Electronics Material Officer

Electronics Material Officer Course






















This lesson topic presents information common U.S. Navy radar systems and equipment.

The LEARNING OBJECTIVES of this LESSON TOPIC are as follows:

4.5 Describe shipboard Combat Systems Electronic RADAR equipment as related to:

a. Safety

b. Physical characteristics

c. Purpose

d. Limitations

e. Maintenance

f. Installation

g. Components

h. Operations

i. Interfacing

j. Other electronic subsystems

k. Technical documentation

l. Material condition

4.6 Describe the purpose and characteristics of the four fundamental RADAR systems used aboard ships to include:

a. Surface search

b. Air search

c. Altitude determining

d. Fire control

4.7 Describe the characteristics of basic types of RADAR antennas.

4.8 Describe the basic shipboard IFF system to include:

a. Purpose

b. Units

c. Interface

d. Modes of operation

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








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

Written Lesson Topic presentations in the Module Booklet:

1. Lesson Topic Summary

2. Narrative Form of Lesson Topic

3. Lesson Topic Progress Check

Additional Materials:

1. Assignment Sheet

2. Answer Booklet


1. Shipboard Electronics Material Officer, NAVEDTRA 12969

2. Electronic Technician 3 & 2, NAVEDTRA 10197

3. EIMB Radar

4. Electronic Technician Supervisor, NAVEDTRA 12411

5. NEETS Module 18, Radar Principles, NAVEDTRA 172-18-00-84








This lesson topic will introduce you to a variety of radar systems and equipment that you may be materially responsible for as EMO. The review of radar principles includes basic radar theory, terminology, types, and detection methods. An overview of common radar equipment and factors that affect radars will also be presented. The lesson narrative is organized as follows:

Radar Systems/Equipment

A. Radar Principles Review

B. Basic Radar System

C. Factors Affecting Radar Performance

D. Target Resolution

E. Radar Types

F. Radar Systems/Equipment

G. Radar Indicators

H. IFF Equipment

I. Radar Distribution Switchboards










Basic radar principles, components, and operating characteristics/parameters were covered in Division Officer Course. A brief review is provided here. Radar is an acronym for Radio Detection and Ranging. Shipboard radar systems are used primarily for early detection of surface or air contacts and to gather data, such as range, bearing, altitude, and speed of targets. They are also used for general surveillance, navigation, and for controlling ship's own aircraft and small boats. The principle on which radar operates is similar to the reflection or echo of a sound wave or signal. The strength, or loudness, of an echo depends mainly on signal strength, distance to the reflecting surface, ability of the surface to reflect sound waves, and the hearing acuity of the listener. Radar uses RF waves to take advantage of this principle by radiating a high power RF beam from a directional antenna. A signal echo is returned from contacts in the path of the beam and detected by a sensitive receiver. The echoes are then presented visually on an indicator (screen). The radar system gives an indication of target distance (range) by measuring the time between the transmission of the signal and the return echo; and an indication of target direction by the bearing of the directional antenna.



Standard radar frequency bands and their related frequencies are shown in Table 4.2-1. A comparison of communications and radar functions is provided.

Table 4.2-1 Radar Band Use


BandFrequency Communications Function Radar Function





B 250 - 500 MHz VHF/UHF Communications


C 500 MHz - 1 GHz UHF Communications IFF, Missile Guidance, Early Warning


D 1 GHz - 2 GHz UHF Communications


E 2 GHz - 3 GHz UHF Communications Missile Guidance, Early Warning,

Target Acquisiiton


Table 4.2-1 Radar Band Use (Continued)



BandFrequency Communications Function Radar Function


F 3 GHz - 4 GHz SHF Communications Missile Guidance, Early Warning,

Data Transmission


G 4 GHz - 6 GHz SHF Communications Target Tracking


H 6 GHz - 8 GHz SHF Communications Target Tracking, Anti-Aircraft

Fire Control, Target Acquisition,

Navigation, Missile Homing


I 8 GHz - 10 GHz SHF Communications Fire Control, Target Tracking and

Acquisition, Navigation, Missile Homing, Air Intercept, Data Transmission, Surface Search,

Missile Guidance, Airborne Mapping

and Reconnaissance, Airborne Search and Bombing


J 10 GHz - 20 GHz SHF Communications


K 20 - 40 GHz SHF/EHF Communications


L 40 - 60 GHz EHF Communications


M 60 - 100 GHz EHF Communications


N 100 - 200 GHz EHF Communications




The visual data required to determine and track a target's position is supplied by a specially designed cathode ray tube (CRT) installed in a unit known as a plan position indicator (PPI). Bearing, range, and (for aircraft) altitude are necessary to determine target position. Generally, two scopes are used: one for bearing and range, and another for range and altitude. With the addition of an IFF interrogator, altitude information is available at any PPI. Certain consoles provide bearing, range, and altitude information.


The antennas of most radars are designed so that they radiate energy in one lobe that is moved by moving the antenna itself. The general shape of a lobe is shown in Figure 4-2.1. The shape of the lobe is such that the echo signal strength varies more rapidly with a change of bearing on the sides of the lobe than near the axis. Therefore, the echo signal varies in amplitude as the antenna rotates. At antenna position A (Figure 4.2-1), the echo is relatively small, but at position B (Figure 4.2-2), where the lobe axis is aimed directly at the target, the echo strength is maximum. Thus, the bearing of the target can be obtained by training the antenna to the position at which echo is greatest. In actual practice, manipulation of the antenna in this manner could alert an enemy unit that it has been detected and denies remote indicators full use of the radar for search purposes. However, this technique is widely used in weapons control and guidance radar systems in manual and automatic modes.















Figure 4.2-1 Bearing Determination, Antenna Position A














Figure 4.2-2 Bearing Determination, Antenna Position B



The successful use of pulse modulated radar systems depends primarily on our ability to measure distance in terms of time. Radio frequency energy radiated into space travels at the speed of light; i.e., 186,000 miles per second, 162,000 nautical miles per second, or 328 yards (yds) per microsecond (msec). When it strikes a reflecting object, it is redirected, with no loss in time.

The constant velocity of radio frequency energy is used in radar to determine range by measuring the time required for a pulse to travel to a target and return.

For example, assume that a 1 msec pulse is transmitted toward an object that is 32,000 yds away. Figure 4.2-3, View A, shows conditions at the instant the pulse is radiated. When the pulse reaches the target, it has traveled 32,800 yds at 328 yds/msec. Therefore, 100 mseconds have elapsed. View B shows the pulse arriving at the target where it is reflected back to the antenna. Since the return path is also 32,800 yds, it takes 100 mseconds for the pulse to return to the radar antenna (View C). The total elapsed time is 200 mseconds for a distance of twice the actual range of the target. Therefore, velocity is considered to be one half of its true value, or 164 yds/msec; i.e., time x 164 = range or 200 x 164 = 32,800 yards.






































Figure 4.2-3 Range Determination



Altitude is determined by using either height-finding radar or fade charts.


Height-Finding Radar

Height-finding radar uses a very narrow vertical beam, which is moved up and down electronically or mechanically to pinpoint targets. The electronic method, shown in Figure 4.2-4,

produces a frequency scanning pattern along the vertical plane. Lines originating at the antenna depict the number of beam positions required to ensure complete coverage. Each beam position corresponds to a slightly different radiated frequency, which is set at a specific angle or step in relation to the base of the antenna. When the antenna base is stable, the initial radiated frequency sets up the top beam. A slight change in frequency activates the second beam, and the process continues until the entire plane is covered. When the antenna base is unstable, error signals are introduced by components of the system. A change then results in the transmitted frequency, compensating for ship's pitch and roll and ensuring a complete search of the vertical plane.














