Identification Friend-or-Foe (IFF) is a secure aircraft identification system that detects and tells us the difference between the friendly and hostile aircrafts. These systems use electromagnetic and RF devices to communicate with the aircrafts and identifying friendly, foe or neutral forces in airspace. It is central to military operations as it allows armed forces to recognize friends and identify potential hostiles. The equipment acts as a sophisticated digital version of a ‘challenge and response’ password system, sending out a challenge signal and checking it receives a correct password.

An IFF system is a is a two-channel system, using the 1030 MHz band for interrogating signals and the 1090 MHz band for response signals. This can be divided into four modes, two for military use and two for both military and civilian aircrafts:

• Mode 1 - This mode has 64 reply codes, and is used in military air traffic control to determine what type of aircraft is answering or what type of mission it is on.
• Mode 2 – This mode is only for military use, it has 4096 possible reply codes that identifies the 'Tail Number' of an aircraft. 
• Mode C - Automatic Altitude Reporting Mode. Returns the current altitude of the aircraft.
• Mode 3/A - This is the standard air traffic control mode. It is used with the automatic altitude reporting mode (Mode C), to provide identity and positive control of all aircraft flying under instrument flight rules. FAA ground interrogators normally interlace modes by alternately sending Mode 3/A and Mode C to the aircraft, receiving continuous identity and altitude data from the controlled aircraft.