IFF Questions and Answers

This material was originally published on a Litton site.  That page is no longer available.  I found the same material at http://www.dean-boys.com.  I decided to reproduce it here because of its value and because it is not readily available. - GC

Q. What is IFF and how does it work?

A. With supersonic aircraft and swift antiaircraft missiles, there is no time to identify friendly forces by visual means. IFF (Identification Friend or Foe) is an electronic system which can determine the intent of an aircraft with the speed of the fastest computer. A related system is used for air traffic control at civilian airports. Litton is a world leader in the development and manufacture of military IFF systems.

Q. How do present day IFF Systems Work?

A. Modern IFF is a two-channel system, with one frequency (1030 megahertz) used for the interrogating signals and another (1090 megahertz) for the reply. The system is further broken down into four modes of operation, two for both military and civilian aircraft and two strictly for military use.

FAA regulations require that all aircraft, military or civilian, flying at an altitude of 10,000 feet or higher in U.S. controlled airspace, must be equipped with an operating IFF transponder system capable of automatic altitude reporting (this is the reason that two of the modes are used by both military and civilian aircraft).

Each mode of operation elicits a specific type of information from the aircraft that is being challenged. Mode 1, which has 64 reply codes, is used in military air traffic control to determine what type of aircraft is answering or what type of mission it is on.

Mode 2, also only for military use, requests the "tail number" that identifies a particular aircraft. There are 4096 possible reply codes in this mode.

Mode 3/A is the standard air traffic control mode. It is used internationally, in conjunction with the automatic altitude reporting mode (Mode C), to provide positive control of all aircraft flying under instrument flight rules. Such aircraft are assigned unique mode3/A codes by the airport departure controller. General aviation aircraft flying under visual flight rules are not under constant positive control, and such aircraft use a common Mode 3/A code of 1200. In either case, the assigned code number is manually entered into the transponder control unit by the pilot or a crew member.

Altitude information is provided to the transponder by the aircraft's air data computer in increments of 100 feet. When interrogated in Mode C, the transponder automatically replies with the aircraft altitude. FAA ground interrogators normally interlace modes by alternately sending Mode 3/A and Mode C challenges thus receiving continuous identity and altitude data from the controlled aircraft.

After takeoff, the aircraft soon leaves the departure zone. At this time, the pilot is instructed via radio to contact a specific enroute controller on a specific radio frequency. The enroute controller provides additional flight instructions and may assign a new Mode 3/A code in the event of conflicts in his control zone. On a transcontinental flight, the aircraft passes through dozens of such zones until it is handed over to the approach controller at its destination.

In dense terminal areas, that is, where many aircraft are flying in a small area, the pilot may be asked to "Squawk I/P." The pilot then presses the I/P switch on the transponder which shows up as a unique display and helps pinpoint the aircraft's exact position. Specific Mode 3/A code are reserved to signify aircraft emergencies and radio failures.

It's all a matter of timing:

The code signal sent by the interrogator system consists of two pulses spaced at a precisely defined interval. (A third pulse that has nothing to do with the coding of the query is actually used for interference suppression reasons.) In Mode 1, the interval between the first and last pulse is 3 microseconds; in Mode 2, it is five microseconds; in Mode 3/A, it is eight microseconds; and, in Mode C, it is 21 microseconds. The airborne transponder contains circuitry that discriminates between these various timings and automatically sends back the desired reply. 

The transponder replies are also in the form of a pulse, though in this case, there are 12 information pulses that are digitally coded as "ones" and "zeros." The total number of reply code combinations therefore, is 4,096. The reply codes are entered by means of four code wheels on the transponder control unit. The reply pulses generated by the transponder are decoded by the interrogating system and are typically displayed as needed on the primary radar scope near the blip that represents the aircraft that has been challenged. Thus, the aircraft controller can monitor the track of each aircraft through his zone and know its identity, altitude and position at all times.

Q: What is the history and development of IFF? 

A: Throughout time, it has always been important to know who one's friends are. Nowhere has this been more obvious than in military conflicts, where for centuries, flags, banners, insignia and uniforms have allowed adversaries to distinguish their cohorts from others who might have less friendly intentions. And, in darkness, when those visual means of identification were impossible, the business of using passwords and countersigns achieved the same end.

