What is a secondary surveillance radar and how does it work for aircraft identification?

A secondary surveillance radar (SSR) works for aircraft identification by interrogating aircraft transponders with coded signals and receiving coded replies, unlike primary radar (which detects aircraft by reflected energy). SSR does not measure the aircraft's radar cross-section; instead, it communicates with a cooperative transponder aboard the aircraft. The SSR system consists of: a ground-based interrogator (transmits interrogation pulses at 1030 MHz; the interrogation encodes a mode (Mode A for identification, Mode C for altitude, Mode S for selective addressing)), and an airborne transponder (receives the 1030 MHz interrogation, decodes the mode, and transmits a coded reply at 1090 MHz; the reply contains: Mode A: a 4-digit octal squawk code assigned by ATC (4096 codes available), Mode C: the aircraft's pressure altitude (in 100-foot increments), and Mode S: a unique 24-bit aircraft address plus additional data (altitude, call sign, airspeed, heading; extended squitter provides ADS-B data)). The SSR determines the aircraft's azimuth from the antenna beam direction (like primary radar) and range from the round-trip time (interrogation to reply: approximately 3 us + round-trip propagation delay). The SSR provides identification (which aircraft is which) that primary radar cannot; primary radar combined with SSR provides a complete air traffic picture (position + identity + altitude).