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

markus.juenemann at gmail.com markus.juenemann at gmail.com
Wed Jan 10 04:45:35 UTC 2007


Thanks to everyone for their very useful replies. Unfortunately I won't
be able to perform some of the tests, e.g. comparing two GPS receivers
and their 1 PPS output, because of their remote and distant location.
Also the comment about the satellites position in relation to the
receiver explains how the time error might possibly much smaller than I
thought.

I am still trying to find out how the GPS receiver (a Trimble
Thunderbolt) behaves in "stationary" mode and under what circumstances
it reports or auto-corrects errors. The error reporting unfortunately
gets masked by the ntpd process.

It might be (I need to verify this!), that the Thunderbolt stops
providing timing information if it is in "non-stationary" mode and can
see too few satellites (less than three). Some of our sites actually do
get into situations where they can only see 2 satellites. This might be
due to bad positioning of the GPS antenna and also must be
investigated.

Anyway, again thanks heaps for your replies

Markus

Darren Dunham wrote:
> 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
> satellite?
>
> 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
>          < This line left intentionally blank to confuse you. >




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