Dani
I don't claim that the transponder transmissions tell us anything definitive, but I would expect that the aircraft's GPS receiver(s) would have an update rate of 10Hz for both position and speed, so a transponder transmission will contain the latest and very recent data.
I don't know how aviation GPS receivers calculate velocity, but I would expect it to be based on doppler frequency from the SV signals as extracted by the correlators. Pretty accurate in terms of ground speed, usually to 0.1 x displayed velocity units for a consumer GPS.
If anyone here knows the B738 avionics well enough to comment on whether I'm being realistic that would be useful.
The guy with the receiver near EHAM tracked TC-JGE over a period of 37 minutes (may not have been continuously, I don't know), and received 1042 Mode-S messages from this aircraft during that period. On average that's one message every 2.1 seconds.
With my own Mode-S receiver, if I have a single B738 in range I see between 3 and 7 messages received per second, so the update rates are pretty high for an ADS-B equipped aircraft, in a dense environment though a fair percentage of these will overlap in time with other aircraft so there is a larger chance of missing a given message.
When the FDR data becomes known we can see whether this report is anywhere near the truth.
Last edited by Feathers McGraw; 26th Feb 2009 at 12:41.
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