[ntp:questions] Accuracy of NTP - Advice Needed

Chris Albertson albertson.chris at gmail.com
Fri Dec 30 01:26:49 UTC 2011


People really do need to read the paper rather then guess.

Yes, As some have said, normally GPS is not accurate enough for this
level of work but they are not using GPS in the normal way.  What they
do is agree on ONE specific GPS satellite that happens to be visible
at both locations.  Each site measures the difference between its own
Cesium atomic and the clock aboard the GPS sat.    They need to
account for Doppler shift, and path delay through the atmosphere.
They can double check they got the path delay right by watching a
second GPS that is in common view.  The method works well because
several sources are self canceling.

Someone said that were simply using better and more expensive GPS
receivers.  No,  that is NOT the case.  (BTW very good GPS receivers
are not expensive, cheaper than consumer car navigation GPSs because
they don't need a graphic display, map data or a pretty box.   (For
example, these are only $60 each if you call and ask:
http://www.synergy-gps.com/images/stories/pdf/m12mt_brochure.pdf )

This short introduction describes the method used to sync two clocks
using "GPS common view"
http://tf.nist.gov/time/commonviewgps.htm.
It is short and easy to read.




On Thu, Dec 29, 2011 at 7:26 AM, Terje Mathisen <"terje.mathisen at
tmsw.no"@ntp.org> wrote:
> Danny Mayer wrote:
>>
>> No, they use synchronized Cesium atomic clocks for time accuracy. GPS is
>> only used to get a fix on the location and I'm not sure that 10's of
>> centimeters is good enough for what they are trying to prove. I'd have
>> to look closely at the methods used and the data to even have a clue as
>> to what is needed and I have touched that stuff in years.
>
>
> Danny, how do you think they keep those atomic clocks synchronized?
>
> How do they _verify_ that they actually stay in sync (to a single-digit ns
> level) over the entire length of the experiment(many months)?
>
> Even Hydrogen Masers won't give you that performance over a year or so, you
> have to have some way to sync them either to each other or to UTC.
>
> Terje
> --
> - <Terje.Mathisen at tmsw.no>
> "almost all programming can be viewed as an exercise in caching"
>
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-- 

Chris Albertson
Redondo Beach, California


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