IFF Mark X was the NATO standard military identification friend or foe transponder system from the early 1950s until it was slowly replaced by the IFF Mark XII in the 1970s. It was also adopted by ICAO, with some modifications, as the civilian air traffic control (ATC) secondary radar (SSR) transponder. The X in the name does not mean "tenth", but "eXperimental". Later IFF models acted as if it was the tenth in the series and used subsequent numbers.
For most of World War II the standard IFF system used by the allied air forces was the IFF Mark III. Mark III responded on the same frequency as the trigger signal, returning a selected pulse pattern. Originally, the Mark X was simply a version of Mark III operating at a higher frequency, which has several practical advantages. Three return patterns, or Modes, were available. As it was being introduced, the new Selective Identification Feature, or SIF, allowed the response signal to be modified with bit encoding, providing the ability for each aircraft to produce a unique response using octal digits. This was initially handled through a separate box that connected to the original Mark X. For a brief time in 1957 this was known as IFF Mark XI before becoming IFF Mark X (SIF).
As the civil aviation market grew in the 1950s, Mark X was selected as the standard transponder system as the Air Traffic Control Radar Beacon System, or ATCRBS. For this role, a new series of four Modes was introduced, A through D. A is essentially identical to Mode 3, and these are now referred to as Mode 3/A. Mode C responds with a four-digit code encoding the pressure altitude in increments. Combining information from a radar with Mode A and C responses, the ATC system can build a complete picture of the airspace without the need for height finders or 3D radars. Using Mark X for the civilian role also allowed existing military users to be routed within the civilian network, as well as allowing civilian aircraft to use an existing and well-tested transponder design.
Mark X retained a key problem that was present in all IFF systems to date; the aircraft transponder would respond to any interrogation signal on the proper frequency with no way to tell if it was a friendly transmitter. This allows an enemy force to query the transponders and use triangulation to determine their location, or simply count the responses to look for increased activity. Military users had long desired a system that encoded both the interrogation and response, allowing the transponders to ignore signals from interrogators that did not present the right code. This led to the development of IFF Mark XII and its associated Mode 4 which began to be deployed in 1970.
History Mark III
The first IFF system to see widespread multinational use was the British IFF Mark III, which appeared in early 1942 with the Royal Air Force and then used for the rest of the war by the US and Canada as well. This was a simple system that listened for broadcasts on a narrow band of frequencies, amplified the incoming signal using a regenerative receiver, and rebroadcast the result. The regenerative design was extremely simple, often consisting of a single vacuum tube. The ground station used an "interrogator" to send out pulses in synchronicity with a radar unit, and mixed the received signal into the one from the radar to produce a unified display. On most radar displays, the IFF signal would lengthen the "blip" or cause additional blips to appear.
Mark III had the serious limitation that it would respond to signals from any broadcast in the 176 MHz range. It was long feared that the Germans would send out their own interrogation pulses to trigger the IFF, and then use a radio direction finder to locate the aircraft. The British did this to German night fighters using a system known as Perfectos, forcing the Germans to turn off their IFFs and causing many friendly fire incidents. It seemed logical that the Germans would return the favour, but this was rarely the case; while ground-based radio reconnaissance units were known to track British aircraft by their IFF on occasion, their success was greatly mitigated by turning the IFF transponder off while over enemy airspace. The Germans could not do the same, as they were almost always flying over their own airspace.
A more practical concern was that the IFF signals were in the middle of the widely used existing VHF radar bands; moving to a new frequency would help reduce potential interference. Moving to a higher frequency would have the added advantage of allowing the use of smaller antennas. Another issue with Mark III was that the transponder responded on the same frequency as the interrogation pulse, so other IFFs might hear the response signal and trigger their own, resulting in cascade of replies. This was particularly problematic near airports, where aircraft clustered and could hear each other's signals. Using separate send and receive frequencies would solve this, but the regenerative design worked by feeding back a received signal, so it could not be easily adapted to respond on a different frequency.
US efforts, Mark IV The US Naval Research Laboratory (NRL) had also been considering the IFF concept, and had come up with a concept similar to Mark III in that it used its own private frequency for interrogations, 470 MHz. Unlike the British designs, the response signal was on a separate frequency, 493.5 MHz. This avoided one IFF triggering another, but at the cost of requiring a complete separate transmitter system. When the US and British formed the joint Combined Research Group at the NRL, this system was given the name Mark IV.
A complication was that the response frequency was close enough to the 600 MHz frequency of the German Würzburg radars that there was concern that those radar's pulses might trigger the transponder, causing a new blip to appear on the Würzburg's radar display and thus immediately revealing their operational frequency. With British systems already in widespread use, the decision was made to adopt Mark II and Mark III for US aircraft.
