[ntp:questions] NTP Performance on WINNT

Richard B. Gilbert rgilbert88 at comcast.net
Mon Jul 20 23:24:28 UTC 2009

Unruh wrote:
> "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 
>> longitude.
>> 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. 

I did this years ago and I think it might have been while DOD was still 
encrypting the low order bits in the data.

It might be faster and easier today but what I have works and I'm not 
going to mess with it!

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