IFF Mark III, also known as ARI.5025 in the UK or SCR.595 in the US, was the Allied Forces standard identification friend or foe (IFF) system from 1943 until well after the end of World War II. It was widely used by aircraft, ships, and submarines, as well as in various adaptations for secondary purposes like search and rescue. 500 units were also supplied to the Soviet Union during the war.
Mark III replaced the earlier Mark II which had been in service since 1940. Mark II had an antenna that received signals from radar systems, amplified them, and returned them. This caused the blip on the radar display to become larger, indicating a friendly aircraft. As the number of radar systems on different frequencies proliferated through the mid-war period, the number of models of Mark II had to do the same. Aircraft could never be sure their IFF would respond to the radars they flew over.
Freddie Williams had suggested using a single separate frequency for IFF as early as 1940, but at that time the problem had not become acute. The introduction of microwave radars based on the cavity magnetron was the main reason for adopting this solution, as the Mark II could not easily be adapted to respond on these frequencies. In 1942, a new frequency band, between 157 and 187 MHz, just below most VHF radars, was selected for this role. The only downside to this design is that the radar itself no longer provided the trigger signal for the transponder, so a separate transmitter and receiver was needed at the radar stations.
The Mark III began to replace the Mark II in 1942 and 1943, in a somewhat lengthy switchover period. It was also used as the basis for several other transponder systems such as Walter and Rebecca/Eureka, which allowed suitably equipped aircraft to home in on locations on the ground. These found use for dropping paratroopers and supplies in Europe, locating downed aircraft, and other roles. Several newer IFF designs were trialled, but none of them offered enough of an advantage to warrant a switchover. Mark III was replaced by IFF Mark X over an extended time starting in 1952.
History IFF Mark I and II
IFF Mark I was the first IFF system to see experimental use, with a small number of units installed in 1939. Mark I was a simple system that listened for signals on the 5 meter band used by Chain Home radars and responded by sending out a short pulse on the same frequency. At the Chain Home station, this signal would be received slightly after the reflection of the station's own broadcast signal, and was more powerful. The result was that the aircraft's blip on the radar display grew larger and stretched out. The same blip would be produced if the radar was tracking a group of targets in formation, so the transponder also had a motorized switch that turned the signal on and off, causing the blip to oscillate on the display. Mark I was used only experimentally, with about 50 sets completed in total.
The problem with Mark I was that it operated only on the 23 MHz Chain Home frequency. By 1939 there were already several other radars being introduced that operated on different frequencies, notably the 75 MHz used by the GL Mk. I radar and the 43 MHz used by the Royal Navy's Type 79 radar. To address this, development of the IFF Mark II began in October 1939 and the first units were available in early 1940. This used a complex mechanical system to select among several separate radio tuners and sweep through each one's band of frequencies, ensuring it would hear the radar signal from any of the systems in service at some point in the 10 second cycle. Mark II was the first system to be operationally deployed, and was widespread by late 1940.
Even as Mark II was being deployed, it was clear that the number of radars being introduced would shortly present a problem even for that system. In 1940 Freddie Williams had suggested that the IFF systems should work on their own frequency band instead of trying to listen for every possible radar that might come along. This would also have the advantage that the radio electronics would be much simpler, eliminating the complex mechanical switch and multiple tuners. At the time it was not considered a serious enough problem to warrant a change, instead the Mark II's would be given different tuners depending on which radars they were expected to encounter. It was not long before there was a profusion of different versions of the Mark II covering different combinations of radars.
IFF Mark III
After the 1941 introduction of the cavity magnetron operating in the 3 GHz range this process could not be continued. These frequencies required entirely different electronics to detect and amplify. It was at this point that Williams's suggestion was first taken seriously. During the development of the new Mark III in 1941, Vivian Bowden was in charge. Converting the Mark II to this new concept was straightforward; they simply removed all of the existing tuner equipment and replaced it with a much simpler one tuned to a single band. The chosen band was 157 to 187 MHz, which the motorized tuner swept through every two seconds.
Things were not so simple on the radar station's side. Since the radar signal itself was no longer the trigger for the IFF transceiver, a new transmitter had to be added, known in British terminology as an interrogator. To ensure the signals remained in synchronicity with the radar, the interrogator had a trigger input that was fed a small amount of the radar signal so that the ground station sent out its interrogation pulse at the same time as the main radar signal. The aircraft's transponder received and rebroadcast the interrogation pulse. This signal was received by the respondor at the radar station. The second transmitter and receiver quickly gave rise to the name "secondary radar", which remains in use to this day.
