NTDS, to ensure accuracy in target tracking. Ship movements are automatically recorded by computer programs for such applications as firing solutions and Link 11 position reporting. Ship attitude (pitch, roll, and heading) is measured and transmitted to various display and user points. Either electronic or mathematical computer stabilization is accomplished depending on the system. Pitch and roll, for example, are used by NTDS, missile, sonar, gun, and TACAN systems for stabilization data and certain equipment reference platforms. Heading is used by the EW, direction finding, sonar, and radar systems for true and relative bearing displays. Ship navigation and attitude data are provided by various equipment, depending on ship class.

A distinction must be made between navigation in the traditional sense, and tactical navigation. Traditional navigation and piloting are concerned primarily with safe maneuvering of the ship or aircraft with respect to natural hazards, e.g. shoals and reefs. Tactical navigation is not concerned, directly, with maintenance of the ship in navigable waters. In fact, for purposes of tactical navigation, absolute position is unimportant except to the extent that it supports determination of the relative location of targets and cooperating, non-hostile platforms.

Tactical navigation is assumed to deal primarily with the process of fixing the location of the platform to accomplish the following:

Electronic navigation is a form of piloting. Piloting is that branch of navigation in which a ship's position is determined by referring to landmarks with known positions on the earth. These reference points may consist of the bearing and distance to a single object, cross bearings on two or more objects, or two bearings on the same object with an interval between them. Position in electronic navigation is determined in practically the same way as in piloting. However, there is one important difference. The landmarks from which the ship's position is determined need not be visible from the ship. Instead, their bearings and ranges are obtained by electronic means (either radar or radio). The advantages of electronic navigation are obvious. A ship's position may be fixed electronically in fog or heavy weather when visual bearings are impossible. Moreover, an electronic fix may be based on transmitting stations located far beyond the range of clear visibility. This lesson topic presents the following navigation systems:

The Ship's Inertial Navigation System (SINS) is a highly accurate, all-weather, self-contained navigation system used in aircraft carriers, submarines, and surface combatants equipped with SM-2(ER), AEGIS, or Tomahawk systems. SINS is a highly sophisticated dead reckoning device. It continuously computes the latitude and longitude of the ship (after initial latitude, longitude, heading, and orientation conditions are set into the system) by sensing acceleration. This method is in contrast to the LORAN and Omega systems, which fix the ship's position by measuring position relative to some known object. Because SINS operates independently from celestial, sight, and radio navigational aids, the system has the major advantage of security over other types of navigation systems. Note that while, for relatively short periods of time, inertial navigation systems are extremely accurate, accuracy degrades with time. SINS requires periodic ship position inputs (fixes) from electronic, celestial, or dead reckoning external sources, such as NAVSAT or GPS. The inertial navigation system also has the following advantages:

The basic components of an inertial navigation system (Figure 4.4-9) are the accelerometers, gyroscopes, servosystems, and computers (not shown).

Accelerometer/Gyroscope

The accelerometer measures changes in speed or direction along the axis in which it lies. Its output is usually a voltage proportional to the acceleration to which it is subjected. A set of two accelerometers (oriented North-South and East-West, respectively) is mounted on a gyro-stabilized platform. This platform keeps the accelerometers in a horizontal position despite changes in the ship's movements. The accelerometers are attached to the platform by an equatorial mount (gimbal) whose vertical axis is aligned parallel to the earth's polar axis. This permits the N-S accelerometer to be aligned along a longitude meridian, and the E-W accelerometer aligned along a latitude meridian. A three-gyro-stabilized platform is maintained in the horizontal position regardless of the pitch, roll, and yaw of the ship. When the ship's heading changes, gyro signals will cause servosystem motors to operate to stabilize the platform.

Figure 4.4-9 Stable Platform with Inertial Components

High performance servo systems keep the platform stabilized to the required accuracy, based on gyro input signals.

The SINS computer (AN/UYK-20) integrates positional information with other combat systems, i.e., fire control and sonar.

