2.3. AIRCRAFT IDENTIFICATION AND POSITION REPORTING WITH WIDE AREA MULTILATERATION
As more and more new systems derived from the CNS/ATM concept8 come into the daily operational field, the reader may hear about Multi-lateration. At first sight, it seems to be another engineering slang word. It is, and the following is a general description to demystify the Wide Area Multi-lateration (WAM – Multi-lateration technique applied in wide surveillance areas).
The Multi-lateration technique uses the same principle as the localization of a mobile phone with ground stations (i.e. triangulation).
Different ground antennas receive a signal from the aircraft. Each antenna receives the signal at different time due to the relative distance between the antenna and the signal emitter. A central processing unit connected to the ground
7 Only some transponders capable of ADS-B are capable to transmit the SQWK code. Refer to 2.2.6 –
DO-260 and DO-260A.
8 Refer to Part I of the Getting to Grips with FANS, issue III, April 2007 for a description of the
CNS/ATM concept.
antennas calculates the aircraft position from the Time Difference Of Arrival
(TDOA) of the signal at the different ground antennas. With 2 antennas, the TDOA corresponds to a 3D hyperboloid on which the aircraft is located. The calculation of the 3D aircraft position requires 4 antennas. The central processing unit calculates the intersection of the 3 hyperboloids. A configuration with more than 4 antennas permits to calculate an average position with a higher accuracy.
The determination of a 3D position with a 3-antenna configuration requires an additional source (e.g. barometric altitude from Mode C transponder reply) for the aircraft altitude. However, the resulting position is less accurate than the one determined with a configuration of 4 antennas.
The Multi-lateration technique may be either passive (by listening transmissions from the transponder like ADS-B OUT) or active (by interrogating the aircraft like ELS/EHS).
Many signals from the aircraft are available (e.g. SSR, Mode S, DME, etc). The following characteristics drive the choice of the signal:
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The capability of the signal to provide the aircraft identification
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The availability of the signal
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The quality of the signal.
To fulfill the aircraft identification and position reporting functions, the use of transponder and ADS-B signals appear the most appropriate solution. The minimum avionics equipage would be a Mode A/C transponder for active Multi-lateration technique. The embodiment of Mode S transponder or ADS-B avionics would enable the passive Multi-lateration technique.
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