[ntp:questions] Accuracy of NTP - Advice Needed

jimp at specsol.spam.sux.com jimp at specsol.spam.sux.com
Sun Jan 1 23:53:39 UTC 2012


Danny Mayer <mayer at ntp.org> wrote:
> On 12/29/2011 8:38 PM, Dennis Ferguson wrote:
>> 
>> On 29 Dec, 2011, at 23:26 , Terje Mathisen 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.
>> 
>> Yes, they use GPS to compare the clocks to each other.
>> 
>> One of the articles I read even identified the GPS receiver they use.  I think
>> it was a Septentrio PolaRx3eTR PRO (or maybe the older model which that one
>> replaced).  Those receivers take a 10 MHz and 1 PPS reference in from the atomic
>> clock so that they can produce GPS carrier phase measurements with respect to
>> the local clock's time.  Making these measurements simultaneously at both
>> locations gives you data you can post-process to determine the time difference
>> between the two clocks, independent of the GPS system time.  The GPS signals
>> are used only as markers that can be measured at both locations.
>> 
> 
> They used Septentrio PolaRx2e GPS receivers in both places along with a
> Symmetricom Cs4000 Cs atomic clock. All of this raises additional
> questions for which I'd have to dig into the references for answers. For
> example, both ends are underground and they are likely to use heavy
> shielding around the sites of the source and target so how are they even
> getting a GPS signal through in the first place? Are they getting signal
> or did they set up an external antenna in which case they would have to
> also figure out the distance of the antenna from the receiver (which
> part of the antenna?). This is not an easy physics experiment and the
> errors involved can easily overwhelm the result.

Given the size of a GPS antenna, which part they measure from is down in
the noise level.

And yes, if you actually were to read all the documents available, you would
find the antennas are outside and they did measure the distance to the
antenna.

How else would they be able to know were the expirement was in relation
to a known point, i.e. the antenna, and the time delay down the coax if
they didn't make such measurements.

 

-- 
Jim Pennino

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