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Airports that operate an ASDE-X system require aircraft to operate transponders in mode C while moving on the surface.

While I understand the general purpose of operating the transponder to correlate with the other information ingested by the ASDE-X system, why is altitude (encoded in mode C) important for tracking surface movement?

Wouldn't mode A suffice?

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  • $\begingroup$ Problem - many airliners do not transmit Mode A or Mode C when they're on the ground, it's inhibited. Does the ASDE work off Mode S ? $\endgroup$ – RAC Aug 29 '19 at 8:00
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Without knowing the altitude, it is very difficult (or impossible) to accurately calculate the position of a transponder by using multilateration (which is what ASDE-X uses). In addition, without knowing the altitude, ASDE-X would be unable to determine whether an aircraft is on the airport's surface, or passing overhead.

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