IFF - Information Friend or Foe

Frequency Range   10xx MHz
Power Out            watts

How it works

Basically, the system works on an interrogation and reply system, based on pulse coded transmissions. For instance, with IFF or Mode 4, the interrogator generates a pulse train, modulates it onto a carrier and transmits it; then the transponder receives and decodes the pulse train, generates an appropriate response (another pulse code train) and transmits it so that the interrogator can receive the reply and (hopefully) get whatever information they need, which in the case of Mode 4, is a proper response based on cryptographics information.

The military IFF system supports Modes 1, 2, 3/A, C and 4. Modes 1 and 2 are older security modes that have been replaced in utility by Mode 4. Mode 3/A is used by air traffic controllers (both military and civilian). The tower has an interrogator which generates a Mode 3/A interrogation. The aircraft receives the interrogation, and replies with a pulse train which corresponds to what is selected on the transponder control. The tower receives the reply and uses the 4 digit number to help identify aircraft. Mode C works on the same principle, but instead of the information being a selectable 4 digit number, Mode C equates to the aircraft's barometric altitude, which is given to the transponder by some other system (CADC or some altimeter encoder). Again, this is used by air traffic controllers. These modes are all SIFS or Selective Identification Features, and are not really IFF, although the military system that incorporates them is generally referred to as IFF. (I'd say the most accurate name for the whole system would be transponder system as opposed to IFF).
Mode 3A and/or C interrogations are transmitted and received.
Aircraft replies with 4-digit numerical response and/or altitude.

The mode that gives IFF its name is Mode 4. First, the interrogator generates a generic pulse train and transmits. The interrogated aircraft receives this generic pulse train and responds with a pulse train with encoded cryptographic information. The interrogating aircraft receives this and if the cryptographic information matches what it has, the aircraft is identified as friendly. If the response is inaccurate, this will identify the receiving aircraft as non-friendly, and the interrogating aircraft might pursue hostile action. So basically, IFF is used by fighter aircraft to distinguish between friendly and non-friendly targets.

Friend     or     Foe
Generic Mode 4 interrogation is transmitted and received. Generic Mode 4 interrogation is transmitted.
Interrogated aircraft responds with proper crypto-encoded pulse train.   Interrogated aircraft does not respond or responds
  with improper crypto-encoded pulse train.


APX-64 IFF R/T RT-728 - The R/T
TS-1843 Test Set - The APX-64 IFF test set
APX-64 IFF antenna switch SA-1474 Antenna switching unit -
C-6280 - The control APX-64 IFF control
Mode 4 computer KIT-1C Mode 4 computer -

Basic Troubleshooting

Sound troubleshooting will save you a lot of headache with this system. Remember that you have a lot of tools at your disposal, namely the built-in test and the TS-4077 transponder test set. First step is to find out if you are having problems with all the modes or just a problem with Mode 4. This narrows your playing field to only 2 LRUs: the Mode 4 Computer and R/T (as well as a possible improperly loaded TS-4077 transponder test set). Before you go changing any LRUs in this situation, check your IFF annunciator. If it's on, then you may need to reload the Mode 4 computer, and if it persistently drops codes it may merely be a bad battery. In the same vein, if you have a Mode C problem, you're looking at the altitude source or possibly an R/T.

If all modes are out, take a peek at the R/T. If only the top few lights are on, then you could have an R/T problem. On the other hand, you may just need to give it more warmup time. If the problem persists, try resetting the AC circuit breakers on the R/T. If the R/T looks okay, it's time to see what the built-in test is showing. In conjunction with the antenna select switch, you can use the built-in test to eliminate the antennas, respective transmission lines or switching unit, since the test set checks VSWR. If you get bad test results only in upper (and maybe both) look at upper transmission elements. If you get bad results in lower, look there. And if you get only get a bad indication in both, look towards your antenna switching unit. Also, on another note, if you are getting good indications with your TS-4077, but the built-in test is showing a faulty system, then look toward the test set. And finally, if the R/T looks good and the test indications are good, don't rule out the possibility of a faulty TS-4077 transponder test set. And before ordering an antenna switching unit or test set, you can confirm whether or not these LRUs are bad by bypassing them. This can also provide a quick fix on a launch, particularly with the test set, since it's not needed for normal operation.

Take the IFF test
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