David Megginson wrote:
Curtis L. Olson wrote:
All the approaches at KLAX have a 1 or 2 nm bias for the DME. Like you say, there is no *requirement* for this, but there are installations that have a bias set up.
That's strange. Here are the DME distances at the runway threshold for the eight ILS approaches at KLAX, according to the NACO approach plates:
ILS 6L: 1.6 DME ILS 6R: 1.8 DME ILS 7L: 2.1 DME ILS 7R: 2.1 DME ILS 24L: 1.9 DME ILS 24R: 1.9 DME ILS 25L: 2.0 DME ILS 25R: 2.0 DME
Those distances suggest that the DME transmitter is not too far from the localizer past the far end of each runway. If there's a negative bias of 1-2 nm, then I think that some of these might be out in the Pacific Ocean, but I'll need to look at a map to check.
Thanks for taking the time to implement this, Curt -- I'm interested in learning more about the whole issue.
I clearly need to learn more about it as well. DME's by default relay the aircraft's message back with a 50ms delay. The dme unit on the aircraft calculates distance based on the total round trip time of it's original transmission, factoring in this extra 50ms delay at the ground station. I think I read somewhere that the distance bias/offset is implimented by adjusting the delay at the ground station to something other than 50ms. Unfortunately I am having trouble digging up any definitive references at the moment, but google showed me a lot of used medical equipment for sale. :-( Anyone remember when google used to be useful?
Curt.
--
Curtis Olson http://www.flightgear.org/~curt HumanFIRST Program http://www.humanfirst.umn.edu/
FlightGear Project http://www.flightgear.org
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