Inmarsats long awaited article about the MH370 calculations is now available:
journals.cambridge.org/download.php?file=%2FNAV%2FS037346331400068Xa.pdf&code=99d6daf127f9c88ca22b52fd9ff5084a I think I've spotted a weakness. The "BFO-Bias" is the frequency error of the aircraft terminals timebase and the calibrate it to 150Hz using communications while the plane is on the ground. The terminal most likely uses a TCXO for the rapid startup. A 150Hz error on a 10MHz TCXO would be 15PPM which would be impressively bad for this kind of application. Its therefore likely that the TCXO is in the 100MHz range giving an error of a more likely 1.5PPM. The aircraft terminal is powered down and possibly in very hostile environmentals for around one hour, prior to the log-on at 18:25:27Z, but Inmarsat assume, without any comment or qualification, that the BFO-bias is still the same and still constant. I don't think those are valid assumptions. Retrace is typically not specified for TCXOs, for a good reason. I don't think it is unreasonable to assume that the TCXO came up with a difference of +/- 1PPM from the previous frequency, after being turned off for an hour, where both temperature and air pressure may have violated specs. That means that the BFO-bias after the 18:25:27Z logon isn't 150Hz, but somewhere in [50...250Hz], and what's more, it's probably not even constant for the first 10-15 minutes after power-on. (In fact, I'm not even sure that the assumption that the frequency offset at ground level can be used at lower air-pressure at flight-level in the first place.) I havn't gone through their math to see what the implications would be, but I think it will vastly ruin the geometry of the fix. Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.