There are several models of SINS on ships today. The MK 3 MOD 6 system has been placed on some aircraft carriers. The newer AN/WSN-1 Dual Mini SINS (DMINS) has been installed on the SSN 688 class submarines and some aircraft carriers. The DMINS represents a considerable improvement in component size (lighter and smaller) and maintenance concept (replacement of entire units instead of individual part replacement). The DMINS is part of the central computer complex (CCC) that provides input/output and central processor unit functions via AN/UYK-7 computers. The CCC integrates the DMINS information with other combat systems. The AN/WSN-5 SINS is an updated "stand-alone" system (external digital processing resources not required for alignment, reset, calibration or navigational functions). It uses an accelerometer-controlled, three axis, gyro-stabilized platform to provide precise output of the ship’s heading, roll and pitch data in analog, dual-speed synchro format to support navigation and fire control systems. Ship’s heading and attitude data are continually and automatically derived while the equipment senses and processes physical and electrical inputs of sensed motion (inertial), gravity, earth’s rotation, and ship’s speed. The system has an uninterrupitable backup power supply for use during power losses, and built in test equipment (BITE) to provide fault isolation to the module/assembly level. The AN/WSN-5 adds an increased level of performance to serve as an inertial navigator and provides additional analog and digital outputs. Additional data includes position, velocity, attitude, attitude rates, and time data, such as: heading, pitch and roll; rate of the aforementioned data; latitude, longitude and GMT.

The NAVSTAR Global Positioning System has become the primary reference navigation system for surface ships, submarines, and aircraft. A joint DOD/DOT policy statement calls for the following to happen:

NAVSTAR GPS is a space-based, radio navigation system that provides continuous, extremely accurate three-dimensional position, velocity, and timing signals to users worldwide. It consists basically of satellites, ground control, and user equipment, as shown in Figure 4.4-14.

Figure 4.4-14 NAVSTAR GPS Major Elements

An observer on the ground will see the same satellite ground track twice each day; however, the satellite will become visible four minutes earlier each day due to a 4-minute per day difference between the satellite orbit time and the rotation of the earth. The satellites are positioned so that a minimum of four satellites are always observable by a user anywhere on earth. The satellites transmit their signals using spread spectrum techniques, using two types of spreading functions: Course Acquisition (C/A) code and Precise (P) code. The C/A code is available to any GPS user, military or civilian; the P code is only available to the U.S. military, NATO military, and other users authorized by the DOD. Both P code and C/A code enable a receiver to determine the range between the satellite and the user. Since only the P code is on both frequencies, military users can make a dual frequency comparison to compensate for ionospheric, propagation delay in the different transmission times. The C/A code user must use a model of the ionosphere that results in a lesser navigation accuracy. Superimposed on both codes is the Navigation-message (NAV-msg), containing satellite ephemeris data (position information), atmospheric propagation correction data, and satellite clock bias information.

In the control segment, satellites are tracked by ground control and their position coordinates and timing information are updated daily. The control segment includes an operations center, four monitor stations, and three ground antennas. The operations center will calculate signal accuracy. The monitor stations passively track the satellites, and the antennas relay data to the satellites.

User equipment is installed in ships, aircraft, and motorized vehicles. GPS can be used as high as 500 miles above the Earth's surface for space shuttle navigation. The most common shipboard receiver is the AN/WRN-6. When the GPS receiver has acquired the satellite signals from four GPS satellites, achieved carrier and code tracking, and read the NAV-msg, it is ready to start navigating. The GPS receiver normally updates its pseudorange and relative velocities once every second. The next step is to calculate the GPS receiver position, receiver velocity, and GPS system time. The GPS receiver must know GPS system time very accurately because the satellite signals indicate to the GPS receiver the time of the transmission from the satellite. The GPS receiver uses system time as the reference time for when it receives the satellite signals. The difference in time between when the signal leaves the satellite and when it arrives at the GPS receiver antenna is directly proportional to the distance between the satellite and the receiver. Therefore, the same time reference must be used by both the GPS satellites and the GPS receiver. The clock in the GPS receiver is not nearly as expensive as the atomic clock used in the satellites, because such a clock would make the receiver too expensive. Instead, a less expensive crystal oscillator is used and the receiver corrects its offset from GPS system time.

