[time-nuts] Positional accuracy of the M12+T
Poul-Henning Kamp
phk at phk.freebsd.dk
Wed Jan 3 17:35:06 EST 2007
In message <20070103222305.B4FECBE00 at ip-64-139-1-69.sjc.megapath.net>, Hal Murr
ay writes:
>
>> A spherical error volume is a crude approximation, actually it is an
>> ellipsoidal with as the height error is usually significantly larger
>> than the other positional errors which also may have different rms
>> errors.
>
>Why is the height error usually larger? Is that just geometry? Do I get
>good height data if there is a satellite close to overhead?
Yes, it is geometry, and yes a sat overhead does help, but not as much
as if you also had one underneath :-)
The lack of signals from the lower half of the space around you
means that no matter where you are, the height solution is never
going to be more than half as good as the horizontal solutions.
In the polar band I mentioned before, we only have sats in a quarter
of the space around us and therefore we're even worse off.
--
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.
More information about the time-nuts
mailing list