The aircraft is interrogated from the ground station by a predetermined series of pulses on the carrier frequency of 1030 MHz; its transponder then transmits a coded reply on a carrier frequency of 1090 MHz. The modes of operation are:
Mode A - an interrogation to identify an aircraft.
Mode C - an interrogation to obtain an automatic height read-out of an aircraft referenced to 1013 hPa.
Mode S - a development of the basic SSR fully compatible with the conventional Mode A and C units, providing air-to-ground, ground-to-air, air-to-air data link and height read-out in increments of 25 ft.
MODE S (Selective Addressing)
Enables data exchange using communications protocols. Mode S data link can serve as an alternative or supplementary method for various ATC services that are traditionally carried out using VHF voice communications. This utilization of data link technology enhances the safety and reliability of the ATC system by minimizing errors associated with voice-based communication. Numerous types of messages and services within ATC can benefit from data link backup, like:
- Flight identification
- Altitude clearance confirmation
- Take-off clearance confirmation
- New communication frequency for sector hand-off
- Pilot acknowledgement of ATC clearance
- Transmission to the ground of aircraft flight parameters
- Minimum safe altitude warning
A mode S transponder is fully compatible with conventional Mode A and C ground units. This means that if a Mode S transponder is interrogated in Mode A, the transponder responds as if it was set in Mode A, providing aircraft identification (transponder code) to the ATC radar screen.
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