Figure 4.2-4 Frequency Scanning


Height-finding radar provides the two mathematical components that are used to determine the altitude of an aircraft, angle of elevation, and slant range. The slant range of an aircraft is the distance of the aircraft from the radar antenna, measured along the radar beam (Figure 4.2-5). When both the angle of elevation and slant range are known, the altitude of the aircraft can be found. The solution may be by calculation, by reference to a graph, or by a computer built into the radar. To find the altitude by calculation, multiply the slant range by the sine of the angle of elevation. Altitude found in this way is not the true height of the airplane above the earth because the calculation is based on the assumption that the earth is flat. However, most height finding radars have a circuit that computes the error due to the curvature of the earth.













Figure 4.2-5 Determination of Altitude

Fade Charts

When height-finding radar is not installed, fade charts can be used to estimate aircraft altitude. This technique uses a combination of air search radar fade zones and reception zones. The positions of lobes and nulls in an antenna pattern remain the same as long as antenna height and radar frequency are unchanged; thus, a given radar installation will have an unchanging radiation pattern. This makes it possible to plot the positions of lobes and nulls on a chart that may be used as an aid in determining the altitude of aircraft targets. To use a fade chart, a radar operator must notice the ranges at which an aircraft disappears in null areas. By applying these ranges to the chart, the operator can estimate the aircraft's altitude. Data for fade charts is determined experimentally by having an aircraft fly at several constant altitudes, while an operator records its observed signal strengths and ranges.



Up to this point, only the pulse modulation method of transmission has been used to show how a target is detected and tracked. Although this is the most common method, two other methods are sometimes used in special application radars. These are the continuous wave method, and the frequency modulation method.

Continuous Wave

The continuous wave (CW) method uses the Doppler effect to detect a target. The frequency of a radar echo changes when the target is moving toward or away from the radar transmitter. This change in frequency is known as the Doppler effect. It is similar to the effect at audible frequencies when the sound from the whistle of an approaching train appears to increase in pitch.

The opposite effect (a decrease in pitch) occurs when the train is moving away from the listener. The radar application of this effect involves measuring the difference in frequency between the transmitted and reflected radar beams to determine both the presence and speed of the moving target. This method works well with fast-moving targets, but not well with those that are slow moving or stationary.


Frequency Modulation

In FM the transmitted frequency is varied continuously and periodically over a specified band of frequencies. At any given instant, the frequency of energy radiated by the transmitting antenna differs from the frequency reflected from the target. This frequency difference can be used to determine range. Moving targets, however, produce an additional frequency shift in the returned signal because of the Doppler effect. This additional frequency shift affects the accuracy of range measurement. Thus, this method works better with stationary or slow moving targets than with fast-moving targets.


Pulse Modulation

Radars using pulse modulation transmit energy in short pulses that vary in duration from less than 1 to 200 mseconds, depending on the type of radar. Echoes are amplified and applied to an

indicator that measures the time interval between transmission of the pulse and reception of the echo. Half the time interval then becomes a measure of the distance to the target. Since this method does not depend on the relative frequency of the returned signal or on the motion of the target, difficulties experienced with the CW and FM methods are not present. The pulse modulated method is used almost universally in military and naval applications. Therefore, it is the only method discussed in detail in this text.



You can understand the operation of modern radar systems by learning the functions of the blocks in the basic pulsed radar system diagram shown in Figure 4.2-6. The heart of the radar system is the modulator. It generates timing pulses (triggers) for use in the radar and associated systems. Its function is to ensure that all subsystems making up the radar system operate in a definite time relationship with each other. It also ensures that the intervals between pulses, as well as the pulses themselves, are of the proper length. Some of the more common pulses furnished by the modulator include transmitter trigger, receiver gate, indicator trigger, associated Identification Friend or Foe (IFF) System trigger, and EW blanking trigger. The rate at which the transmitter is triggered is called the pulse repetition rate (PRR) or pulse repetition frequency (PRF). The transmitter supplies RF energy, often at extremely high power, for short intervals of time. A pulse transformer in the transmitter increases the voltage of the pulse received from the modulator to trigger a magnetron or power amplifier. (Generally, air search and height-finding radars use power amplifiers.) The magnetron or power amplifier oscillates at the designed transmission frequency of the radar for the duration of the pulse. Its output is transmitted to an antenna assembly via a duplexer and transmission line.























Figure 4.2-6 Block Diagram of a Basic Radar System

A duplexer permits the use of a common transmission line and antenna for both transmitting and receiving. The duplexer consists of two electronic switches, the transmit-receive (TR) and the antitransmit-receive (ATR). The TR switch blocks the path to the receiver each time the transmitter is fired, preventing the high-powered pulse from damaging the receiver. The ATR switch directs the received signal to the receiver while blocking it from the transmitter, keeping the signal from being dissipated in the magnetron during the receive interval. Thus, the duplexer not only provides coupling to the antenna system, but also prevents damage to the receiver system, and loss of the return echo in the transmitter. The radar antenna system takes the RF energy pulse from the transmitter and radiates it as a directional electromagnetic beam. It also picks up the returning electromagnetic echo and passes it on to the receiver as an RF pulse, with minimum loss. Modern radar receivers are highly sensitive superheterodyne receivers. The receiver amplifies the weak RF echo and converts it into a video signal. The radar indicator converts the video output of the receiver to a visual display of range and bearing (or in the case of height finding indicators, range, and height).



Internal characteristics of radar equipment that affect range performance are: peak power transmitted, pulse width, pulse repetition rate, transmission line efficiency, antenna height, and

receiver sensitivity. External factors are operator skill; target size, composition, angle, and

altitude; weather conditions; and ECM activity.


In general, the maximum range that can be measured on an indicator is limited by the pulse repetition rate (PRR). This is because with each transmitted pulse the indicator is reset to zero range. Therefore, if the time between transmitted pulses is shorter than the time it takes the transmitted pulse to reach the target and return, the indicator will have been reset and started as a new sweep; thus indicating a false range upon reception of the echo. Pulse width (PW) also affects maximum detection range. The wider the pulse, the greater the average power out, resulting in a greater detection range of small targets. Air search radars usually have a much greater PW than surface search radars. The more sensitive the receiver, the weaker the echo required to produce a target indication. As the receiver sensitivity is increased, which is reflected in a higher minimum discernable signal (MDS), the range at which a particular target can be detected is increased. Target size also affects maximum range. Generally, the larger a target, the greater the range at which it can be detected. Land, particularly high, steep cliffs, can be detected at a much greater range than any other type of target, except, perhaps, high-altitude aircraft. Similarly, a group of aircraft can be detected at a greater range than a single aircraft because of their larger reflecting area. Targets at high altitudes can be detected at a longer range than those at low altitudes simply because it is possible for the radar pulse to reach them.

Another factor affecting the maximum range is antenna height. The distance in nautical miles to the radar horizon (disregarding propagation phenomena) is approximately 1.25 times the antenna height (measured in feet). To determine the detection range of a target, use the formula

1.25 Öh1 + 1.25 Öh2, where h1 is the height of the transmitting antenna, and h2 is the height of the target. The antenna beam width also affects the maximum detection range. A narrow, more concentrated beam has a greater range capability than a wide beam since it provides higher energy density per unit area.

Yet another factor that affects the maximum detection range is the antenna rotation rate. The slower an antenna rotates, the greater the reliability of detection at long range. When the antenna is rotated at ten revolutions per minute (RPM), the energy beam strikes each target for one-half the time it would if the rotation were 5 RPM. The number of strikes per antenna revolution is called hits per scan. During this time, a sufficient number of pulses must be transmitted to return an echo that is strong enough to be detected. Long-range search radars normally have a slower antenna rotation rate than radars designed for short-range coverage.



The closest range at which radar can detect a target is controlled primarily by the duration of the transmitted pulse. Some of the energy of the transmitted pulse leaks directly into the receiver. This overloads and blocks the receiver. At the end of the transmitted pulse, the receiver begins to recover, but recovery is not instantaneous. As long as the receiver is blocked, a saturation signal appears on the indicator, preventing echo pulses from being seen. Modern radar receivers have recovery times measured in hundredths of a microsecond, allowing targets that are at a range just slightly greater than half the transmitted pulse width to be displayed.

