Definition

Secondary Surveillance Radar (SSR). A surveillance radar system which uses transmitters/receivers (interrogators) and transponders.

Source: ICAO Doc 4444 PANS-ATM

Description

Principle of operation

The radar antenna rotates (usually at 5-12 rpm) and transmits a pulse which is received by the onboard equipment (transponder). The transponder sends back a reply based on the interrogation mode. It may contain:

• a four-digit code (Mode A).
• the aircraft level (Mode C).
• other information, e.g. aircraft identification, selected level, etc. (Mode S). 

The information received depends on the interrogation mode (A, C or S) and the transponder selection. Examples: