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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 ...
An SSR ground station transmits interrogation pulses on 1030 MHz (continuously in Modes A, C and selectively, in Mode S) as its antenna rotates, or is electronically scanned, in space. An aircraft transponder within line-of-sight range 'listens' for the SSR interrogation signal and transmits a reply on 1090 MHz that provides aircraft information.
Primary radar determines range and bearing to a target with reasonably high fidelity, but it cannot determine target elevation (altitude) reliably except at close range. SSR uses an active transponder (beacon) to transmit a response to an interrogation by a secondary radar.
The basic concepts from Mode S were then militarized as Mode 5, which is simply a cryptographically encoded version of the Mode S data. The IFF of World War II and Soviet military systems (1946 to 1991) used coded radar signals (called cross-band interrogation, or CBI) to automatically trigger the aircraft's transponder in an aircraft ...
Mode C uses a spacing of 21 μs, and requests the aircraft's pressure altitude, provided by the altitude encoder. Mode 2 uses a spacing of 5 μs and requests the aircraft to transmit its Military identification code. The latter is only assigned to Military aircraft and so only a small percentage of aircraft actually reply to a mode 2 interrogation.
The interrogation signal, sometimes known as the uplink format, consists of two 0.8 μs long pulses on 1030 MHz. The time between the pulses defines which mode is being queried. Mode 1 had two pulses 3 μs apart (±0.2 μs), Mode 2 was 5 μs and Mode 3 was 8 μs. The civil B, C and D were 17, 21 and 25 μs apart.
Transponders can respond with one of several different "modes" determined by the interrogation pulse from the radar. Various modes exist from Mode 1 to 5 for military use, to Mode A, B, C and D, and Mode S for civilian use. Only Mode C transponders report altitude.
This interrogation-and-response cycle may occur several times per second. [ 6 ] [ 2 ] The TCAS system builds a three dimensional map of aircraft in the airspace, incorporating their range (garnered from the interrogation and response round trip time), altitude (as reported by the interrogated aircraft), and bearing (by the directional antenna ...