In 1942, the Combined Research Group began the development of a new system based on the basic Mark IV pattern but with a further increase in operational frequency to 1.03 GHz for the interrogation and 1.09 GHz for the responses. This Mark V was to be the basis for IFF worldwide in the post-war era, and so was also known as the United Nations Beacon, or UNB. UNB adaptors for existing Mark III sets were produced and saw service with the US forces, but were not adopted elsewhere. The UK withdrew from the program in October 1945, believing another war was at least ten years off. The US only used UNB for a brief time because a new experimental concept, known as Mark X, was rapidly maturing.
Mark X The main difference between Mark X and earlier IFF systems was the use of two pulses in the interrogation rather than one. A single pulse had been used in the past because the original interrogation signal was the radar beam sweeping past the aircraft, and these were single pulses of energy. With the IFF systems now working on completely different frequencies, compatibility with existing radars was no longer required, so for the Mark X a system using two pulses was adopted.
There were two advantages to this system. The first was that an enemy interrogator would have to match both the frequency and timing of the pulses to trigger a response. This offered a small amount of additional security. Much more important, however, was that by varying the timing of the pulses, different replies could be triggered in the airborne transponder. The original design had three such "modes", Mode 1 was triggered by the interrogator by sending the two pulses 3 μs apart, Mode 2 was 5 μs and Mode 3 was 8 μs (all ±0.2 μs).
The response to these interrogations remained simple; a successful interrogation on Mode 1 or 3 caused a single pulse to be sent in reply, very shortly after the interrogation was received. Since this arrived back at the radar station after the return of the original radar pulse, this signal caused a second blip to appear on the radar screen at a slightly further distance. Mode 2 was similar, but returned two pulses from aircraft and a single delayed pulse from ships.
The aircraft transponder had a switch that set which Mode it was listening for, and would only respond to enquiries for that mode. In practice, the modes were used to identify individual aircraft. In UK use, for instance, most aircraft would set their transponder to Mode 1, which would provide basic IFF indication on their "blip" on the radar display. The flight leader would instead select Mode 3, thereby allowing the ground operator to sort out the entire formation from the individual aircraft within. Finally, Mode 2 was selected on demand to allow the operator to identify a specific aircraft.
In addition to the basic Modes, the system also included an emergency response feature which was selected by the aircraft. When turned on, the aircraft always returned four pulses in reply to an interrogation, no matter what mode the ground station selected.
Although they had withdrawn from the UNB, the UK kept in touch with their US counterparts, and officially adopted Mark X in October 1949, followed by the Canadians the next year. RAF representatives were invited to attend the testing in late 1951, by which point they had already contracted Ferranti to develop equipment that worked on both Mark III and Mark X. Due to delays on both sides of the Atlantic, it was not until the early 1960s that Mark X support was truly universal.
SIF Allied systems had started as transponders which simply reflected the original radar pulse and were unable to encode any sort of customized response message. The US had been studying encoding more data into the return signal during the war, but ultimately felt getting a new system into service as quickly as possible was more important than improving its capabilities, so the original Mark X differed only in simple ways from Mark III.
While development of the basic Mark X was still ongoing, development also began on the new Selective Identification Feature, or SIF. This system was initially implemented as a separate box that plugged into the Mark X and modified its return signals. Instead of two pulses, the SIF unit returned a "pulse train" that contained multiple pulses between start and stop "framing" pulses. Each pulse was 0.45 μs in duration and 1.45 μs apart, and the train as a whole was 20.3 microseconds long. Each group of three pulses is used to encode an octal digit, 0 to 7. In Mode 1 and 3, two sets of three pulses are used, while Mode 2 used all four sets of pulses in the train.
To help identify individual aircraft, ground operators would tell the aircraft to set their IFF to a particular Mode, and then select a two-digit code on the SIF box. In Mode 3, all 64 responses (00 to 77) were possible, but in Mode 1, the second digit was only 0 to 3, for a total of 32 codes. Emergency mode remained, but only worked when interrogated by Mode 1 from the ground station. To indicate an emergency on Mode 3, the user instead dialled in code 77. To ensure the emergency code would be picked up by most interrogators, the operator would set the IFF to emergency and dial 77, thus responding with the same set of pulses on both Mode 1 and 3.
The longer response in Mode 2 was used solely by military users, allowing them to identify individual aircraft. The four digits provided a total of 4096 possible codes, although 7700 was used for emergency, producing the same emergency response as above.
ATCRBS