[ntp:questions] NTP Performance on WINNT

Unruh unruh-spam at physics.ubc.ca
Mon Jul 20 22:27:18 UTC 2009

"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

>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.

Yee gads. My "survey" in my location gives me a location whose bounds are about 3mX3m scatter
at worst. The centre  is about 4m off from where Googleearth claims my rooftop location is, but that could well be
Googleearth's problem. This is with a Garmin GPS18 with a view of about 50% of the full sky.
(roughly 50 N, 120 W) I am surprized your error box is that large. 


>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.

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