Identification friend or foe
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In telecommunications, identification, friend or foe (IFF) is an identification system designed for use during hostile conditions, that enables military aircraft, or both civilian and military air-traffic controllers to distinguish friendly aircraft, vehicles, or forces from the enemy, and also to track them. IFF was first developed during World War II.
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Method of operation
The IFF system traditionally used coded radar signals (also known as interrogations) to automatically trigger the aircraft's transponder in an aircraft "painted" by the radar. A transponder responds,
- in a military aircraft (other military vehicles or unit) equipped with an IFF capable transponder, by returning a correctly coded reply signal only when the incoming interrogation is identified as part of the friendly forces network;
- if no IFF response is generated a civil interrogation is generated and the aircraft, by returning a proper code can then be identified.
In an IFF network both the interrogation and reply are verified as friendly. Interrogation validation prevents an interrogator from using a transponder's replies for tracking purposes.
A cross-band beacon is used, which simply means that the interrogation pulses are at one frequency and the reply pulses are at a different frequency. 1030 MHz and 1090 MHz is the standard frequency pair used internationally.
Benefits
Major military benefits of IFF include:
- Avoiding "friendly-fire" or fratricidal attacks
- Reducing attacks on civilian aircraft
- Improving, by coordination among friendly forces, decisions about tactics
The secondary surveillance radar (SSR) system used in modern air traffic control systems is an outgrowth of the military IFF system used during World War II. The IFF equipment carried by modern military aircraft is compatible with the SSR transponder system used for civilian air traffic control. The non-secure codes are manually set by the pilot but assigned by the air traffic controller.
Civilian benefits of IFF-based transponder technology include:
- Providing automatic information to ground stations via the return signal
- Simplifying target tracking
- Increasing the range of surveillance radar when tracking cooperating aircraft. Signal losses are proportional to the square of the distance with transponders, vs. the foruth-power of the distance with radar skin tracking
- By setting a special 4-digit transponder code, a pilot can unobtrusively signal a hijacking, radio failure or other distress condition without having to make an audible radio broadcast.
IFF modes
There are five major military modes of operation designated for use by NATO (and NATO friendly) military aircraft, plus two civil modes that are jointly used by the military:
- Mode 1 is a nonsecure low cost method used by ships to track aircraft and other ships.
- This is 2-digit octal number. First number is 0-7 and the second is 0-3.
- Mode 2 is used by aircraft to make carrier controlled approaches to ships during inclement weather.
- This is a 4-digit octal number.
- Mode 3 is the standard system also used by commercial aircraft to relay their position to ground controllers throughout the world for air traffic control (ATC). It is known as Mode A in the civil world.
- This is a 4-digit octal number.
- Mode 4 is secure encrypted IFF (the only currently fielded true method of determining friend or foe)
- Received interrogation can be good (i.e will solicit a reply), no code received or wrong code received.
- Transmitted reply indicates the transponder is a friend
- Requires cryptographic capabilities for both the interrogator and transponder
- Mode 5 is a new encrypted mode that has yet to be widely implemented
- Mode "C" produces grey-coded altitude responses. This allows the interrogator to identify the relative postion of the transponder in three dimensional space (radial distance by the response delay and altitude by the response data). This mode is used both by civilian ATC and military.
- Mode S is the newest civil mode featuring very detailed information on the aircraft and its intent, including GPS derived position
- Is an addressed interrogation
- Is widely used for TCAS (Traffic Collision Avoidance System)