That system worked for millennia as long as conflicts were more or less face to face and visual identification was possible. But about 50 years ago, just as World War II began, the widespread use of aircraft caused a dramatic and inexorable change. Now threats could approach with great swiftness. So by the time the visual identification was possible, it was often too late to prevent destruction. And while battle forces were once drawn up on opposing sides of some geographic line, the new battle zones quickly became chaotic mixtures of friendly and hostile forces with many isolated units operating autonomously.

Visual means were and still are an important method of discriminating between friends and enemies. Some children of the late thirties and early forties may well remember studying the aircraft identification handbooks and black silhouettes that enabled them to tell the difference between a Japanese Mitsubishi Zero from an American Lockheed P-38 Lightning or anyone of dozens of other aircraft. But airplanes could also fly at night, and their swiftness made other means of warning a vital necessity.

The earliest forms of radar were then emerging and, although it seemed to offer a solution to the problem, a major drawback soon became evident. The radar could detect incoming aircraft at considerable distance by sending out powerful pulses of radio energy and detect the echoes that were sent back, but it could not tell what kind of aircraft had been spotted or to whom they belonged. It is ironic that the tragic events that occurred at Pearl Harbor might have been changed had the radar been able to identify as well as detect. A U.S. radar station at Diamond Head saw the incoming armada, but it was dismissed as a flight of American aircraft coming in from the mainland.

The Beginning of Electronic Identification

Early in the European conflict of World War II, British airmen were puzzled by the strange behavior of German fighter aircraft. Occasionally and without apparent reason, the German planes would simultaneously roll over. The British eventually intercepted radio signals from the ground that always preceded this maneuver. It was then realized that by rolling over at a predetermined signal the Germans were changing the polarization of the radar reflections picked up by their own ground radars. They created a distinctive blip on the radars that differed from others so German radar operators could identify their friendly forces.

As crude and simple as it was, this constituted the first attempt at an electronic IFF system. It incorporated the basic structure of all cooperative IFF systems that followed: a challenge or question (the coded radio message) and a specific response (the roll over that caused a change in the reflected radar signal).

The First Active IFF Systems

Both British and American forces were working to develop a viable identification system as well. That first German maneuver, which was soon superseded by others, was a passive system in that the returned signal was still just a reflection of the radar energy sent from the ground.

The first active system used by the allies employed radio energy generated onto the target aircraft and then used for the return signal. This is the basic method now used in all modern cooperative IFF systems.

About 1940 an active system, designated the Mk I, was put into service. It used a receiver aboard each aircraft that broke into oscillation and acted as a transmitter when it received a radar signal. Because of the variety of radar frequencies used, it had to be mechanically tuned across the radar bands in order to be triggered by any radar that was illuminating it. This mechanical tuning requirement and other factors limited its performance.

The system was developed further by the addition of a separate transmitter that was tuned through the radar bands simultaneously with the receiver and was triggered by signals from the receiver. This greatly increased the strength of the return signal and the return range. Known as Mk III, it also could be programmed to respond in one of six different codes thus providing some further degree of identification.

Further Refinements

After the war, with rapid technical developments creating new high performance aircraft, the need for efficient and reliable IFF systems led to a long series of further refinements that eventually evolved into the modern IFF systems in use today. Litton's history started when the company began making IFF components in 1951. This activity grew into the development and manufacture of complete IFF systems and support equipment and led to the leadership position that the company now enjoys in this field.

Modern IFF systems are basically Question/Answer systems. An interrogator system sends out a coded radio signal that asks any number of queries, including: Who are you? The interrogator system is frequently associated with a primary radar installation, but it may also be installed aboard a ship or another airplane. The interrogation code or challenge, as it is called, is received by an electronic system known as a transponder that is aboard the target aircraft. If the transponder receives the proper electronic code from an interrogator, it automatically transmits the requested identification back to the interrogating radar. Because it was developed as an adjunct to the primary echo-type detection radar and is usually used in conjunction with a primary radar, the IFF system is also known as secondary radar.

Q. How is civilian air traffic control different than military identification?

A. By the 1960's civilian air traffic in the United States had increased so much that air traffic controllers began to have their own identity crisis. The radar screens in high traffic areas became so cluttered with primary returns that it was becoming difficult to know which blip represented which plane. Nor was the primary radar much use in determining the altitudes at which the various planes were flying. Consequently, a system similar to and compatible with, military IFF systems was authorized and introduced by the civilian air traffic control authorities. Since civilian air traffic control deals only (hopefully) with friendly aircraft, it is more properly called the Air Traffic Control Radar Beacon System. 

Q. What are the future uses and modes of operation for IFF and air traffic control? 

A. In the present IFF system, the beam of interrogation signals sweeps the horizon in synchronism with the primary radar beam and all aircraft transponders that are reached by that beam are triggered to respond whether they are of immediate interest to the air controllers or not. This leads to a certain amount of undesirable self-interference within the system.

The current air traffic control system is also quite labor intensive for both ground controllers and flight crews and relies heavily on two-way voice communications for the transfer of routine data. As air traffic densities increase, the situation becomes even more severe.

To reverse this trend, the FAA has authorized the development of a new system designated Mode S to be implemented for commercial carriers. The system is also being deployed in regions of Europe. The system uses the standard IFF frequencies of 1030 and 1090 megahertz, but both the challenge and reply formats are more complexly coded than in the current beacon system. In particular, each user aircraft will be assigned a permanent Mode S address which will share with no other (more than 16 million addresses will be available). Upon the aircraft's entrance into a Mode S control zone, the address will be automatically elicited by the ground control station and entered into a central computer. Thereafter, the aircraft can be uniquely addressed, thus greatly reducing system self-interference. The reply message will also contain the aircraft address, altitude and other selected data. 

The Mode S system is designed to be compatible with the current air traffic control beacon system, so that the Mode S equipped aircraft can continue to operate in non-Mode S controlled airspace. This will allow the system to be installed in an evolutionary manner. The system also incorporates a number of preplanned growth features that will lead to a highly automated air traffic control system including onboard collision avoidance equipment. Thus the new system will increase flight efficiency and safety.

Military identification is also being investigated for change. During the mid-1980's, a spread spectrum waveform was studied, and a full scale development was begun in the United States. The system was designated Mk XV. In Europe, the NATO countries also participated in a parallel program dubbed NIS (NATO Identification System). The first priority of the new waveform was to provide NATO inter-operability among the NATO countries in identifying aircraft. The cost for full implementation of these systems was prohibitive, and developments were abandoned.

Interest in a new waveform which can also provide data transfer similar to Mode S is still a priority. The NATO nations are continuing in defining a new waveform dubbed Mk XIIA which will use spread spectrum techniques to provide improvements in jamming and garble performance. A strict emphasis is on implementation cost and inter-operability with present Mk X/XII.

Q. What are the secured or cryptographic modes of operation for IFF?

A. The original reason for IFF systems came about was to identify friendly forces in a battlefield environment. For that reason, it is essential that hostile forces not be able to use the system to identify themselves as friendly even if the physical IFF equipment should fall into their hands. Litton supplies military IFF equipment, including the most advanced encryption systems, that prevent unauthorized use. 

The secure mode is used exclusively for military purposes. This mode uses a very long challenge word which contains a preamble that tells the transponder it is about to receive a secure message. The challenge itself is encrypted at the interrogator by a separate device that uses various mathematical algorithms to put it in a secure form. The transponder routes the ensuing challenge to a separate device that uses the inverse algorithms to decode the challenge. In effect, each challenge is telling the transponder to respond in a certain way. If the transponder cannot decipher the challenge, it will not be able to respond in the proper way and thus will not be identified as a friend.

To prevent unauthorized use of either the interrogation equipment or the transponders if they should fall into hostile hands, a key code must be periodically entered into each device. To eliminate the chance of a random guess by a hostile target corresponding with the proper response, each identification consists of a rapid series of challenges each requiring a different response that must be correct before the target is confirmed as a friend. A very high degree of security to the identification system is ensured through the use of key codes and powerful cryptographic techniques.