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Upon interrogation, Mode S transponders transmit information about the aircraft to the SSR system, to TCAS receivers on board aircraft and to the ADS-B SSR system. This information includes the call sign of the aircraft and/or the aircraft's permanent ICAO 24-bit address (which is represented for human interface purposes as six hexadecimal ...
Several new modes were also introduced during this process. Civilian modes B and D were defined, but never used. Mode C responded with a 12-bit number encoded using Gillham code, which represented the altitude as (that number) x 100 feet - 1200. Radar systems can easily locate an aircraft in two dimensions, but measuring altitude is a more ...
Around busy airspace there is often a regulatory requirement that all aircraft be equipped with altitude-reporting mode C or mode S transponders. In the United States, this is known as a Mode C veil. Mode S transponders are compatible with transmitting the mode C signal, and have the capability to report in 25-foot (7.5 m) increments; they ...
SSR antenna of Deutsche Flugsicherung at Neubrandenburg, in Mecklenburg/Western Pomerania Transponder in a private aircraft squawking 2000. Secondary surveillance radar (SSR) [1] is a radar system used in air traffic control (ATC), that unlike primary radar systems that measure the bearing and distance of targets using the detected reflections of radio signals, relies on targets equipped with ...
Depending on the type of interrogation, the transponder sends back a transponder code (or "squawk code", Mode A) or altitude information (Mode C) to help air traffic controllers to identify the aircraft and to maintain separation between planes. Another mode called Mode S (Mode Select) is designed to help avoiding over-interrogation of the ...
Mode 3/A is used to identify each aircraft in the radar's coverage area. Mode C is used to request/report an aircraft's altitude. Two other modes, mode 4 and mode S, are not considered part of the ATCRBS system, but they use the same transmit and receive hardware. Mode 4 is used by military aircraft for the Identification Friend or Foe (IFF ...
The DME receiver in the aircraft searches for reply pulse-pairs (X-mode = 12-microsecond spacing) with the correct interval and reply pattern to its original interrogation pattern. (Pulse-pairs that are not coincident with the individual aircraft's interrogation pattern e.g. not synchronous, are referred to as filler pulse-pairs, or squitter ...
The original design had three such "modes", Mode 1 was triggered by the interrogator by sending the two pulses 3 μs apart (±0.2 μs), Mode 2 was 5 μs and Mode 3 was 8 μs. [ 10 ] The response to these interrogations remained simple; a successful interrogation on Mode 1 or 3 caused a single pulse to be sent in reply, very shortly after the ...