A Secondary Surveillance Radar (SSR) system uses transmitters/receivers (interrogators) and transponders. A pulse-coded interrogation message is sent from the ground to the aircraft. The transponder responds in accordance with the pulse-coded interrogation in the form of a pulse train that is a series of pulses, which carry the appropriate information. The transmissions are achieved in the UHF band where all ground interrogation pulses are processed at a 1 030 MHz carrier frequency and all transponder responses are completed on the 1 090 MHz carrier channel.

The two main modes of operation (excluding Mode S) are:

• Mode A. An interrogation to identify an aircraft.
• Mode C. An interrogation to obtain an automatic height read-out of an aircraft.

Three pulses (P1, P2, and P3) are always transmitted to differentiate between interrogations. The spacing between P1 and P2 is fixed at 2 μs. The spacing between P1 and P3 is 8 μs for a Mode A and 21 μs for a Mode C interrogation. The transponder detects the time interval between the pulses P1 and P3 which determines the mode of interrogation.