When a high-powered radar is operated within a few miles of land, targets cannot be tracked into a short range because side lobe echoes (Figure 4.2-7) clutter the first mile or two of the scope. Targets within the area producing sea return (false return of signals from the nearby sea) usually produce very strong echoes. Receiver gain can be reduced so that actual echoes will stand out

















Figure 4.2-7 Side Lobes


from sea return. Sensitivity Time Control (STC) is a modification of the receiver in which receiver gain is reduced for the first few thousand yards of each sweep and then restored to normal for the remainder of the sweep. The reduced gain at short range provided by this modification decreases sea return and prevents side lobe echoes from obscuring the start of the trace. The minimum range at which high-flying aircraft can be tracked depends on the vertical coverage of the radar antenna. In most search radars, little energy is radiated directly overhead or at large elevation angles.


Anomalous propagation is a general term applied to all nonstandard radar or radio propagation. It is the net effect of certain variables that may result in extremely long or short ranges (particularly in radar), often changing from one to the other or back to normal in a matter of hours or days. Anomalous propagation is caused by changes in atmospheric conditions, principally in temperature and moisture content. Under normal conditions, radar energy travels in a near line-of-sight path, with some bending possible. Under normal conditions the temperature and moisture content of the atmosphere decreases from the surface of the earth to the higher altitudes. At a certain height above the surface, depending on the height of the body of warm air, the temperature will be greater and the moisture content will be less than at the surface. This results in a sharp temperature inversion and a pronounced decrease in moisture content above the layer of cool air. Under these conditions, radar waves are refracted more than normal and tend to follow the surface of the water in a surface duct, whose upper and lower

boundaries are the warm, dry air and water surface. Therefore, surface targets and low flying aircraft may be detected at greatly increased ranges because of the duct formed between the

water and the warm air mass. When the duct is formed in the atmosphere instead of along the surface, particularly when the duct angles upward, radar surface coverage is adversely affected. In this case the duct formed in the strata of warm air will have no connection with the surface of the water. If the duct tilts upward, the trapping of the energy in the duct may increase high-angle coverage but seriously reduce low-angle or surface coverage.

Another problem exists because of radar holes that are caused by atmospheric conditions. Targets previously undetected could suddenly appear at a very close range, or even go totally undetected except visually. Another phenomena is radar ghosts or false targets. These can be caused by atmospheric conditions, clouds, sea return, and birds. False targets can cause confusion and concern to radar operators. Occasionally, a process called multipath may hamper the evaluation of targets and identification of ghosts. Multipath occurs when two signals, one direct and the other bounced or sea-reflected, return from a target. If the path of a target causes a land mass or island to come between the target and the radar temporarily, the reflected signal may be interrupted, leading to drastic changes in radar range, and the target may disappear. This may cause an operator to conclude that the radar had been tracking a ghost when the target was actually real. Anomalous propagation can lead operators and technicians to believe that radar equipment is malfunctioning. It is important to ensure that equipment is operating properly before poor performance is blamed on atmospheric conditions. If accurate records of equipment performance are kept, they can be reviewed to indicate that the equipment is operating normally when anomalous propagation is suspected.

Overall performance can be verified with an echo box. An echo box checks transmitter and receiver performance and in some applications includes a check of the antenna and transmission line. A preferred method of checking radar system performance is a measurement of output power with a power meter and a measurement of receiver sensitivity with a signal generator. Radar performance figure checks, usually PMS checks, are the most reliable test.



Target resolution is a radars ability to distinguish between targets that are close together in either range or bearing. Weapons control radar, which requires great precision, should be able to

distinguish between targets that are yards apart. Search radar is usually less precise and only distinguishes between targets that are hundreds of yards or even miles apart. Resolution is usually divided into two categories: range and bearing resolution.


Range resolution is the ability of a radar to resolve between two targets on the same bearing, but at slightly different ranges. The principal factors that affect range resolution are pulse width, receiver gain, and indicator range scale. A high degree of range resolution requires a short pulse, low receiver gain, and a short-range scale. When two targets are on the same bearing, the minimum distance they must be separated to show as two echoes is slightly greater than one-half the pulse length. The theoretical range resolution of a radar system can be calculated from the following formula: range resolution = PW x 164 yds/msec. Although pulse width is the primary factor in determining range resolution, the amount of receiver gain used also affects the resolution. Echoes from two targets that are close together may merge into a single indication when the gain setting is high, but they may separate into individual blips when the gain is reduced. The third factor effecting range resolution is the range scale used. On a long-range scale, a separation of a few hundred yards will not be apparent. In fact, two adjacent blips will seem to blend into one. If these same echoes can be displayed on a short-range scale, a small separation will be visible.



Bearing, or azimuth, resolution is the ability of a radar system to separate objects at the same range but at different bearings. The degree of bearing resolution depends on radar beam width and target range. Range is a factor in bearing resolution because the radar beam spreads out as range increases. Two targets at the same range must be separated by at least one beam width to be distinguished as two objects.



Due to different design parameters, no single radar set has been produced that can perform all of the radar functions required by combatant ships. As a result, the modern warship has several radar sets, each performing a specific function. A shipboard radar installation may include surface search, navigation radar, air search radar, a height finding radar, and various fire control radars. These radar installations include some form of support system; dry air, cooling water, electrical power, etc. These support systems are described in Lesson Topic 4.5.


The primary functions of a surface search-radar are:

l Detection and determination of accurate ranges and bearings of surface targets and lowflying aircraft

l Maintaining a 360 search for all targets within LOS distance from the radar antenna

Since the maximum range requirement of a surface search radar is primarily limited by the radar horizon, higher frequencies are used to permit maximum reflection from small target-reflecting areas such as ship masthead structures and submarine periscopes. Narrow pulse widths are used to permit a high degree of range resolution at short ranges, and to achieve greater range accuracy. High pulse repetition rates are used to permit maximum definition of targets. Medium peak

powers can be used to permit detection of small targets at LOS distances. Wide vertical beam widths permit compensation for pitch and roll of ownship and detection of low flying aircraft. A narrow horizontal beam width permits accurate bearing determination and good bearing resolution. The following are applications of surface search radars:

l Indicate the presence of surface craft and help determine their course and speed

l Coach fire control radar onto a surface target

l Provide security against attack at night, during conditions of poor visibility, or from behind a smokescreen

l Aid in scouting

l Obtain ranges and bearings on prominent landmarks and buoys as an aid to piloting, especially at night and in conditions of poor visibility

l Facilitate station keeping

l Detect lowflying aircraft

l Detect certain weather phenomena

l Detect submarine periscopes

l Control small craft during boat or amphibious operations


Navigation radars fall into the same general category as surface search radars. As the name implies, navigation radars are used primarily as an aid for navigating and piloting the ship. This type of radar has a shorter operating range and higher resolution than most surface search radars.



The primary function of an air search radar is to detect aircraft targets and to determine their ranges and bearings over relatively large areas while maintaining a complete 360 surveillance from the surface to high altitudes. Air search radars have the following general characteristics:

l Relatively low radar frequencies - permit long range transmissions with minimum attenuation

l Wide pulse width and high peak power - aid in detecting small targets at great distances

l Low pulse repetition rates - permit greater maximum measurable range

l Wide vertical beam width - helps ensure detection of targets from the surface to relatively high altitudes, and to compensate for pitch and roll of ship

l Medium horizontal beam width - permits fairly accurate bearing resolution while maintaining 360 search coverage


The following are applications of air search radars:

l Warn approaching aircraft and missiles before they can be sighted visually to determine the direction from which an attack may develop, launch fighters in time if air attack is imminent, and ready antiaircraft defenses in a timely manner

l Monitor enemy aircraft and vector Combat Air Patrol (CAP) aircraft for intercept

l Provide security against sneak attack at night and during low visibility

l Control aircraft on a specific geographic track (such as an antisubmarine barrier or search and rescue pattern)



