IFF Mark X

It was also adopted by ICAO, with some modifications, as the civilian air traffic control (ATC) secondary radar (SSR) transponder.

Mark III responded on the same frequency as the trigger signal, returning a selected pulse pattern.

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.

[1] 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.

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.

[7] A more practical concern was that the IFF signals were in the middle of existing VHF radar bands; moving to a new frequency would help reduce potential interference.

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.

When the US and British formed the joint Combined Research Group at the NRL, this system was given the name Mark IV.

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.

[1] UNB adaptors for existing Mark III sets were produced and saw service with the US forces, but were not adopted elsewhere.

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.

[10] 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.

[11] Although they had withdrawn from the UNB, the UK kept in touch with their US counterparts, and officially adopted Mark X in October 1949, followed by the Canadians the next year.

[12] Allied systems had started as transponders which simply reflected the original radar pulse and were unable to encode any sort of customized response message.

This system was initially implemented as a separate box that plugged into the Mark X and modified its return signals.

Although the accident was ultimately the fault of the DC-8 crew, among the contributing factors was that the air traffic controllers were unable to positively identify the aircraft.

For uncontrolled aircraft, codes generally use only the first two digits so that they could also be read on older military ground equipment.

[17] In the military, Mark X had the significant drawback that it continued to respond to any interrogator, allowing it to be used by enemy forces to triangulate aircraft.

Now an airborne transponder could check to see if the interrogation pulse was from a valid friendly source, and ignore anyone that did not present the proper code.

Work on a spread spectrum IFF Mark XV started in the US, but was cancelled in 1990 due to rising costs of the estimated 17,000 required units.

This allows the ground station to periodically send out a signal similar to Mode 3/A, but receive unique codes for every aircraft.

From then on, the interrogator can send out position and altitude calls with specific codes, thereby having only the selected aircraft respond.

Mode S also adds a series of long response formats that allow text messages and other information to be sent.

[24] Ground-based interrogators normally cycle through the different modes in order to collect complete information, this is known as the interlace pattern.

Most general aviation aircraft in North America are told to "squawk 1200", meaning that they should set their transponder to 1200, whereas in the rest of the world, 7000 is used for this same purpose.

SIF also adds a single additional optional pulse, Special Purpose Identification or SPI, which is sent 4.35 μs after the F2.

SPI is triggered manually by the transponder operator to identify a single aircraft by pressing a button.