This change also realized two additional advantages. Radar signals were typically horizontally polarized which improved the interaction with the ground or sea surface. This also meant the antenna on the aircraft should ideally be horizontal as well. This was not easy to arrange, on the Supermarine Spitfire, for instance, the antenna was stretched along the fuselage toward the tail, and only operated properly if the aircraft was flying roughly perpendicular to the radar so the antenna was visible. With the move to a separate transmitter, the signal could be vertically polarized instead. Mark III antennas were a simple quarter-wave unipole projecting down from the bottom of the aircraft, which provided excellent omnidirectional reception as long as the aircraft was not upside down.
The other advantage was that the return pulse no longer had to be short or singular. With Mark II the IFF signals were displayed on the same display as the radar signals, so if the IFF returned too many of these signals or ones that were too long, they could hide the blips from other aircraft on the display. With Mark III, the signal was separately received and did not have to be sent to the same display. Generally, the signal was sent through an inverter and then sent to a second channel on the radar's cathode ray tube. The result was a normal radar display on top (or bottom) half of the screen, and a second similar display below it (or above) with the IFF signals only. This allowed the Mark III to send back longer pulses as they no longer overlapped aircraft reflections which were above the axis. This made the signals both easier to see as well as allowing them to be modified in order to identify individual aircraft or provide security.
Another problem that had been seen in Mark II as the number of radar sets in use increased was that the number of interrogation signals being received began to swamp the transponder's ability to reply. A related issue made tracking distant targets difficult; in the case where two aircraft were being interrogated by a single radar, their responses would not overlap because the more distant aircraft was not triggered until the signal reached it at a later time. However, if the nearer aircraft was being interrogated by more than one radar, its responses to those other radars might occur at the same time as the other aircraft's response to the first, masking it. Mark III fixed both of these problems. The first was addressed by adding a delay so the transponder responded only after receiving 4, 5 or 6 pulses. The second was somewhat more complex; as the interrogation rate increased, the Mark III began to lower its output signal, so that more distant aircraft signals were not masked.
The new design also included a number of detail improvements, most notably a new power supply for the transponder. This allowed the crews to adjust the strength of the return signal while the aircraft was on the ground (or on the deck of an aircraft carrier) and no adjustments were needed in flight. This greatly improved the reliability of the system.
In service
Shortly after Bowden took over the development of the Mark III, he was summoned by the Commander in Chief, Fighter Command, Hugh Dowding. Dowding stated
They responded by working night and day until the system was completed, which was "introduced quickly" and went into production at Ferranti in Manchester. A big trial was carried out in Pembrokeshire with transponders installed in all sorts of aircraft. This successful demonstration was one of the reasons the US Army Air Force selected Mark III for their own aircraft, instead of their own designs that were somewhat more sophisticated. This led to a massive production effort in the US, where Bowden was sent to help get things started. At one point, Hazeltine Corporation was building more IFF units than all other radars in the US combined.
IFF only works if the aircraft being queried is carrying it; this makes the switchover from one IFF to another a difficult affair as it has to be carried out all-or-nothing in any given area of operations. This was almost impossible to arrange and led to great confusion. For instance, during the Operation Avalanche period in September 1943, the anti-aircraft cruiser reported that over a period of a month they interrogated Mark I, Mark II, Mark IIG, Mark IIN and Mark III, as well as many friendly aircraft that displayed no IFF at all. Mark III was still considered a qualified success during this era.
One of the few modifications to the basic Mark III was the Mark IIIG, also known as ARI.5131 in the UK or SCR-695 in the US. This combined the normal Mark III transponder with a second one tuned to the frequency of the newer ground control radars, notably the AMES Type 7 at 209 MHz. A motorized switch was used to turn on the second frequency for of a second, once every second. This produced a signal similar to the one from the original Mark I but because Type 7 used a plan-position indicator display, the result was a series of small blips on either side of the target return. This was known as the "crown of thorns". A further version, Mark IIIQ or ARI.5640, does not appear to have been deployed.
Beacon use
James Rennie Whitehead used the Mark III electronics to produce beacons that responded on the 176 MHz frequency of ASV Mk. II radar. These were placed at naval bases and Fleet Air Arm airfields, allowing aircraft to use their anti-shipping radars as long-range navigation systems. As they only responded to a single frequency, they were more like the original Mark I in a technical sense, but used the Mark III internals to gain all the advantages of the newer electronics and production capability. When the Blind Approach Beacon System (BABS) was introduced on 173.5 MHz, the ASV beacons had to move to 177 MHz. A similar system for RAF airfields was quickly adopted by the night fighters, operating on the 212 MHz of the AI Mark IV they carried.