The primary function of height-finding radars (also referred to as three-coordinate or 3-D radar) is to compute accurate ranges, bearings, and altitudes of aircraft targets detected by the air search radar. Height-finding radars are used by the ship's air controllers to direct fighter aircraft for interception of air targets. The main differences between the air-search radar and the height-finding radar are that the height-finding radar has a higher transmitting frequency, higher power output, a much narrower vertical beam width, and requires a stabilized antenna for altitude accuracy. Applications of height-finding radar include the following:

l Obtain range, bearing, and altitude data on enemy aircraft and missiles to assist in the control of CAP for intercept

l Detect low-flying aircraft and track aircraft over land

l Determine range to land

l Detect certain weather phenomena and track weather balloons

l Provide input to fire control for director control



Tracking Radars

Radars that provide continuous positional data on targets are called tracking radars. Most tracking radar systems used by the military are also fire control radars; the two names are often

used interchangeably. Fire control tracking radar systems usually produce a very narrow, circular beam. Fire control radar must be directed to the general location of the desired target because of the narrow beam pattern. This is called the designation phase of equipment operation. When the radar beam is in the general vicinity of the target, the radar system switches to the acquisition phase. During acquisition, the radar system searches a small volume of space in a prearranged pattern until the target is located. When the target is located, the radar system enters the track phase of operation. Using one of several possible scanning techniques, the radar system automatically follows all target motions. The radar system is said to be locked on to the target during the track phase. The three sequential phases of operation are often referred to as modes and are common to the target-processing sequence of most fire control radars. Typical fire control radar characteristics include a very high PRF, a very narrow pulse width, and a very narrow beam width. These characteristics, while providing extreme accuracy, limit the range and make initial target detection difficult.


Missile Guidance Radar

A radar system that provides information used to guide a missile to a hostile target is called a guidance radar. Missiles use radar to intercept targets in three basic ways. Beamrider missiles follow a beam of radar energy that is kept continuously pointed at the desired target. Homing missiles detect and home on radar energy reflected from the target. The reflected energy is

provided by a radar transmitter either in the missile or at the launch point and is detected by a receiver in the missile. Passive homing missiles home on energy that is radiated by the target. Because the target's position must be known at all times, a guidance radar is generally part of, or associated with, a fire control tracking radar. In some instances, three radar beams are required to provide complete guidance for a missile. The beamriding missile, for example, must be launched into the beam and then must ride the beam to the target. Initially, a wide beam is radiated by a capture radar to gain (capture) control of the missile. After the missile enters the capture beam, a narrow beam is radiated by a guidance radar to guide the missile to the target. During both capture and guidance operations, a tracking radar continues to track the target. Applications of fire control radars include:

l Gun and missile control

l Detect lowflying aircraft

l Assist in radar navigation

l Track weather balloons

l Furnish range and bearing data for calibrating search radars



Ship's radars cano perform a variety of functions. For example, most height finding radars can be used as secondary air search radars, and in emergencies, fire control radars have served as surface search radars. Because there are so many different models of radar equipment, the radars and accessories described in this chapter are limited to those common to a large number of ships,

and to those that are replacing older equipment currently installed in the fleet.


As mentioned earlier, the principal function of surface search radars is to detect surface targets and lowflying aircraft and determine their range and bearing. The most common surface search radars in use today are the AN/SPS-67(V)1, AN/SPS-10(F) (Figure 4.2-10), AN/SPS-55 (Figure 4.2-11), and the AN/SPS-64(V) (Figure 4.2-12).

Radar Set AN/SPS-67(V)1

The AN/SPS-67(V)1 radar is a two-dimensional (azimuth and range) pulsed radar set primarily designed for surface operations with a secondary capability of anti-ship-missile and lowflier detection. The radar set operates in the 5450 to 5825 MHz range, using a coaxial magnetron as the transmitter output tube. The transmitter/receiver is capable of operation in a long (1.0 sec), medium (0.25 msec), or short (0.10 msec) pulse mode to enhance radar performance for specific operational or tactical situations. Pulse repetition frequencies (PRF) of 750, 1200, and 2400 pulses/second are used for the long, medium, and short pulse modes, respectively. The AN/SPS-67(V)1 radar will be the primary surface search and navigation radar with limited air search capability, and will eventually replace the existing AN/SPS-10 series radars on some classes. The construction of the radar set is primarily solid state, with the exception of the transmitter magnetron and the receiver TR device. Miniature and microminiature technology are used extensively throughout the radar set. Standard electronic module (SEM) architecture is incorporated in the set design to the maximum extent possible. (The SEM program, established within the Navy Material Commands, provides standardization of modular plug-in cards for all electronic systems.)

The radar set includes a built-in test equipment (BITE) subsystem that will locate 80% of the failures, to a maximum of four modules, within the Video-Processor, and the receiver transmitter. Faults are indicated on light emitting diode (LED) index indicators, and the condition of each indexed test point is displayed on readout indicators as GO, MARGINAL, or NO-GO. In addition, the BITE subsystem provides the maintenance operator with an interactive test mode, which permits the selection of a series of sensor test points for monitoring purposes, while making level or timing event adjustments. Power and VSWR are monitored on an on line basis.

The BITE subsystem is designed to have its own self-check mode, which is performed automatically on a periodic basis. The BITE circuitry will not degrade the performance of the system during normal operation, or in the event of a failure in the BITE circuitry. The major units of the AN/SPS-67(V)1 are described below. Refer to Figure 4.2-9.

The Radar Set Control, Unit 1, is a bulkhead-mounted unit that contains a SEM rack, a power supply, and the controls and indicators necessary to operate the radar set in all modes of operation. It has lighted pushbutton switches and indicators, potentiometers, and a 3-digit LED display. The Receiver-Transmitter, Unit 2, is a bulkhead-mounted unit that contains all of the system's microwave components, a SEM rack, and subassemblies. Cooling air enters and exhausts via louvered openings and is forced through the unit by two blowers. The Video Processor, Unit 3, is a bulkhead-mounted unit that contains the two SEM racks, two control panels, plug-in power supplies and some chassis-mounted parts. Cooling air enters and exhausts via four louvered openings and is forced through the unit by two blowers. The Antenna Controller, Unit 4, provides remote control of the prime power to the antenna. It consists of a









































Figure 4.2-9 AN/SPS-67(V)1


three phase remotely actuated power relay and a thermal relay located in a housing. An overload indicator and a manual overload reset button are located on the front panel. The Antenna Safety Switch, Unit 5, protects maintenance personnel from electrical shock, radiation exposure, and antenna rotation while they work on the antenna by interrupting power to the antenna and inhibiting the radar transmitter. It is mounted near the antenna. The Antenna Assembly, Unit 6, radiates the pulses of microwave energy from the magnetron and directs the echo signals to the

receiver through the waveguide. The IFF Band Suppression Filter, Unit 7, is connected to the IFF equipment and minimizes interference from the radar set. The AN/SPS-67(V)1 radar set is compatible with the following equipment:

l Blanker/Video Mixer Group AN/SLA-10

l IFF equipment

l Indicator Group AN/SPA-25, or equivalent

l MK 27 Synchro Signal Amplifier or equivalent

l Multiplexed Unit for Transmission Elimination (MUTE)


The AN/SPS-67(V)1 has the following advantages over the AN/SPS-10F:

l Lowflier detection

l Three selectable pulse widths

l Jitter mode for increased ECM capability

l Sector radiate

l Easier to troubleshoot and maintain due to the use of SEMs

l Transmitter and receiver can be automatically tuned


There are two other configurations of the AN/SPS-67(V), the (V)2 and (V)3. The AN/SPS-67(V)2 is identical to the AN/SPS-67(V)1 with exception of the antenna. This variant uses a standard surface search antenna. The AN/SPS-67(V)3 is ehanced from previous configurations. This radar has a signal processing unit that provides digital moving target indicator (DMTI) capability. The function of the DMTI circuitry is to automatically cancel unwanted fixed echoes (sea clutter, clouds, rain, etc.) and display only moving target signals.


Radar Set AN/SPS-10

The AN/SPS-10F and its associated components are shown in Figure 4.2-10. Its operation is similar to that of the AN/SPS-67(V)1. For additional information, refer to the appropriate technical manual.


Radar Set AN/SPS-55

The AN/SPS-55 is a solid state, surface search and navigation radar capable of detecting targets from as close in as 50 yards and out to 50 miles and beyond, with good target resolution. Figure






































Figure 4.2-10 AN/SPS-10F


4.2-11 illustrates the major assemblies of the radar and their relationship to each other. Radar Set AN/SPS-55 consists of four major units: antenna group, radar receiver/transmitter, radar set control, and box switch. The system generates two selectable pulse widths. The RF frequency is tunable from 9.05 to 10.0 GHz with a minimum peak power out of 130 kW (measured at the magnetron). The linear array antenna, rotating in azimuth at 16 RPM, forms a beam narrow in azimuth (1.5) and broad in elevation (-10 to +10, centered on the horizon). Return target echoes are amplified and detected by the receiver and applied to a PPI. The target information can be displayed in either of two modes, a relative mode where zero degrees bearing on the PPI represents the heading of the ship or a true mode where zero degrees bearing




































Figure 4.2-11 AN/SPS-55


represents true north. The heading marker indicates the bow of the ship in either case. The radar set uses several signal processing circuits to improve operation under certain prevailing conditions:

l Fast Time Constant (FTC) circuit - Reduces clutter by displaying only the leading edge of the echo returns.

l Sensitivity Time Control (STC) circuit - Reduces receiver gain at close-in ranges where clutter is strong, while allowing a gradual return to normal gain at longer ranges where clutter is less.

l Sector Radiate Capability - Allows the operator to limit radiation to a selectable azimuth segment to minimize interference from other ships' radars or ECM equipment.


Radar Set AN/SPS-64(V)9

The AN/SPS-64(V)9 system combines high transmitter power, high pulse repetition rate, narrow antenna beam width, a sensitive receiver, and a digitally enhanced display to provide a bright, accurate, and clearly defined radar presentation. The system is capable of driving one or more AN/SPA-25 indicators, of providing a blanking signal to the AN/SLA-10, and of accepting standard gyro inputs. The AN/SPS-64 is illustrated in Figure 4.2-12. The IP-1282B indicator contains a 12-inch CRT display which, due to a unique signal processor, provides a bright, daylight viewing display. Indicator control functions provide for selectable range scales out to 64 nautical miles, fixed range rings for target range estimation, digital LED readouts of exact target range and bearing, and the ability to offset ownship's position on the display. With a gyro input, the indicator can provide true or relative bearing displays. The RT-1246A receiver/transmitter operates on a fixed frequency of 9375 MHz and has a peak power output of 20 KW. The linear array antenna can rotate at speeds up to 33 RPM, which provides a high degree of target resolution and allows target information to be updated every two seconds.


Miscellaneous Surface Navigation Radars

There are a variety of small radar sets that are used for relatively short range surface search and navigation purposes. The maximum range of these sets is generally 36 miles or less. The low power consumption and small size make them ideal for small craft where space and generator capacity are limited; however, they may also be found installed aboard large ships such as carriers. The indicating units of these radars are normally located on the bridge or in the pilot house, depending upon the vessel in which they are installed. Two of these radars are the CRP-3100 and the LN-66. The type commander must approve the installation of any type of commercial radar equipment, as many of these radars will actually degrade the performance of other installed electronics and weapons systems.



The primary functions of air search radars are to detect aircraft targets at long ranges and to determine their range and bearing. The most widely used 2D radars in the fleet are the AN/SPS-40 and the AN/SPS-49. These radar sets use PPIs for determining range and bearing. The main design features of 2D air search radars are basically the same. They may, however,

vary in frequency, range, type of antenna, and in design detail. All of these radar sets use a moving target indicator (MTI) to discriminate between stationary objects and moving targets. All 2D air search radars transmit long pulses from a generated narrow pulse and then receive and compress the long pulse back into a narrow pulse. This minimizes the peak power requirements of the radar set without impairing range resolution. These modified shaped pulses also reduce interference with other shipboard electronic equipment.


































Figure 4.2-12 AN/SPS-64(V)9

Radar Set AN/SPS-40B, C, D

The AN/SPS 40 B, C, D is a high power, long-range, 2D early warning air search radar designed for use aboard destroyer escort size or larger navy ships. It operates in the 422.4 - 447.5 MHZ frequency range, has a power output of 200 - 300 KW, and is capable of ranges up to 250 nautical miles. Target range and bearing video signals are displayed on PPIs. Normal radar operation is performed from the radar set control located in CIC. In addition, remote-local switching permits operation of the radar set from the equipment room. An integral IFF and radar feed antenna is used, eliminating the requirement for having a separate antenna for each function. A typical AN/SPS-40 is depicted in Figure 4.2-13.

The AN/SPS-40 B, C, D provides two modes of operation; the long-range mode and lowflier detection mode. In the long-range mode, the transmitted signal is expanded to 60 msec and the received signal is compressed to 1.0 msec. In the low filter detection mode, the transmitted signal is a 3 msec pulse, the antenna speed is increased from 7.5 to 15 RPM, and the PRF is changed on alternate antenna scans. The narrow pulse enables targets to be detected at short range. Pulse expansion techniques permit operation at a lower transmitted peak power with the same average power as conventional systems, without sacrifice of range detection performance and range resolution. These techniques reduce the susceptibility of the radar set to jamming. The set also contains MTI systems, providing target discrimination against clutter from sea or shore return. Moving target indicator systems distinguish between reflections (clutter) from stationary objects whose frequency spectrum duplicates that of the transmitter, and moving targets whose spectrum is doppler shifted.


Radar Set AN/SPS-40B, C, D with DMTI

In radar set AN/SPS-40 B, C, D, with field change 8 installed, a digital moving target indicator (DMTI) automatically eliminates unwanted clutter, selecting only objects moving with some minimal radial velocity as targets. Ship and antenna scanning motion, cause the radar beam to

shift to a slightly different range and azimuth on each pulse, changing the relative phase relationship on a pulse-to-pulse basis, and causing the echoes from stationary objects to appear to the receiver as minor moving targets. The DMTI processor compensates for motion by examining the video returns of clutter for pulse-to-pulse phase shift, signal periodicity, and by establishing amplitude thresholds. The DMTI provides this radar system with a substantial improvement in the ability to detect targets flying over land, and small targets in a strong clutter environment. The heart of the AN/SPS-40 B,C,D with DMTI is the receiver. The receiver provides the radar with three major functions: signal generation, signal processing, and timing synchronization. As a signal generator, the receiver provides the transmitter with the low power radar pulse to be amplified by the transmitter and radiated into space by the antenna. As a signal

generator, the receiver performs MTI processing of the received radar returns. As a timing synchronizer, the receiver provides triggers to switch the radar units between transmit and receive operations.


Radar Set AN/SPS-40E

The AN/SPS-40E uses the same DMTI processing and pulse compression/expansion techniques described above. The main difference between this model and the others is that it is equipped












































Figure 4.2-13 Radar Set AN/SPS-40 B, C, D


with a solid state transmitter and an improved cooling system. It also incorporates BITE features that aid in troubleshooting and fault isolation.

