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 100 foot (30 m) 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.
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