Yes, seilfly.
Aircraft equipment sends a signal.
Ground transponder receives the signal a short time later.
Ground transponder deliberately delays the signal by 50 microseconds.
Ground transponder sends the signal back.
Aircraft receives the signal a short time later.
Aircraft does the time-speed-distance calculation.
To "zero-range" the signal, the 50 microsecond delay is reduced by whatever time is appropriate to adjust the zero-distance point to the end of the runway. The maximum adjustment is therefore (300 million m/s) x (50 millionths of a second) = 15km (approximately!)
Cheers,
O8
Last edited by Oktas8; 22nd August 2009 at 10:00.
Reason: Fix my awful maths...