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

Darren Dunham ddunham at redwood.taos.com
Tue Jan 9 16:10:09 UTC 2007

markus.juenemann at gmail.com wrote:
> 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.

Well, there's an angle factor as well.  If an individual satellite were
on the line between the receiver and the assumed location (and therefore
very near the horizon), then the error would be equivalent to the offset
distance.  But for any other location, the difference should be reduced
by a factor of cos*theta.  So the assumed time offset from an
approximately overhead satellite will be significantly less.

> 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.

I think that depends a great deal on the exact unit you have and its
behavior.  What does it do in zero-d when multiple satellites are
received and they are in conflict (due to position error)?  

Depending on the site, how likely is it to require a low-horizon

You might try sci.geo.satellite-nav as well.  Someone there might have
more information about how your unit would react.

Darren Dunham                                           ddunham at taos.com
Senior Technical Consultant         TAOS            http://www.taos.com/
Got some Dr Pepper?                           San Francisco, CA bay area
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