GPS ranging is based on triangulation using known positions of satellites as reference points. However, knowing the exact position of the satellite is a difficult task, so GPS relies heavily on transmission time. Position with reference to a satellite is based on the length of time the transmitted signal takes to get from the satellite to the receiver. We know the velocity of RF energy and the frequency of the transmission. Therefore, we must only solve a simple equation to find our position relative the satellite. The GPS can determine position fixes within 50 feet or less and is accurate to within a tenth of a meter per second for velocity and 100 nanoseconds for time. Other characteristics of the GPS are:

GPS receivers are able to automatically convert from one grid system to another at the users choice. Although this system will make some other navigation systems obsolete, inertial navigation will remain because it is self-contained and cannot be jammed. Additionally, the inertial navigation and global positioning systems complement each other. The GPS can be used to align the inertial navigation system, and in turn, the inertial navigation system can assist the GPS in its satellite acquisition and tracking. Although each satellite continuously transmits time, position, and velocity information, a receiver must process signals from four satellites to get full use from the GPS.

A shipboard navigation system that uses GPS is the AN/WRN-6(V) shown in Figure 4-4.17. This system computes accurate position coordinates, elevation, speed, and time information from signals transmitted by GPS satellites. In the P mode, it has an accuracy of 16 meters and in the C/A mode it has an accuracy of 100 meters, although better results have been reported by individual users. Table 4.4-1 shows the various configurations of the AN/WRN-6(V). The two major components of the set, the receiver and the indicator control are described below. The other units depicted in Figure 4.4-16 (page 4-4-25) perform functions similar to those same units in other systems. For more detailed information, refer to the appropriate equipment technical manual.

Table 4.4-1 AN/WRN-6(V) Variants

The R-2331/URN is the receiver/processor for the AN/WRN-6(V). The electronic assemblies are housed in a shock and damage resistant chassis and accessed via a cover assembly. Input/output connections are provided on both the front and rear of the chassis. Three C cell alkaline batteries are used to save critical memory contents when primary power is lost. The R-2331/URN also contains operator controls and indicators. The C-11702 indicator control is used as an operator input/output interface for the system.

The AN/SRN-25(V)1 (Figure 4.4-17) is a highly integrated navigation system that uses data from GPS and Transit satellites and Omega ground stations to compute position. The AN/SRN-25 contains a single-channel Transit receiver, a two-channel GPS receiver, a three-channel Omega

Figure 4.4-16 AN/WRN-6(V)

receiver, a data entry keyboard, a display unit, a digital processor, a power supply, and a battery backup system for use in case of temporary loss of external power. The navigation program for

the digital processor is permanently stored in a read-only memory. There are separate reference

oscillators for the Transit and GPS systems. An RF frequency converter module allows interfacing to an external frequency standard in the AN/SRN-25. All components except the antennas are located within the console chassis. No external units are required; however, interface connections are provided on the back panel so that external equipment may be connected if desired. With the shutdown of the Transit and Omega systems, this equipment is not widely dispersed in the fleet.

Console

Figure 4.4-17 AN/SRN-25(V)1 Radio Navigation Set

The AN/SRN-25(V)6 will be the newest version of the GPS navigation system. Operation will be the same as the AN/SRN-25(V)1 except the (V)6 will not take data inputs from Transit

satellites. With an additional circuit card installed, the option of taking LORAN inputs will be available. GPS and Omega data are still received as inputs to the AN/SRN-25(V)6.

The TACAN system is a polar-coordinate type radio air-navigation system that provides a TACAN-equipped aircraft with:

All air-capable ships have TACAN installed. Usually, a meter in the aircraft indicates, in nautical miles, the distance of the aircraft from the surface beacon. Another meter indicates the direction of flight, in degrees-of-bearing, to the geographic location of the surface beacon (Figure 4.4-18). Pilots can use the bearing and distance from a specific beacon, identified by its identification signal to fix their geographic position.