Radar Set AS/SPS-49(V)

The AN/SPS-49(V) radar (Figure 4.2-14) is a narrow beam, very long range, 2D air search radar that primarily supports the AAW mission in surface ships. The radar is used to provide long range air surveillance regardless of severe clutter and jamming environments. Collateral functions include air traffic control, air intercept control, and antisubmarine aircraft control. It also provides a reliable backup to the three-dimensional (3D) weapon system designation radar. The AN/SPS-49(V) radar is, or will be, installed in most medium to large naval ships. The AN/SPS-49(V) radar operates in the frequency range of 850 - 942 MHZ. In the long range mode, the AN/SPS-49 can detect small fighter aircraft at ranges in excess of 225 nautical miles. Its narrow beamwidth substantially improves resistance to jamming. The addition of coherent side lobe canceller (CSLC) capability in some AN/SPS-49(V) radars also provides additional resistance to jamming/interference by cancelling the jamming/interference signals. The moving target indicator (MTI) capability incorporated in the AN/SPS-49(V) radar enhances target detection of low-flying high speed targets through the cancellation of ground/sea return (clutter), weather and similar stationary targets. In 12 RPM mode operation, this radar is effective for the detection of hostile low flying and "pop-up" targets. Features of this set include:

l Solid state technology with modular construction used throughout the radar, with the exception of the klystron power amplifier and high power modulator tubes

l Digital processing techniques used extensively in the automatic target detection modification

l Performance monitors, automatic fault detectors, and built-in-test equipment, and automatic on line self test features


There are currently eight configurations of the AN/SPS-49(V):




Ship Class


With Coherent Sidelobe Canceller (CSLC) that electronically cancels jamming

CV, CVN, CG, DDG 993, LHD 1, DD 997, LSD 41


Without CSLC



(V)1 system modified to interface with a Radar Video Processor (RVP)



(V)2 system modified to interface with RVP




































Figure 4.2-14 AN/SPS-49(V)



Ship Class


(V)1 system modified to provide an automatic target detection (ATD) capability and improved ECCM features

New Threat Upgrade (NTU)



(V)3 system with double shielded cables and a modified cooling system

CG 47


(V)5 system with a (V)6 cooling system

AEGIS Platforms


(V)5 system enhanced to include the AEGIS

Tracker modification kit

AEGIS Platforms



Among the height-finding radars currently installed aboard navy ships, the most common are the AN/SPS-52, AN/SPS-48, and the AN/SPY-1. These radars are normally the primary source of target information for weapons systems. The 3D radar functions much like a 2D system, but provides an elevation search pattern in addition to horizontal and vertical search patterns. Most radars present only range and bearing, so their beams are narrow in azimuth and broad in the vertical plane. The beams of height-finding radars are quite narrow vertically, as well as horizontally. Azimuth is provided as the antenna rotates continuously at speeds varying up to 15 RPM. The antenna may be controlled by the operator for searching in a target sector. Air search, 3D radars determine altitude by scanning the vertical plane in discrete increments (steps). This may be done mechanically or electronically (electronic is the most frequently used). In electronic scanning, the radiated frequency is changed in discrete increments, causing the radar beam to be radiated at different elevation angles. Each elevation angle or step has its own particular scan frequency. A computer can then electronically synchronize the radiated frequency with the associated scan angle to produce the vertical height of a given target. The 3D radars also use a range height indicator (RHI), in addition to the PPI used with the 2D radars.



CCA and GCA radar systems are essentially shipboard and land-based versions of the same radar. Shipboard CCA radar systems are usually much more sophisticated than GCA systems due to the movements of the ship and more complex landings. Both systems, however, guide

aircraft to a safe landing at zero visibility. Aircraft are detected and observed during the final approach and landing sequence. Guidance information can be supplied to the pilot as verbal radio instructions, or to the automatic pilot. Three CCA systems currently are installed aboard carriers: the AN/SPN-42, AN/SPN-43, and AN/SPN-44.

Radar Set AN/SPN-42

The AN/SPN-42 is a computerized automatic carrier landing system (ACLS) radar that provides precise control of aircraft during their final approach and landing. The equipment can automatically acquire, control, and land a suitably equipped aircraft on aircraft carriers under severe ship motion or weather conditions. A new ACLS system coming to the fleet is the AN/SPN-46, which incorporates the latest technology, improving reliability and operability.


Radar Sets AN/SPN-43, 43A

The AN/SPN-43 provides azimuth and range information from 50 miles to a minimum range of 250 yards at altitudes from radar horizon to 30,000 feet. Special indicators in the Carrier Air Traffic Control Center enable operators to direct aircraft along a predetermined azimuth to a point approximately one-quarter mile from touchdown. At this point the aircraft is "handed-off" to the final approach controller who uses the AN/SPN-42.


Radar Set AN/SPN-44

The AN/SPN-44 is a range-rate radar set that computes, indicates, and records the speed of aircraft making a landing approach to a carrier. Both true and relative speed are indicated.



The purpose of a radar indicator or repeater is to act as the master-timing device in analyzing the return radar system video, and provide that information to various locations physically remote from the radar set. Each indicator has the ability to select the outputs from any desired radar aboard the ship. This is done by the use of a radar distribution switchboard. The switchboard contains a switching arrangement which has inputs from each radar (and associated IFF system) aboard ship and provides outputs to each repeater. The radar desired is selected by turning a switch on the repeater. For the repeater to present correct target position data, it must have the following inputs from the radar selected: trigger pulses, video, and antenna information. The trigger (timing) pulses from the radar ensure that the sweep on the repeater starts from its point of origin each time the radar transmits. As discussed earlier, the repeater displays all targets at their actual range from the ship based on the time lapse between the instant a pulse is transmitted

and the instant a target echo is received. The returning echo is applied to the repeaters from the radar receiver. Antenna information is provided to ensure that the angular sweep position of a PPI is synchronized to the angular position of the radar antenna, in order to display contact bearing/azimuth information. The most common displays are the A scope (range-only indicator), PPI scope (range-azimuth indicator), and RHI scope (range-height indicator). The "A" scope (Figure 4.2-15) is not normally considered a radar repeater, but rather an auxiliary display. Its use is limited because of a range-only capability.

























Figure 4.2-15 "A" Scope Presentation


The PPI scope (Figure 4.2-16) is the most common radar repeater. It is a polar coordinate display of the surrounding area, with ownship represented by the origin of the sweep, which is normally located in the center of the scope, but may be offset from center on some sets. The PPI uses a radial sweep pivoting about the center of the presentation, resulting in a map-like picture of the area covered by the radar beam. A relatively long-persistence screen is used so that targets remain visible until the sweep passes again. Bearing is indicated by the target's angular position in relation to an imaginary line extending vertically from the sweep origin to the top of the scope. The top of the scope is either true north (when the radar is operating in true bearing), or ship's heading (when the radar is operating in relative bearing).

The RHI scope (Figure 4.2-17) is used with height-finding radars to obtain altitude information. The RHI is a two-dimension presentation indicating target range and altitude. The sweep of an RHI originates in the lower left side of the scope and moves across the scope, to the right, at an angle the same as the angle of transmission of the height-finding radar. Targets are displayed as vertical blips and the operator determines altitude by adjusting the movable height line to the point where it bisects the center of the target blip. Target height is then read directly from the altitude dials (counters). Vertical range markers are provided to estimate target range.

Many repeaters on Navy Tactical Data System (NTDS) equipped ships are being replaced with multipurpose consoles; however repeaters are still irreplaceable on ships not equipped with NTDS and as a backup to NTDS consoles. NTDS Consoles are addressed in Lesson Topic 4.3.


























Figure 4.2-16 PPI Scope Presentation






















Figure 4.2-17 RHI Presentation


Remote Indicators AN/SPA-25 A, B, C, D, E, F

Although there are several models of the AN/SPS-25 and, with the exception of the AN/SPA-25G, they all perform in the same manner. The only difference is the technology of their circuit components. The earlier models use large components and electromechanical devices; whereas the newer models rely more on solid state electronic technology.

The remote indicator AN/SPA-25 (Figure 4.2-18) is a transistorized general purpose PPI designed for use with any standard navy search radar system having a PRF of 10 to 5000 PPS. The indicator can display radar information from any one of several radar systems on a 10-inch CRT. The indicator group incorporates continuous range variation from .5 to 300 miles.



























Figure 4.2-18 AN/SPA-25 with Dead Reckoning Auxiliary Unit


Bearing can be determined using (1) the electronic cursor and azimuth scale or (2) the electronic cursor and a direct-reading electronic readout. Range may be determined by using (1) the range rings or (2) the electronic range strobe and a direct-reading electronic readout. The range strobe can be used on either the electronic cursor or the video sweep. When used on the video sweep, the strobe appears as a movable range strobe.

