Non-traditional PAR Using SSR Transponder Reply
There are systems that provide PAR functionality without using primary radar. These non-traditional PAR systems use transponder multilateration, triangulation and/or trilateration.
One such system, Transponder Landing System (TLS) precisely tracks aircraft using the mode 3/A transponder response received by antenna arrays located near the runway. These antennas are part of a measurement subsystem that is used to precisely determine the aircraft 3-dimensional position using TOA, DTOA and AOA measurement techniques. The aircraft position is then displayed on a high-resolution color graphics terminal that also shows the approach centerline and the glide path. A GCA controller is then able to use this screen for reference to issue GCA instructions to the pilot.
The signal strength for the secondary surveillance radar subsystem of a non-traditional PAR is not attenuated by rain since the frequency is within the long range band, L-band. Therefore a non-traditional PAR does not experience noticeable rain fade and in the case of the TLS has an operational range of 60 nm.
Read more about this topic: Precision Approach Radar
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