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

shoppa at trailing-edge.com shoppa at trailing-edge.com
Tue Jan 9 18:58:39 UTC 2007

On Jan 9, 6:18 am, markus.juenem... 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.

For receivers that can be "programmed" with their initial position,
being 100km off will prevent the GPS from ever really locking onto a
satellite constellation. The satellite timing with relation to the
almanac will not be making sense.

With all modern timing receivers, such a receiver will report back that
it does not have a good lock, but it may return a time that is
semi-plausible. The time may be entirely derived from a ticking crystal
rather than any recent GPS reading.

> 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.

It is important to determine that the 512bps clock synchronizer will
shut down the transmitter if the GPS unit is reporting "never locked"
or "unlocked too long".

If the synchronization comes from say a PPS output, then putting two
units side-by-side, one correctly configured and one incorrectly
configured, will give you some information. Typically if you cold-start
a GPS the PPS will start ticking about once per second at a rather
random phase, but then when it locks the PPS phase jumps to be correct.
If it never locks it still might be ticking the
roughly-once-a-second-but-out-of-phase PPS.


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