[ntp:questions] Re: Effect of Antenna Cable Delay

Tim Shoppa shoppa at trailing-edge.com
Sat Sep 27 18:50:06 UTC 2003


hmurray at suespammers.org (Hal Murray) wrote in message news:<vn9mlpq6nq9a03 at corp.supernews.com>...
> >    Well, with GOES, the satellite is geosynchronous, so the distance
> >between the satellite and the receiver is constant. The only thing you can't
> >correct for is the slight variability in the speed of light over the portion
> >of that distance that's through the atmosphere.
> 
> How much does the atmospheric delay change?  Is there any way
> to correct for that?  (Say by using the outside temperature,
> or the time of day.)
> 
> How stable is the actual satellite orbital position?  I think
> they wander around slighty.  Is that significant for accurate timing?

Yes, it is significant.  For the GOES satellites, the timecode isn't
actually generated on the satellite, it comes from the base station (for
at least one of the satellites the base station is Wallops Island... maybe
the one over the Western US is controlled from there too.)  So the relevant
delay is the time from Wallops island to the satellite and back to the
base station.  This means that any propogation variation is roughly
doubled.

Even taking into account the propogation delay for the current satellite
position the best accuracy you can get is something like 10 microseconds.
In my experience, the vast majority of GOES receivers are not set up
properly (admittedly it's a lot harder than setting up a GPS receiver)
and I see day-to-day offsets in the several millisecond range.

In any event, the GOES time service has recently
become officially "unsupported" but the receivers I interface with are
about as [in]accurate as they ever were.

Tim.



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