Identification of Friend and Foe System in brief

Fact for 17/6/2021



The Identification, Friend or Foe (IFF) military system and a similar civilian system, the Secondary Surveillance Radar (SSR), are both radar-based aircraft/flying body recognition secondary radar systems. A primary radar system depends on receiving a radar echo reflected passively by a target, whereas, in a secondary radar system, a transmitted radar signal is used to trigger a response from specialised programmable equipment in the target.
IFF/SSR is used by air defence (AD) units, AD aircraft and civil radars to identify radar echoes. AD radars on the ground are usually cross-linked by secure data channels to air traffic control ground installations. Maximum bandwidth within the IFF/SSR ambit is provided to military aviation. It is quite secure, because a coded response to a military radar challenge in India is not going to reach South Africa, being limited to Line Of Sight. But, with so many advances in technology and AWACS/SatNav/GPS integration, the dispersion of the radar-frequency based challenge and response may well increase. Its detection range depends on the strength of the pulse, its duration and altitude
The ground (or AD aircraft) transmitter, known as the interrogator, transmits a coded interrogation signal (challenge) which is received and decoded by transponders in the targeted aircraft/body. Depending on the mode to which the transponder is set, a coded reply signal is transmitted back to the interrogator. This reply signal is decoded and shown on the radar display along with the primary radar response as Friendly or Unknown.
Further vocal or visual interaction will clear any doubts. Such foul up is rare, but happens now and then. I have been scrambled to intercept what my No 2 and I could see from 100 km away was an airliner way off track. We had to get to M 1.4 to catch up with the ac which was at M0.84, slow down and show our missiles to the doping Capt till he woke up, saw me and gave me a thumbs-up. His 2nd Officer must have done the necessary spadework since we were recalled shortly thereafter.
Cockpit Control Panel:  The interface is small, about 2.5cm x 12.5 cm. It has four windows, each with a corresponding up & down scroller. The power switch is also placed on the panel. The windows are placed serially, i.e., A-B-C-D and each of these windows has its own interpretation. It is so programmed that it can show only codes identified by numbers 1-7. The maximum no of codes actually available is 4096. The interrogator will see a response like 4172.
IFF and SSR, though quite different, operate on the same principle. All IAF aircraft are now fitted with transponders which can operate with both systems. Civil ac may not use military modes, unless in a dire emergency or if hijacked. IFF is now being used more frequently in the IAF.
In most countries abroad, where traffic density is high, you will not be permitted to get airborne if your IFF is defective. It is used on the ground also, since civil airfields can have multiple ac milling around with two or more runways in use. Aircraft on ground are identified by ground radar and controlled visually by the standard ATC

The basic interrogation signal is transmitted at a frequency of 1030 MHz and the response is at 1090 MHz. Civil and Military Aviation have their own operating algorithms, with one common mode, called Mode A for civil ac and Mode 3 for military ac. The IAF Rafale that just flew in would have been on mode 3 from France to India. Moreover, it would have incorporated the mandatory Traffic Alert and Collision Avoidance System (TCAS).
The military IFF Modes are very tightly controlled. These are the secro-channels for military ac of a country. Military ac have to transmit raw data quickly for identification while civil ac can take their time and pack in a lot of data in just one transmission. We are dealing in micro and milliseconds here. For instance, the common Modes 3/A are considered broad at a spacing of 8 μs. Civil ac have responses taking up to one second. There is no law banning mated frequencies of 1040-2000 MHz and the like! But then, you will not be able to leave your own airspace
The transmitted code (4172) is displayed in a sleeve on the ac as seen on the PPI radar display. It can be dropped temporarily by de-cluttering the screen and calling it back when required.
There are a total of seven normal reply modes, three for military use and five for civil use, with one overlap. Of the 5 civil modes, only three are used. Of these three, one is dedicated to altitude. The transponder, in association with an encoding altimeter, replies with a code indicating the aircraft’s height conforming to global standards.  Air traffic controllers can confirm that aircraft are maintaining, vacating, reaching, or passing assigned flight levels, and to monitor the vertical separation between transponding aircraft, without requiring to talk.
Of the two dormant modes, one incorporates the TCAS.
In military aviation, the challenge format is changed from day to day, or at shorter periods. The response is also changed, mated to the challenge. Take an ac at Ambala. The code number to select would have come at the beginning, while taxying out.  It could be changed just before takeoff. Let’s say the ac is allotted freq no 4648. The ac will accept challenges from Ambala Radar, or from affiliated AD radar units, which are (should be) data-linked to the ATC. Or that ac may be given a fresh code. Halwara Radar will accept the Ambala code as its data would have been updated by Ambala and the sector radar.
These days, frequency hopping is also used as a cloak. There is plenty of space and scope. ECCM measures like pulse compression and staggered replies are common. Our data-link is fine. The only ac that have a problem are the MI-17, MiG-21 and the odd MiG-29. As you are aware, all radar controllers on separate consoles identify and allocate numbers to ac flying. Their altitude and heading comes down the data chain into a sleeve around the ac. We lost a chopper early 2019 because they had not updated their codes for the day, when there was pandemonium on the ground. They were not expecting a war-like scenario and stayed out of the loop, to their avoidable bad luck.
In new-gen ac, IFF is a HOTAS function, managed by the weapons computer and routed to the radar computer for update and dispatch. The ac have their own small antennae that look like shark fins.
One problem area is Interference. Since all IFF/SSR equipments work on the same transmit and receive frequencies, any interrogator can trigger any transponder which is within range and selected to the appropriate mode. Thus any ground station can receive replies from transponders interrogated by other nearby ground stations. These unwanted replies appear as interference or 'fruit'. It is more than possible that a PAF radar unit will try and spoof an IAF aircraft to get Sigint. This interference is removed by Defruiting. Adjacent interrogators are operated at different pulse recurrence frequencies (PRFs) and comparator circuits only pass replies at the correct home station PRF.               
There are other problem areas too, but they fall into complex zones which I will examine later