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On Thu, 8 Jan 2004, Jared Sherman wrote:

>running before it is launched.>
>
> From everything I've read, actually 3 atomic clocks on the old birds, and 4
>on the new birds.

The Block II and IIA spacecraft have 4 atomic clocks: 2 cesiums and 2
rubidiums. The newer Block IIR spacecraft have 3 clocks -- all rubidiums.

>So "GPS time" will be the result of what one bird has
>decided three or four atomic clocks really say, and a normal 3D fix that has
>polled 4 birds will be presenting a time "concurrence" from 12-16 atomic
>clocks. Which are corrected as needed from ground control, I believe. I think
>the compensation for leap seconds, etc, is in the signal that is sent, what
>is it, once every 14 minutes by the satellites?

Only one clock in each satellite is active at any one time. GPS System Time is
based on the time kept by all active satellite clocks and the clocks at the
monitor stations. It's a "paper" time scale which is kept close to UTC(USNO)
but without leap seconds. The satellite clocks are infrequently adjusted but
the offset with respect to GPS Time is monitored and uploaded to the
satellites for rebroadcast in the navigation message. The navigation message
also includes the current leap second information. This information is
accessed even by old GPS receivers.

-- Richard Langley
   Professor of Geodesy and Precision Navigation

>Not a bad way to access that many million dollars worth of clocks.

===============================================================================
 Richard B. Langley                            E-mail: lang@unb.ca
 Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
 Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
 University of New Brunswick                   Fax:      +1 506 453-4943
 Fredericton, N.B., Canada  E3B 5A3
     Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
===============================================================================

   

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