Figure 4.4-18 TACAN Aircraft Indications

The distance measuring concept used in TACAN equipment is an outgrowth of radar-ranging techniques; i.e., determining distance by measuring the round-trip travel time of pulsed RF

energy. The return signal (echo) of the radiated energy depends on the natural reflection of the radio waves. However, TACAN beacon-transponder facilities, located at specific geographic

positions, generate artificial replies rather than depending upon natural reflection. The airborne equipment generates timed interrogation pulse pairs that are received by the surface TACAN system and decoded. After a 50 µsec delay, the transponder responds with a reply. The round trip time is then converted to distance from the TACAN facility by the airborne DME. The frequency and identification code provides the geographic location of the transmitting beacon.

In principle, the TACAN transponder need only reply to aircraft interrogations to supply the necessary distance data. However, the total pulse output of the transmitter would constantly vary according to the number of interrogating aircraft. For azimuth information to be supplied, the average power supplied to the antenna must be relatively uniform over time. To accomplish this, the transponder is operated on the constant-duty-cycle principle. In this method of operation, the receiver has automatic gain and squitter (noise-generated output) controls that maintain the receiver at a constant pulse output. If few interrogations are being received, the squitter and gain of the receiver will increase and add noise-generated pulses until the constant pulse output is obtained. If more interrogating aircraft come into range, the gain and squitter will decrease to maintain constant pulse output. If more than 100 aircraft interrogate, typically only the strongest 100 will generate replies from the transponder.

To provide aircraft with positive identification of the replying transponder, an identification feature is necessary. To meet this need, an identification code is transmitted at approximately

½-minute intervals. This is done by momentarily interrupting the transponder distance data and squitter-generated output and substituting pulse groups spaced at a 1350 pause rate. Each pulse group contains two sets of 12 µsec pulse pairs spaced 100 µsec apart. The duration of the identification pulse groups varies to represent Morse-coded characters.

The timing of the transmitted pulses is used to supply distance information to the aircraft. This leaves amplitude modulation as another medium for the transponder to convey information to aircraft. The TACAN beacon-transponder modulates the strength of the pulse to convey bearing information by producing a specified directional-radiation pattern rotated around a vertical axis. This signal, when properly referenced, identifies the aircraft direction from the TACAN facility. This, and distance data, give a 2-point fix for specific aircraft location.

The composite TACAN signal is composed of 2700 interrogation replies and noise pulse pairs per second, plus 180 north burst pulse pairs per second, plus 720 auxiliary burst pulse pairs per second for a total of 3600 pulse pairs per second, or 7200 pulses per second.

Priorities have been established for transmission of the various types of TACAN signals. These priorities are as follows:

Therefore, the identification group, replies, or squitter will be momentarily interrupted for the transmission of either the main or auxiliary reference group. The transmission of replies or squitter will be interrupted every 37.5 seconds during the transmission of an identification code dot or dash.

There are a number of different types of TACAN equipment aboard ship that perform the same function, although they are physically different in appearance. The AN/SRN-15A, AN/URN-20B(V))1, and AN/URN-25 represent the old and new TACAN radio sets in the fleet today.

The single Radio Set AN/URN-20C(V)1, (Figure 4.4-19) and dual Radio Set AN/URN-20C(V)2 are TACAN radio sets intended for ship or shore installation. The functions performed by the single and dual sets are identical. The dual set, however, includes two transponder groups, one additional monitor, and three additional line voltage regulators.

Figure 4.4-19 AN/URN-20C(V)1 (Single Set)

The local control units, interconnecting wiring, and the test monitor control (TMC) RF components of the dual set are different from those of the single set. The additional equipment in the dual set makes it possible to continue operating the TACAN set if one transponder group should fail. The radio set can operate in either the "X" or "Y" mode. In the X mode, the set transmits both distance measuring and bearing information. In the Y mode, only distance measuring information can be transmitted, although an equipment field change being tested may make transmission of all information feasible.

The radio set can provide individual distance measuring service for up to 100 interrogating aircraft simultaneously. Since bearing and identification signals are delivered spontaneously and not in response to interrogations, an unlimited number of properly equipped aircraft can derive this information from the radio set over a line-of-sight range up to 300 nautical miles.

