IFF Mark III

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 impetus for adopting this solution, as the Mark II could not easily be adapted to respond on these frequencies.

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.

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.

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.

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.

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.

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.

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.

[12]They responded by working night and day until the system was completed, which was "introduced quickly" and went into production at Ferranti in Manchester.

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.

[14] 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.

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.

[15] James Rennie Whitehead used the Mark III electronics to produce beacons that responded on the 176 MHz frequency of ASV Mk.

These were placed at naval bases and Fleet Air Arm airfields, allowing aircraft to use their anti-shipping radars as long-range navigation systems.

[16] 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.

The transponder would then reply to the pulses of the fighter's radar, providing a powerful signal that could be received at ranges as great as 100 miles (160 km).

[18] In June 1941, a battery-powered version of the same equipment was used by Robert Hanbury Brown in a demonstration for the RAF Army Co-Operation Command.

Not only did their RAF Bristol Blenheim easily find it, but it also attracted the attention of a night fighter that just happened to be flying in the area and saw an odd return on their display.

When the Co-Operation Command observers complained that it was a setup, their Blenheim repeated the trick a second time after the transponder was moved.

This was a small version of the beacon system that was carried aboard aircraft life rafts and activated if they were forced down on water.

This made a blip appear on all of the other aircraft's ASV Mark II radar displays which they would then use to find the indicated area.

A less important problem was that as electronics improved it became possible to move to higher frequencies in the UHF region, which allowed for smaller antennas and thus less drag on the aircraft.

There were concerns that a Würzburg might trigger the Mark IV and cause a reply on their display, immediately revealing the presence of the system and its working frequencies.

[23] Bowden stayed on in the US, joining the NRL group in 1942 to begin development of the further improved Mark V, later known as the United Nations Beacon or UNB.

This allowed the ground operators to instruct the aircraft to change their transponder to a specific channel so that they could be sure they were receiving the signals from their interrogator and not an enemy broadcaster.

[24] The fact that the Soviet Union had been supplied with 500 Mark III units was a serious concern for US Navy planners.

It was assumed that the Soviets would use these units during the Korean War, and this caused the concern that an aircraft carrier might find itself being attacked by a group of planes displaying proper IFF responses.

In May 1951, the US Far East Air Force ordered its units to not assume an aircraft displaying Mark III was friendly.

In July 1951, Scott-Moncrieff stated that "identification has been one of the more unsatisfactory features of this war" and in August the decision was made to treat all aircraft as friendly to avoid friendly-fire incidents.