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

Richard B. Gilbert rgilbert88 at comcast.net
Tue Jan 9 14:56:45 UTC 2007

markus.juenemann at gmail.com wrote:

> 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
> position!

In practice the single satellite usage should never be necessary.  There 
were, at last reports, some twenty-seven Navstar (GPS) satellites in 
orbit.  The likelihood of only one being visible is infinitesimal!! 
There are normally eight or nine above the horizon and five or six well 
above the horizon.

> 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.

I believe that both the magnitude and sign of the error would depend on 
the position of the satellite.   For certainty, consult a mathematician. 
  (I can usually count to twenty with my shoes off)!

Some GPS timing receivers; e.g. the Motorola Oncore receivers, offer a 
feature called TRAIM or Time Receiver Autonomous Integrity Monitoring in 
which excess satellites are used as a cross check.

It's customary, when setting up such sites, to have the receiver do a 
"site survey" which means that it calculates positions every second for 
anything from two hours to thirty days and does a least squares fit to 
determine the most probable position.  A two hour site survey might 
leave you with an uncertainty of two or three hundred feet.  A thirty 
day site survey would probably refine the position to within two or 
three feet.

If you actually did site surveys, you are probably all right.  If 
someone read the latitude and longitude off a map, you should probably 
do a site survey.

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