A Mode S sensor has two methods of interrogation: All-Call and Selective. All-call interrogations are transmitted regularly at a steady rate in a similar way to conventional SSR. Any Mode S transponder that is not ‘locked out’ will reply to an all-call interrogation, transmitting its unique 24-bit aircraft address. In this way, the interrogator acquires targets not previously detected.
Once a transponder is known to the interrogator and its track has been established, it can be ‘locked out’. This prevents the transponder from replying to any more all-call interrogations from that or any other Mode S sensor with the same identifier code, and it will then only respond to Selective interrogations. However, it will continue to respond to interrogations from other Mode S sensors with a dissimilar Identifier Code and also to Mode A/C sensors.
Lockout is a new concept for SSR and is one of the major factors in radically reducing reply rates and, thus, reducing interference. To prevent the potential for undesirable, uncontrolled lockout of targets, a number of safeguards have been built into the international standards for both interrogators and transponders to ensure that lockout is handled in a fail-safe manner.
Selective interrogations make use of the unique 24-bit aircraft address and can be sent out close to the azimuth where the aircraft is expected to be. No other aircraft that happens to be in the radar beam at that time will reply. The aircraft addressed will reply with its Mode A code (assuming one has been assigned), aircraft identification and altitude. The type of reply is controlled by the interrogator, but in either case only a single reply is required because there is no ambiguity as to which aircraft the reply belongs. Extra interrogations can be made to ensure that at least one reply is received and that the azimuth performance is maintained.