[ntp:questions] NTP Performance on WINNT

Richard B. Gilbert rgilbert88 at comcast.net
Mon Jul 20 16:36:53 UTC 2009

David J Taylor wrote:
> Richard B. Gilbert wrote:
> []
>> Assuming a GPS *TIMING* receiver, and that it can be installed with
>> the antenna having a good view of MOST of the sky, you can expect
>> results that are *almost* as good as the atomic clocks on board the
>> satellites!
>> You do need to run a "site survey" to establish your location as
>> exactly as possible!  Once your location is known, the calculation of
>> the "speed of light delay from your selected satellite to you can be
>> done quickly and easily.
> I didn't have such difficulties when using today's sensitive GPS devices 
> - e.g. GPS 18x LVC:
>  http://www.gpsw.co.uk/details/prod2402.html
> Having a view of, say, a 180-degree arc of southern sky is probably good 
> enough.  I have mine indoors, sitting on top of a PC on the top floor of 
> my building.  No separate antenna.  The earlier GPS 18 LVC sits on the 
> roof.  The GPS determines its location all by itself - no site survey 
> required.  Microsecond accuracy.
That is what a "site survey" is.  The GPS determines its location a few 
hundreds of times over the course of a day and does a "least squares" 
calculation to get a reasonable approximation of your latitude and 

Cellular phone base stations do a more extended site survey, thirty days 
instead of one.  This gives them a MORE reasonable approximation.  When 
I did my site survey (24 hours) the locations plotted a locus about 300 
feet long in an East-West direction and with a width of about thirty feet.

If I had had the patience and if I had required sub microsecond accuracy
I could probably have trimmed that locus further.  . . . .

It is enough, for me, that my little herd of computers agree as to 
approximately what time it is.

More information about the questions mailing list