[time-nuts] Inmarsat needs a time-nut
kb8tq at n1k.org
Fri Oct 17 07:39:21 EDT 2014
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.
> On Oct 17, 2014, at 5:22 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> Inmarsats long awaited article about the MH370 calculations is now
> 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 Kamp | UNIX since Zilog Zeus 3.20
> phk at 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.
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