This is mitigated by combining the SSR with a PSR. In case of transponder failure the SSR will receive no reply and will therefore not discover the target. The SSR relies on the onboard equipment to discover aircraft.These can interfere with the signal, though, thus sometimes making detection of real targets more difficult. Therefore, terrain, buildings and weather phenomena (clouds and precipitation) do not produce clutter, as is the case with PSR. Consequently, even if an echo on 1030 MHz is received, it is not processed by the system. SSR is immune to clutter as it uses different frequencies for interrogation (1030 MHz) and replies (1090 MHz).Targets are easier to distinguish due to the different SSR codes.The information provided is not limited to range and bearing from the antenna but also includes additional data based on the transponder mode of operation (A, C or S).This is because the transmitted signal only needs to reach the aircraft, while the PSR needs to emit a signal strong enough to reach the aircraft and travel back to the antenna. Requires much less power to achieve the desired range, in comparison to PSR.SSRs may be used as interrogators for passive MLAT/ WAM arrays. The azimuth is taken from the antenna position. The range is calculated by knowing the time difference between the interrogation and the reply (the speed of propagation is the speed of light).
When the reply is received, aircraft position (range and bearing) is determined. As a result, when the controller observes that two aircraft are separated by 1000 feet, it means that this separation exists regardless of the altimeter settings of the two aircraft. This is a mitigation against human error. Note that the level received from the transponder is always in respect to standard pressure (1013.25 hPa, 29.92" Hg) regardless of the altimeter setting selected by the pilot.
The reason for using Mode A more frequently is that the identity of the aircraft (the SSR code) is of greater importance to the controller. Typically, two Mode A interrogations are followed by a Mode C interrogation. For example, interrogation in Mode A will receive a reply in mode A even though the transponder may have Mode C or Mode S capability and an interrogation in Mode C will not trigger a response from a Mode A transponder. The information received depends on the interrogation mode (A, C or S) and the transponder capability. aircraft identification, selected level, etc.
#ATC DELAY MEANING CODE#
The transponder sends back a reply containing at least a code (if operating in Mode A) but more often this is combined with level (mode C) or other information, e.g. The radar antenna rotates (usually at 5-12 rpm) and transmits a pulse which is received by the onboard equipment ( transponder). Source: ICAO Doc 4444 PANS-ATM Description A surveillance radar system which uses transmitters/receivers (interrogators) and transponders.
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