[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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