Remote Indicator AN/SPA-25G

The AN/SPA-25G is an advanced navigation, air search, and tactical situation radar indicator for both CIC and bridge environments. The unit is entirely solid state with the exception of the CRT. It increases the operator's capabilities while decreasing the work load through a unique information display and efficient human-machine interface. The indicator solves all range, bearing, and plotting tasks associated with target tracking, navigation, estimated point of arrival (EPA), and air traffic control. Formerly manual plotting and range and bearing calculating tasks were done on the AN/SPA-25G by pushing buttons, moving its shiftstick control and reading and viewing the solution(s) on the screen. This indicator may be a sit-down version resembling Figure 4.1-18, or it may be a stand-up version as shown in Figure 4.2-19.






























Figure 4.2-19 AN/SPA-25G




Range-Azimuth Indicator AN/SPA-66

The AN/SPA-66 is a general purpose PPI (Figure 4.2-21). It is normally used to support AAW operations, but may be used as an ordinary PPI if necessary. This indicator can be connected to other AN/SPA-66s, with one acting as the master station and the others linked to it.



















Figure 4.2-20 AN/SPA-50A





















Figure 4.2-21 AN/SPA-66



IFF equipment is used to permit a friendly craft to identify itself automatically before approaching closely enough to threaten the security of other friendly craft. The basic steps of this identification are challenge, reply, and recognition. Two sets of IFF equipment are used to perform the identification process. These are the interrogator (recognition) and transponder (identification) sets. AIMS is an acronym for Air Traffic Control Radar Beacon; Identification Friend or Foe, Mark XII System. The Mark XII system is capable of challenging in five different modes (1, 2, 3/A, 4, and C). Each mode is assigned a specific function. Modes 1, 2, and 4 are assigned for military use only. Modes 3/A and C are assigned for civilian and military use. The various modes have the following uses:

l Mode 1 - Used as directed by field commands. Thirty-two response codes are available.

l Mode 2 - Used to identify a specific airframe or ship. Four thousand ninety-six response codes are available.

l Mode 3/A - Within CONUS, used as identity codes for air traffic control. Outside CONUS, used as identity codes for purposes assigned by operational commanders. Four thousand ninety-six response codes are available.

l Mode 4 - Used for secure identification of friendly platforms. The reply for this mode is generated automatically according to a preset cryptographic key list.

l Mode C - Used to determine the altitude of aircraft. This is automatically derived from the aircraft's altimeter.


The four operational uses for IFF equipment are:

l Anti-Air Warfare (AAW) uses Modes 1, 2, 3/A, & 4 to provide complete identification of airborne platforms.

l Air Control uses Modes 2, 3/A, & C to provide necessary data for control of friendly aircraft.

l Air Traffic Control (ATC) uses Modes 2 and 3/A for departure and approach of carrier aircraft.

l Surface Identification uses Modes 1, 2, 3/A, & 4 for complete identification of friendly surface platforms.


Additionally, the transponder provides the shipboard interrogator operator with special warnings, both audible and visual, upon receipt of any of the following special purpose replies: Emergency (indicates aircraft in trouble), Communication Failure (indicates aircraft with inoperative communications equipment), and Special Purpose Identification/Position Identification (manually

activated special response by aircraft upon verbal request by ground/ship air control operator).



The Mark XII IFF system includes all Mark X equipment, such as interrogators, transponders, and decoders, plus additional equipment such as interrogator side lobe suppression switches and drivers, defruiters, and crypto computers. The interrogator transmits a coded challenge in the form of a pulse pair on the frequency of 1,030 MHz. The spacing between the pulses is determined by the mode of operation. The transponder is a receiver-transmitter combination that automatically replies to a coded challenge. The reply is a series of coded pulses, which are transmitted omnidirectionally at a slightly different frequency than the interrogator frequency (1,090 MHz). The receiver section of the transponder receives and amplifies signals within its bandpass, decodes correctly coded signals, and automatically keys the transmitter to send certain prearranged reply signals on its designated transmit frequency. The receiver section of the interrogator receives the coded reply signals from the transponder of the target craft and processes the reply for display on an indicator. The coded reply from a friendly craft is normally displayed on the PPI just beyond the radar blip as a dashed line, as shown in Figure 4.2-22. Naval Tactical Data System (NTDS) display consoles use symbology and numerics to indicate the transponder responses.

The interrogator operates in a manner similar to a radar transmitter and receiver. Bearing information is obtained by using a small directional antenna attached to or rotated in synchronization with the air search radar antenna (Some radars have IFF antennas integrated into their antennas). Range information is obtained by determining the time lapse between the transmission and the reception of a reply. IFF synchronization triggers are normally received from the modulator of the radar set with which the IFF equipment is being used. The IFF interrogator operates at fairly low peak power (1 to 2 kW). High output power is not required, as the pulses transmitted by the interrogator do not have to return to the transmitting unit. Instead, they are transmitted on a one-way trip to the target. After the transmitted pulses are detected by the friendly target's transponder, a different set of pulses is transmitted by the target's transponder for the return trip. A ship may have one or more interrogator sets, but will have only one transponder. Normally, interrogators and transponders aboard ship function independently, with the only interconnection between the two being a suppression (blanking) signal to inhibit the transponder from replying to the ship's own interrogators.


Interrogations and Replies

Air traffic control and code monitoring for friendly aircraft and surface craft are done by the use of SIF modes (Modes 1, 2, and 3/A). These modes (and Mode C) interrogations consist of two pulses spaced at a characteristic interval for each pulse, with a third pulse added for ISLS operation. Each Mode 1, 2, or 3/A transponder reply is a binary code contained between two bracket (framing) pulses, which are present in every reply, regardless of code content. Each reply code corresponds to a unique four-digit decimal code. For Mode 1 replies, the first digit may be any number from 0 to 7, inclusive, and the second digit any number from 0 to 3 inclusive, with the remaining two digits normally 0. For Mode 2 and 3/A reply codes, each of the four reply digits may assume any value from 0 to 7, inclusive.
























Figure 4.2-22 Fundamentals of IFF operation


Mode C replies are also binary codes contained between bracket pulses similar to those for the SIF modes. Mode C replies may represent any altitude from 1000 feet to 126,700 feet in 100 foot increments, the Mode C reply being derived from an encoder linked to the aircraft altimeter. Shipboard transponders are wired to reply to Mode C interrogations with back pulses only (Code 0000). Modes 1, 2, 3/A and C replies cannot in themselves be separated according to mode. The fact that the interrogator "knows" which mode it has interrogated allows replies to be separated and identified with the proper mode. Secure identification of friendly aircraft and surface vessels by the Mark XII system is provided through the use of Mode 4. Mode 4 interrogations are encoded multipulse trains, which consist of four (sync) pulses and an ISLS pulse, followed by up to 32 information pulses. Upon receipt of a valid Mode 4 interrogation, the transponder section processes the interrogation and sends out a time-coded three pulse reply. The interrogator section also converts the reply to one pulse, and time decodes it for presentation on the indicators if it is a valid reply.

Military emergencies for Modes 1 and 2 are called 4X (four train) emergencies. Mode 3/A military emergency replies consist of a combined 4X and 7700 code. A Mode 3/A civilian emergency reply is simply a 7700 code, without the 4X code. In addition, a Mode 3/A, 7600 reply code designates a radio communications failure for both civilian and military replies. There are no emergency replies for Mode C or Mode 4. When desired, a transponder can be made to transmit an identification of position (I/P) reply for Modes 1, 2, or 3/A interrogations. This reply is decoded to mark on an indicator a particular aircraft with which the interrogator system operator has voice communication. A pilotless aircraft containing a transponder will transmit an X-pulse reply to Modes 1, 2, or 3/A interrogations. The X-pulse reply consists of a normal mode

reply code plus an additional pulse occupying the center position of a reply train. X-pulse replies are unique to pilotless aircraft. (Mode C replies will not contain an X-pulse.)


Interrogator Section

A simplified block diagram of the interrogator section of a representative Mark XII IFF system is shown in Figure 4.2-23. The major units of the interrogator section (with the exception of the video decoder group) are usually mounted in a rack, as shown in Figure 4.2-24, and located in the radar equipment room.






















