[ntp:questions] Re: MAXDISPERSE is too high! was Re: is there a way to "lock" the drift frequency
Tim Shoppa
shoppa at trailing-edge.com
Mon Dec 8 18:55:05 UTC 2003
Terje Mathisen <terje.mathisen at hda.hydro.com> wrote in message news:<br1vgc$5lg$2 at osl016lin.hda.hydro.com>...
>
> Yes: 4 us/day difference between a sat clock and one at sea level.
>
> I.e. the sat clocks are tuned before launch to be off by those 4 us, so
> that they will be correct while in orbit.
At least on the Block II GPS sats, the correction is tunable (via
a synthesizer on the satellite) after launch. Ashby's indicates that
this was done because of some concern that the GR predictions wouldn't
be correct:
At the time of launch of the first NTS-2 satellite
(June 1977), which contained the first Cesium clock
to be placed in orbit,
there were some who doubted that
relativistic effects were real. A frequency
synthesizer was built into the satellite clock
system so that after launch, if in fact the
rate of the clock in its final orbit was that
predicted by GR, then the synthesizer could
be turned on bringing the clock to the
coordinate rate necessary for operation. The
atomic clock was first operated for about 20
days to measure its clock rate before turning
on the synthesizer. The frequency measured
during that interval was +442.5 parts in
10^12 faster than clocks on the ground; if
left uncorrected this would have resulted in
timing errors of about 38,000 nanoseconds
per day. The difference between predicted
and measured values of the frequency shift
was only 3.97 parts in 10^12, well within
the accuracy capabilities of the orbiting
clock. This then gave about a 1% validation
of the combined motional and gravitational
shifts for a clock at 4.2 earth radii [the
radius of the satellite's orbit].
Rest of paper is at
http://xxx.lanl.gov/abs/gr-qc/9702010
If I were program manager of the (then largely military) GPS program,
I'm not sure I'd trust a bunch of freaky long-haired GR theorists on such
matters either :-)
Tim.
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