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A transponder (XPDR) is a receiver/transmitter which will generate a reply signal upon proper interrogation; the interrogation and reply being on different frequencies. (ICAO)
Transponders first came into use to enable military authorities to identify friendly aircraft, which transmitted a coded signal when interrogated by military radar. This was known as IFF (Identification Friend or Foe).
Subsequently, transponders have come into widespread use in civil as well as military aviation. It is now standard practice to allocate a specific transponder code to each aircraft flying in controlled airspace so that the ATCO can readily identify a specific aircraft on a crowded radar screen, using SSR (Secondary Surveillance Radar).
By international agreement, 2000 is used for aircraft which have not been assigned a transponder code, although in some parts of Europe, 7000 is used for this purpose. Details of standard codes in different countries may be found in national AIPs.
Special codes are used in emergency, as follows:
- hi-jack (7500);
- loss of communication (7600); and,
- general emergency (7700).
Mode A, C, S and Ident
Civil aircraft may be equipped with transponders capable of operating in different modes:
- Mode A equipment transmits an identifying code only.
- Mode C equipment enables the ATCO to see the aircraft altitude or flight level automatically.
- Mode S equipment has altitude capability and also permits data exchange.
Mode C or S equipment is a mandatory requirement for many busy areas of controlled airspace.
Transponders have an “Ident” facility which causes the aircraft radar response to stand out when the pilot operates the Ident switch in the cockpit. This should only be opoerated upon ATC request.
Transponders and ACAS
ACAS operation requires that both aircraft - the interrogator and the target - are equipped with operating transponders. An aircraft equipped with ACAS will receive the following information depending on the type of transponder with which the target aircraft is equipped: