A process by which aircraft replies accepted by the interrogator-responsor are tested by means of storage and a comparator for synchronism with the interrogation-repetition frequency. Only replies which are in synchronism (correlate on a repeated basis in range) will be output from the defruiter. Other replies are rejected as “fruit” or false.
ICAO (2004), doc. 9684 Manual of the Secondary Surveillance Radar (SSR) Systems