|NAVAL ORDNANCE AND GUNNERY
VOLUME 2, FIRE CONTROL
RADAR AND OPTICS
| D. Special Purpose Equipments
16D1. Identification, friend or foe (IFF)
The age-old problem of distinguishing friend from enemy in warfare increased greatly when radar and aircraft came into combat use. Radar supplied target information at greater than visual ranges and it was therefore necessary to identify targets, both surface and air, as early as possible in order to take the initiative. As deadlier and faster aircraft have been developed, the identification problem has increased. Although a radar can detect aircraft at long ranges, a radar scope shows aircraft, whether friend or enemies, only as spots of light. With modern, high-speed aircraft the captain of a ship cannot afford to wait until the aircraft are identified by visual means before he prepares his ship for battle. He must identify the aircraft at a great distance from the ship; and if he does not identify the aircraft, he must assume them to be enemies and order his aircraft to intercept them as far away from the ship as possible.
The problem of identification led to the development of an electronic system that permits friendly forces to identify themselves automatically before approaching near enough to threaten the security of other naval units. The foregoing electronic system is called, appropriately enough, identification, friend or foe, or 1FF. In practice, the “foe” part of the name could be omitted because, as a foe does not carry identifying equipment, he is identified by his lack of identification rather than by his identification.
The IFF system consists basically of a pair of special transmitter-receiver units, one set aboard the friendly ship and the other aboard another friendly unit. Because space and weight aboard aircraft are limited, the airborne system is smaller and lighter and requires less power than the shipboard transmitter-receiver. The airborne equipments are automatic and do not operate until triggered by a signal from a shipboard unit.
IFF systems are designated by mark numbers. In order to avoid confusion between IFF systems and fire-control systems, the IFF mark number is a Roman numeral (Mark III) whereas the fire-control number is an Arabic numeral (Mark 29).
1. Theory of operation.
The IFF system operates as follows:
An air search radar operator sees an unidentified target on his radar scope. He turns on the IFF transmitter-receiver, which transmits an interrogating, or “asking” signal to the airborne transmitter-receiver. The interrogating signal is received by the airborne unit, which automatically transmits a characteristic signal called an identification signal. The shipboard system receives the signal, amplifies it, and displays it on the radar scope, or on a separate indicator scope.
When the radar operator sees the identifying signal and identifies it as the proper one, he knows that the aircraft is friendly.
However, if the aircraft does not reply when interrogated, or if it sends the wrong identifying signal, the ship must then assume the target is an enemy and defensive action must be taken.
2. Security aspects. You can easily see from the foregoing text that the enemy could do a tremendous amount of damage if he were able to imitate the characteristics of our IFF identification signals. This he might readily do if he could obtain a transmitter-receiver unit with codes that were not complicated, and not varied enough to be secure in themselves. For this reason, strict security measures are observed with regard both to identification signals and to IFF equipment. The identifying signal usually is (1) coded, (2) changed frequently, and (3) given a high security classification.
IFF equipment is: (1) specially designed, (2) wired to destroy itself on the impact of a crash or at the will of the control personnel, and (3) given a high security classification.
If these security measures are observed carefully, the enemy cannot: (1) discover what the coded signal is, (2) use the right signal at the right time, (3) obtain a piece of our equipment, or (4) design a similar piece of equipment.
3. Types of IFF equipment.
Early IFF equipments were of two types: (1) the Interrogator-Responsor, and (2) the Identification Set.
The Interrogator-Responsor performs two functions. (1) to transmit an interrogating signal, and (2) to receive the reply.
The Identification Set, known as a transponder, also performs two functions. (1) to receive the interrogating signal, and (2) to reply automatically to the interrogating signal by transmitting an identifying signal.
4. Types of interrogation. There are two types of interrogation-direct and indirect. The interrogation is direct when the interrogating signal that triggers the transponder is a pulse from radar equipment. The interrogation is indirect when the interrogating signal is a pulse from a separate recognition set operating at a different frequency from that of the master radar.
Early 1FF systems used direct interrogation. However, direct interrogation proved unsatisfactory because the transponder was required to respond to radars that differed widely in frequency and pulse-repetition frequencies. Therefore, the later IFF systems use indirect interrogation within a special frequency band reserved for IFF operation.
Due to the security classification of the current IFF systems, this text will not attempt to present any technical details of IFF operation. Although the World War II models are now obsolete and unclassified, it is felt that to discuss them here may cause confusion in an officer’s mind when and if he comes in contact with the current equipment.
Radio countermeasures (RCM) prevent the enemy from using his radar and communications equipment effectively, produce false signals on the enemy receivers, and prevent the enemy from using countermeasures on our own radar and communications equipment (counter-countermeasures). The foregoing shows that an enemy tries to reduce the effect of our countermeasures and also tries to develop countermeasures equipment of his own to deny us the use of our radar equipment. Therefore, the development of training in use of countermeasures equipment is a continuing process.
In order to use countermeasures most effectively against an enemy radar, you should know the following things about the enemy radar facility. (1) frequency, (2) pulse width, (3) pulse-repetition frequency, (4) peak power, (5) receiver bandwidth, (6) time constants of the receiver coupling circuits, (7) antijamming features, (8) amount of shielding, (9) type of indicator, (10) antenna beamwidth, (11) type of scan, and (12) use of the radar. The things you should know about an enemy communications system are. (1) frequency, (2) type of modulation, and (3) receiver bandwidth. Some of the foregoing information is obtained by analyzing the enemy transmission, and other information must be obtained by examining captured equipment.
Special equipment has been developed for use in analyzing r-f transmissions. This equipment includes search receivers, which search all the frequencies that can be used; panoramic adaptors, which measure the frequency, strength, and type of modulation of a transmission; and pulse analyzers, which measure the pulse rate and width. The pulse analyzer and the panoramic adaptor are used in conjunction with the search receiver.
