[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
>> 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:
> 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.
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