Identification of Friend and Foe System




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. It is obviously 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, which depends on the strength of the pulse and its duration, will increase correspondingly.

The two systems are normally operated in conjunction with a primary radar, the two aerials being either co-mounted, or arranged to scan synchronously. The arrangement of a typical system is shown in Fig 1. 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. 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. As a matter of interest, it was a Lufthansa 747.

There are four windows, each with a corresponding up & down scroller. The power and Ident switches are self-explanatory. The function of the separately and on-command used Ident (Identification) switch is also explained later . The windows are placed serially, i.e., A-B-C-D and these are explained in the para on The Confusing Bit below. The actual dimensions are quite small, of the order of 2.5 cm x 12 cm.

IFF and SSR are actually quite different, but the operating principle of each is basically the same and IAF aircraft are now fitted with transponders which can operate with both systems. All civil ac have them already, as part of its assembly kit. IFF is now being used more frequently in the IAF. That is because we have a lot of western world aircraft. In most countries abroad, where traffic density is high, you cannot get airborne if the 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 consists of a pair of pulses, each pulse having a width of 0.85 μs. The separation of the pulses determines the mode of interrogation. IFF has three operating modes, 1, 2, and 3. Mode 3 is made available as the common frequency (A Mode) with civil aviation. This is where you hear the command: Squawk 4238 ordering the pilot to select 4238 on the IFF. Civil aviation has B/C/S modes also. B mode is blank and is a back- up to the A mode. The S mode, the widest of all, is heavily restricted and part of it contains the much-in-the-news Collision Avoidance System. The term Squawk is used only in the common modes, A & 3. The Rafale pilot who just came in would have squawked 10-12 times outside India, perhaps twice in India.

Modes 1 and 2 are also very tightly controlled. These are the secro-channels for 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. Modes 3/A are fairly broad at a spacing of 8 μs.

A normal reply is transmitted on a frequency of 1090 MHz (1030 + 60) and consists of 14 pulses separated in 20.3 μs. There is no law banning mated frequencies of 1040-2000 MHz and the like! The 1st and 14th are data-free and serve as entry/exit warnings, exactly as on a bar-code. That leaves twelve information pulses in between. 

The Confusing Bit

Pulses 1 & 14 are called Framing Pulses, F1 & F2. The 12 data pulses are placed in four groups of three each, designated A, B, C and D. Within a group, the pulses are annotated 1, 2, and 4, (A1, A2, A4; B1, B2, B4, etc), each pulse representing one digit of a 03-digit binary number. No 3 is not used. This way, the presence or absence of pulses allows each group to represent a decimal number from 0 to 7. For example, in group A:

In place of A, one could have B, C or D.                
It is thus possible to transmit 4096 codes, the code being set on the control device. For example, to transmit a code of 4167 the transmitted pulses would be 4 in the A window, 1 in the B window, 6 in the C window and 7 in the D window

The transmitted code is displayed in a sleeve on the ac as seen on the PPI radar display

Normal Reply Modes 

There are a total of seven normal reply modes:

Military Mode 1
Military Mode 1 replies comprise the framing pulses and the information pulses reflecting the Mode 1 code set on the cockpit control panel.

Military Mode 2
Military Mode 2 has the same form as Mode 1. However, the code is not selectable in flight, but is preset on the transponder unit.                    

Common Mode 3/A
Mode 3/A has the same form as Mode 1, but the controller has a separate set of code selection switches so that replies can be made to Mode 1 and Mode 3/A simultaneously. Mode 3/A is the mode normally used by ATC agencies to establish and maintain the identity of an aircraft, to assist in the transfer of control between agencies, and to supplement primary radar information.   

Civil Mode B
Mode B is the same as Mode 3/A but can only be used as back-up.

Civil Mode C
Mode C is used for the automatic reporting of altitude. The transponder, in association with an encoding altimeter, replies with a code train indicating the aircraft’s height relative to the ICAO 1013.25 HP pressure datum. The code uses 11 of the 12 information pulses and a change occurs every 100 ft. Mode C is used by air traffic controllers to confirm that aircraft are maintaining, vacating, reaching, or passing assigned flight levels, and to monitor the vertical separation between transponding aircraft, without recourse to ground/air communication.

Civil Mode D
Mode D is not currently used.      

Civil Mode S
Mode S was introduced to support the automation of some ATC functions. Its full title is 'Mode Select' and it provides a two-way data link facility. Mode S has all the functionality of Modes A and C. One feature of the system is that each aircraft fitted with Mode S is assigned a unique address code. The address, together with the other information, is transmitted once per second in a signal known as a 'squitter'. This signal can be received by ATC units and other Mode S capable aircraft. Mode S contains the mandatory Traffic Alert and Collision Avoidance System.

Identification Replies

In addition to the normal replies discussed so far, identification replies may be initiated, when required, by the pilot. In civil aviation, these identification replies consist of the normal code followed by a pulse (or pulses) which is transmitted for 20 seconds after operation of the toggle. The switch is normally marked IDENT. Transmission of the signal enables the ground controller to carry out rapid identification of a particular aircraft among the many which may appear on the display, operating in the same mode

Military Ident Reply
This reply is given only on military Mode 1 when the HOTAS control over IFF  is operated. It consists of the selected information pulse train followed by a second identical pulse train with the second F1 pulse 4.35 μs after the first F2 pulse. Nothing stops the pilot from using Mode 3, if appropriate. I do not know if the MiG-29 & SU-30 MKIs have this HOTAS ability. The MiG-21 does not.

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.

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 panic 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.

Problem Areas and Solutions

-  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 . Defruiting is the process whereby this interference is removed. Adjacent interrogators are operated at different pulse recurrence frequencies (PRFs) and comparator circuits only pass replies at the correct home station PRF.       

-  Garbling: Because the length of a transponder code train is about 20 μs it is not always possible to decipher replies from aircraft closer than 2 or 3 miles to each other on a radial from the interrogator. The reply signals may garble and the decoder equipment can cause the generation of false targets between the aircraft or cause cancellation of all or part of either or both actual returns. False targets or cancellation may occur even though altitude separation between aircraft exists. Circuits in the decoder equipment are used to cancel garbled replies, and controllers will often ensure that only one aircraft within a formation has a transponder operating.             

-  Mode Interlacing: In order to use the different modes for various functions, it is necessary for them to be transmitted separately from each other, and on a sequential basis. The mixing of the mode transmissions is known as mode interlacing. Each mode is selected at its PRF rate, and each mode sequence is selected at aerial rotation rate. Generally speaking, the use of more than three-mode interlace is not satisfactory operationally, since the number of hits per scan for each mode transmitted falls to a non-effective level. However, the interlacing of four or five modes can be achieved if necessary. 

-  Three-pulse Sidelobe Suppression: In secondary radar, sidelobes are effective at greater ranges than in primary radar since transponder transmissions are detected rather than target echoes. It is thus necessary to suppress any interrogator sidelobes which would be capable of triggering responses and the 3-pulse sidelobe suppression system has been adopted as the international standard technique.

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