Hi One would *hope* that they can do the same process with an aircraft on a known route and get data that makes sense. If that “fact check” is missing from the report … not good.
Bob > On Oct 17, 2014, at 5:22 AM, Poul-Henning Kamp <[email protected]> wrote: > > 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 > [email protected] | 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 -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