The full story of countermeasures in World War II will probably never be told. After the advent of effective radar, both the Allied and Axis powers knew that some type of countermeasures must be developed. The Allied powers finally won because of an all-out continuous effort, and close cooperation among the scientists of the Allied nations. Time after time countless lives were saved by the use of countermeasures against enemy radars and communication networks. In the instance of the Normandy invasion, great masses of false targets were created in different locations to deceive the enemy; in the invasion area jamming equipment was used.
Radar countermeasures (RadCM) fall into two distinct types: nonelectronic, which consist of reflectors, strips of aluminum foil, decoys and prediction devices; and electronic which consist of search receivers, panoramic adaptors, pulse analyzers, jamming transmitters, signal generators, and noise modulators.
1. Nonelectronic countermeasures. Units called “corner reflectors” are used to present strong echoes to enemy radar signals. When, placed carefully in many locations, they return strong echoes that appear to the enemy operator to be a large naval force.
Rope” was the code name for long streamers of aluminum foil. This foil, cut in lengths of about 400 feet, is dropped by aircraft within range of an enemy radar. The foil twists and turns as it falls, thus presenting many different effective lengths to the enemy radar. Some of these lengths are highly resonant at the frequency of the radar and, therefore, appear as strong target signals.
“Decoys” consist of a wide variety of devices. Some of the most effective are balloons towing strips of aluminum foil. These strips, which vary in length, present strong reflections over a fairly wide band of frequencies. Other decoys are aircraft towing streamers of metal foil and floats carrying wire mesh or foil reflectors.
“Window” is the code name for short strips of tuned aluminum foil. The foil is cut to slightly different lengths so that it reflects at the frequency of enemy radar. The strips are packaged and dropped over enemy territory. While fluttering to the ground, they present a multitude of targets to the enemy radars. Thus, enemy searchlight and tracking radars follow the strong echoes presented by the window and cannot be made to track on the lesser echoes presented by the aircraft.
The Radar Prediction Device (RPD) also proved to be a very effective CM device during World War II. The RPD was simply a relief map of the enemy territory with a flashlight bulb at the location of the enemy radar set. The shadows cast by mountains or other features of the terrain indicated weak detection or blind spots in the enemy radar beam. When a blind spot was found, aircraft used it as an avenue of approach to avoid early detection.
2. Electronic countermeasures (ECM). Many electronic equipments have been developed for use in analyzing enemy transmissions and in jamming enemy receivers. Most of these equipments fall under the general headings of: (1) search receivers,
(2) panoramic adaptors, (3) pulse analyzers, (4) jamming transmitters, and (5) jamming modulators. In addition to these just mentioned, several decoy transponders were developed that operate like racons and return a strong signal when triggered directly by a radar pulse. Thus, the transponders will appear on the enemy radar screen as a large target signal.
ECM has two general classifications: passive and active. Passive ECM is merely using the receiving equipment to detect the presence of the enemy by the use of his radar or radio r-f transmissions. Active ECM is the addition of transmitting equipment to the receiving equipment to jam the enemy transmissions.
3. Anti jamming measures. Antijamming measures, or counter-countermeasures (CCM) are used to reduce the effect of enemy jamming on our own equipment. In receivers some of the most important CCM devices are special filters that pass only the important parts of echo signals, thus rejecting as much of a jamming signal as possible. In the transmitters a great many of the radar equipments have tunable magnetrons (oscillators) whose frequency may be varied at regular intervals to prevent an enemy jamming transmitter from locking on its frequency.
16D3. Radar beacon principles
RAdar beaCONS, called RACONS, are similar in operation to the transponder equipment. They are passive until triggered by a radar signal. When triggered by suitable radar pulses, the beacon emits a coded series of pulses which appear on an aircraft’s radar indicator and identify the beacon. A navigation fix can be obtained from one or more racons of known position.
Racons were used extensively in World War II on the ground, in ships, and in planes for air navigation, ship navigation, paratrooper rendezvous, aircraft rendezvous, bombing, shore bombardment, and amphibious operations. A modern use of racons is in pilotless aircraft, A beacon in a pilotless aircraft gives a means of following the trajectory of the missile.
The racon is a direct type of transponder and therefore is designed to cover a wide range of frequencies and to respond to many different pulse rates and durations. In order to operate with racon equipments, radars (particularly those employed in aircraft) must have a beacon-reception circuit to make possible the reception of the beacon response.
When a search radar aboard an aircraft interrogates a racon, the range and bearing of the racon reply are noted on the radar scope. As racons are placed in known geographical locations, the position of the ship or aircraft with relaxation to the known position of the racon establishes a fix.
16D4. Airborne early warning radar
The U. S. Navy now has in extensive use radar equipments called Airborne Early Warning (AEW) systems. These are special shipboard and aircraft radar systems that work together as a single unit.
In these systems the aircraft is able to re-transmit or relay to the shipboard unit the information from the plane’s search radars. The ship then has a PPI display from the aircraft’s search radar equipment. It can be easily understood how this will extend the range of radar for the ship by great distances. For example, a plane at a 1,000-foot altitude will have a minimum radar detection range, on a target 50 feet high, of 55 miles. If the plane is relaying radar information to a mother ship 50 miles away, then the ship has an effective search range of 105 miles in the plane’s direction. If a relay is directly over a mother ship at 5,000 feet, then the ship has an effective 360° search range, to radar horizon, of 100 miles.
Operators on the shipboard equipments have units similar to 1FF systems which identify his own ship in the plane’s pattern of presentation. AEW systems make use of off-center PPI units and operators are thus able to have either the plane, the ship or the target as the center unit on the screen.