[ntp:questions] Time accuracy in relation to position accuracy

markus.juenemann at gmail.com markus.juenemann at gmail.com
Tue Jan 9 11:18:10 UTC 2007

I have a real scenario problem regarding GPS/NTP time synchronisation
and since I am not such an expert I'd like to ask the GPS wizards here
for help.

As discussed in this group before there are GPS based reference clocks
which can be put into so called "fixed" or "stationary" mode. In simple
terms this means that the GPS will calculate its precise position only
during startup and not continuosly during operation. In return it will
happily provide GPS based time even if only a single satellite is in
view. Since the GPS receiver is stationary, it simply remembers its

This all works really well as long the stored position information is
accurate. I was wondering, how inaccurate the time provided by the GPS
receiver will become if the position information is incorrect by, for
example, 100km. Is it correct to assume that this is equivalent to the
time the GPS radio signal takes to travel 100km? If so, the time would
be wrong by approximately(!) 0.3ms.

This issue is actually related to a real problem we have with a GPS
synchronised simulcats radio paging network. For some unknown reason
(this might be non-technical, e.g. operator error) we had a GPS
reference clock in stationary mode with a position error of about
200km. If this would translate into a time error of at least 0.6ms this
would be sufficiently bad to corrupt messages transmitted
simultaneously by several paging transmitters at 512 bps (512 bps means
that in the most trivial encoding scheme one bit takes about 2.5ms to
transmit - 1000ms divided by 512).

The essential question is whether we have to check about 200 GPS
receivers for incorrect position data or whether this wouldn't really
cause any problem.

Thanks a lot


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