responsor

A responsor is a receiving unit in a radar or radio interrogation system, paired with an interrogator transmitter to extract useful data from transponder replies. When an interrogator sends a coded pulse train toward a target equipped with a transponder, that transponder automatically bounces back a reply signal. The responsor sits at the interrogator location, waits for and captures these return signals, then decodes them into usable information: target identity, altitude, range, or other parameters the transponder was equipped to send.

In secondary radar systems, particularly air traffic control SSR (Secondary Surveillance Radar), the interrogator and responsor work as a single functional unit. The interrogator transmits on 1030 MHz; the transponder on the aircraft replies on 1090 MHz; the responsor receives and processes that 1090 MHz reply. The responsor must filter out ground clutter, reject false targets, and decode Mode A, Mode C, or Mode S replies fast enough to handle dozens of aircraft in range simultaneously. This requires stable crystal oscillators, sensitive receivers with good noise figure, and logic circuits capable of pulse-by-pulse analysis.

Responsors differ from simple receivers because they are specifically designed to work with interrogation protocols. They expect signals in particular formats at particular times and discard anything that does not match. A generic radio receiver passively listens; a responsor actively correlates incoming pulses against the interrogation it just sent, rejecting stray signals and dramatically improving signal-to-noise ratio in a crowded radio environment.

The term survives mainly in older radar and aircraft identification documentation. Modern systems use more specific labels such as receiver, decoder, or signal processor. However, responsors remain at work in thousands of SSR ground stations and maritime identification systems worldwide, still pulling vital target information from transponder echoes.