Figure 4.2-23 Mark XII IFF System Interrogator Section


Interrogator Set AN/UPX-23

The AN/UPX-23 interrogator set provides RF interrogations for the various modes, receives the transponder replies to these interrogations, and processes them into proper video signals for use by the decoders and indicators. This equipment is normally used with a radar set, to which its operation is synchronized.


Cryptographic Computer KIT-1A/TSEC

The computer encodes the Mode 4 challenges for transmission by the interrogator, and decodes the Mode 4 transponder replies received by the interrogator. The code changer key, KIK-18, is used to insert the Mode 4 code into the computer.





























Figure 4.2-24 Mark XII IFF Interrogator Equipment



The defruiter (Interference Blanker MX-8757/UPX or MX-8758/UPX) removes nonsynchronous transponder replies and receiver noise from the IFF video. The nonsynchronous replies (termed "fruit") are generated by omnidirectional transmissions from transponders answering to interrogators other than the one receiving the reply, and are not legitimate replies.


Pulse Generator

The pulse generator provides the IFF system pretriggers which initiate IFF challenges for the enabled modes. For "slaved IFF" systems, the pretrigger generator synchronizes IFF interrogations with the associated radar, and for "black IFF" systems, the pretrigger generator produces triggers internally. (Slaved IFF systems are IFF systems that are associated with radar systems, and black IFF systems are those systems not associated with radar systems.)

RF Switching Group AN/UPA-61

The AN/UPA-61 provides Interrogator Side Lobe Suppression (ISLS) and RF switching operation

for the Mark XII system. Targets at close ranges to an interrogator set will reply to side and back lobes of the antenna as well as the main antenna beam. This causes the target to appear for

nearly 360 close to the origin of display (ring around). The function of ISLS is to prevent ring-around by inhibiting transponder replies to interrogations from the side lobes of the IFF antenna.


Control Monitor

The control monitor serves as a remote control and a remote monitor for the interrogator section.

Video Decoder AN/UPA-59

The video decoder, AN/UPA-59, processes the pulse coded replies received by the interrogator set and provides video output to an indicator. The AN/UPA-59 has a variable configuration. The most common configuration uses a video decoder, an intratarget data indicator, and an alarm monitor. The video decoder provides control signals for the interrogator to indicate challenges in the various modes, and accepts reply video for decoding and processing. (Mode 4 reply video is fed directly through the decoder with no processing). The decoder also accepts radar video from an associated radar and routes this video directly to the indicator, or mixes it with IFF video for display. In addition, the decoder contains active decoding circuitry to display information for the intratarget data indicator. The intratarget data indicator provides readouts of codes for Modes 1, 2, and 3/A replies, plus direct altitude readouts for Mode C replies. The alarm monitor, BZ-173/UPA-59(V), contains a loudspeaker and indicator lights for producing audible and visual alarms when IFF emergency signals are decoded.



The standard Mark XII IFF system antenna is the AS-2188/UPX. This antenna has both sum and difference input jacks for radiating RF (from the AN/UPA-61 switch and driver) into the proper patterns for ISLS operation. In installations where the antenna rotary joint will not pass the switching bias, the AS-2188/UPX will transmit a sum pattern only, with a separate AS-177/UPX omnidirectional antenna transmitting the difference RF. Some installations use an integral antenna to transmit and receive both radar and IFF, with difference IFF being transmitted on a separate AS-177/UPX.


Antenna Pedestal Group AN/UPA-57

The antenna pedestal group, AN/UPA-57, is capable of self-synchronous operation at variable rotation rates up to 15 RPM. It is also capable of manual operation whereby an operator (at a PPI) can rotate the antenna to any desired position. In addition, the pedestal group may be slaved to a radar system antenna. This application is used for radars that cannot have the IFF antenna mounted on the radar antenna, but are otherwise compatible with the IFF system. The

pedestal group consists of a control power supply unit, a manual pedestal control unit, an antenna pedestal assembly, and a pedestal disconnect mast switch.

The control supply unit is located below deck and develops all of the power required for the pedestal group. When slaved to a primary radar, the control power supply unit accepts the radar synchro information (via the radar switchboard) and is capable of being slaved to rotation rates of 2 to 30 RPM. When free run operation is selected (on the front panel), the unit drives the pedestal assemble to a variable rate of up to 15 RPM. In conjunction with the manual pedestal control, the unit is also capable of positioning the antenna to any azimuth from a remote position. The manual pedestal control is usually located at the PPI. Front panel controls provide for selection of slave, free run, and manual operation. The antenna pedestal assembly is capable of mounting the AS-2188/UPX or any other 10-foot antenna designed to mount on the same platform. The pedestal disconnect mast switch is located above deck and removes all power from the pedestal assembly when it is activated.


Transponder Subsystem AN/APX-72

The transponder section accepts challenges from other platforms and provides coded identification replies. A simplified block diagram of the most common AIMS shipboard transponder subsystem installation is illustrated in Figure 4.2-25. This configuration applies to all ships carrying an interrogator system. As previously mentioned, certain noncombatant vessels do not have interrogator systems installed and will only have the transponder subsystem. This system uses the KIT-1A cryptographic computer for secure identification.






















Figure 4.2-25 Typical Shipboard Mark XII Transponder Section

Auxiliary Equipment

The AN/UPM-137 or AN/UPM-137A radar test set is a universal IFF test set for calibration, adjustment, and maintenance of all IFF equipment. For transponder only installations, the AN/UPM-136 is used.



The radar distribution switchboard provides a method of selecting and connecting the radar and IFF data to the various indicators, expanding shipboard target video display capabilities. The switchboard inputs are connected to the remote indicators through rotary switch assemblies. It also contains amplifier assemblies that provide sufficient video gain to drive the indicators. There are two switchboards currently used, the SB-1505/SP and the SB-4229.


The SB-1505/SP (Figure 4.2-26) enables any one of ten remote radar indicators to receive radar data from any one of eleven radar receivers. The data from one radar may be fed simultaneously to any number of channels and all channels may be in operation at the same time.



























Figure 4.2-26 SB-1505/SP

All channels function identically; therefore, to avoid repetition, only one channel is described in this section. Each channel consists of one rotary selector switch; one video and four IFF amplifiers; three trigger regenerators; two dc amplifiers; one stabilizer subassembly; one amplifier power supply (which provides dc power for the video and IFF amplifiers, trigger regenerators and DC amplifiers); and a solenoid power supply. The input terminal boards (located on the inside) and coaxial connectors for eleven radar receivers are mounted on the right side (facing the equipment) of the switchboard. The output from each radar receiver is coupled to all rotary selector switches. The channel outputs are located on the left side of the switchboard enclosure. Each channel has two output terminal boards (located on the inside) and twelve output coaxial connectors.

Each radar indicator in the system is associated with a specific channel in the switchboard. Two selector switches for remote switchboard control are located at each radar PPI. One remote switch controls the signal input level for one switchboard video amplifier. The other remote switch controls the position of the rotary-selector switch in the switchboard channel through a 5-wire, 2-wafer, switch-position sensing system. This system orients the rotary selector switch to any one of twelve positions (there are eleven radar receiver input signal positions and an OFF position to patch radar signals to the remote indicator unit. The video, IFF, trigger, and scan signals are coupled through the rotary selector switch from the input coaxial connectors to their respective amplifier stages. The amplifier outputs are wired directly to the output coaxial connectors. The output terminal boards and connectors for each switchboard channel are identical. The switchboard input terminal boards are wired for specific radar receivers.



This radar distribution unit performs the same functions as the SB-1505/SP, with added capabilities. Additionally, this switchboard, uses electronic switching rather than electromechanical. Added capabilities include:

l The capability of selecting one of sixteen input packages, consisting of three radar videos, a data stream, and IFF control with its associated video. They can be selected either locally or remotely. Additionally, these packages can be distributed to any one of nine separate radar indicators.

l The capability of converting the data stream back to analog for older indicators. This switchboard was primarily designed to support the AN/SPA-25G.

l The capability to detect some types of improper operation by means of built-in testing.