The AN/URN-25 is a newer TACAN beacon set that replaces the AN/URN-20B(V)1 sets on many ships. It is smaller and has been improved for modern shipboard use. It consists of two major units, the Transponder Group OX-52/URN-25 and the Control-Indicator C-10363/URN-25, as illustrated in Figure 4.4-20. Each transponder is housed in a cabinet with two vertical drawers, one containing the coder-keyer and the other the receiver-transmitter. The control-indicator displays the status of the transponder and failure alarms, and allows limited control of the transponder from a remote location. It may be mounted in its own cabinet or in a standard 19-inch rack. The AN/URN-25 operates similarly to the AN/URN-20. For details on the AN/URN-25, refer to the appropriate technical manual.

All shipboard TACAN radio sets use the same type of antenna. The most common model is the OE-258/URN which, unlike a radar antenna, does not physically rotate. Older mechanical TACAN antennas have all been replaced with the newer solid-state version.

Shipboard TACAN systems are flight certified by annual/periodic inspections. These inspections are conducted at SESEF (Shipboard Electronic Systems Evaluation Facility) ranges. Where SESEF certification facilities are not available, the FAA (Federal Aviation Administration) will continue to provide flight certification services. In order to obtain FAA certification, the ship must be located no more than 100 NM from the nearest suitable air field which can support jet operations. SESEFs are located as follows:

Figure 4.4-20 AN/URN-25

Note that the Long Beach, CA SESEF facility is being deactivated. An additional SESEF facility is planned for Mayport, FL.

A shipboard TACAN flight inspection is required:

In cases where certification is not possible without inordinately disrupting operations, Fleet Commanders in Chief may authorize, on a case by case basis, operational waivers to aviation facility certification criteria. Certification criteria waivers shall be granted for a specified period of time and shall not exceed one year from the date of the waiver authorization. Follow on waivers shall not be granted until an appropriate inspection of the aviation facility has been accomplished. For ships operating under waivers, corrective measures shall be contained in the list of repairs and/or alterations to be accomplished during the next overhaul. The priority of accomplishing these alterations during overhaul will be established within the Fleet Modernization Program. A waiver shall not be used for those items that are within forces afloat capability to correct.

SESEFs are the primary certification authority. Whereas the FAA inspector must fly to the ship, the ship goes to the SESEF range. SESEFs also provide other services, e.g. antenna pattern checks, communications checks, and ES checks. The advantage of using a SESEF is the simplicity of the procedures and the willingness of SESEF personnel to work with ship's personnel to discuss and resolve problems via the communications net. Once communications is established and the ship is approaching the SESEF facility test area, the ship will verify that the TACAN is transmitting, gyro input is on, and proper magnetic variation is set. SESEF personnel verify that the alignment of the TACAN antenna when ship arrives in test area. The ship will steam in a circle (port or starboard), approximately 1 mile in diameter, at 10 knots.

The results of the test are communicated immediately to the ship. A written report is hand-carried or mailed several days later.

The FAA is the secondary certification authority. Ships located at overseas locations, except Japan, will not be near a SESEF and, consequently, will need to arrange an FAA certification.

Under normal conditions, flight inspections usually require no more than two hours. The sequence of events may vary from flight inspection to flight inspection, but will contain the following events:

At the completion of the flight inspection, and prior to leaving the area, the inspector should inform the ship of the tentative results. Official results are forwarded when analysis of the recordings is complete. If the inspector detects a condition of signal abnormality, i.e., no north burst, no squitter, no distance measurement beyond 5 miles, this information should be relayed to the ship immediately. If the abnormality is not corrected after a reasonable amount of time, the inspection will be terminated. If the inspection is terminated, the ship will complete the necessary repairs on the equipment and request another inspection.

The primary prerequisites required of a ship whose TACAN is to be inspected are:

As EMO, ensure the following:

Instructions for writing messages to request TACAN certification are provided in Instructions and Procedures for Requesting TACAN Flight Certification (NAVELEX EE-172-FA-GYD-010/E120). This instruction is being revised.

PROCEED TO ASSIGNMENT SHEET 4-4-1A IN THE ASSIGNMENT BOOKLET. UPON COMPLETION, TAKE THE ASSIGNMENT BOOKLET TO THE LEARNING CENTER INSTRUCTOR.