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 to the interrogation with a response that is a train of pulses, that can be decoded by the interrogator's receiver to give the appropriate information. The transmissions are made in the UHF band, and all interrogations (ground to air) happen on carrier frequency 1 030 MHz and all transponder responses are completed on the 1 090 MHz carrier frequency.

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

• Mode A. An interrogation to identify an aircraft by a 4 digit code.
• Mode C. An interrogation to obtain an automatic pressure altitude read-out of an aircraft.

Three pulses (P1, P2 and P3) are always transmitted to interrogate an aircraft transponder, and the different pulses have different uses.

P1 and P3 are strong pulses and they are spaced differently to indicate the type of interrogation. 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 that determines which response to send back.

The string of pulses made as a response can either be encoded with the aircraft 4-digit squawk code OR the altitude, never both, so separate interrogations are required to get both sets of information.

The P2 pulse, on the other hand, is always transmitted 2 μs (microseconds) after the P1 pulse, and if its amplitude is stronger than the P1 pulse it means that the aircraft is within an unwanted sidelobe of the SSR interrogator and that the transponder should ignore this interrogation completely.