[ntp:questions] Questions about joining pool.ntp.org

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Wed Aug 31 16:49:53 UTC 2011

unruh wrote:
> On 2011-08-31, Uwe Klein<uwe at klein-habertwedt.de>  wrote:
>> David J Taylor wrote:
>>> How does this square with those who claim 4ns from their GPS devices?
>> Pfft.
>> The defining document is rather old I guess. A lot happened in between.
>> ( I looked into GPS in my diploma thesis ~1987 and not much after that )
> The GPS sattelites with their onboard clocks, etc are also rather old.
> And I do not see how  you can get timing accuracies of 2ns when your
> positional accuracy is 5m. I Think that that 2ns is someone's
> advertising bunf that totally misunderstood the technical arguments and
> simply looked through the documents for the smallest time figure they
> could find without caring what it referred to. That document I mentioned
> talked about a gps receiver in 2003 which claimed 2ns accuracy. And was
> hundereds of ns different from a survey grade instrument.

The 2 ns figure is obviously bulls**t, at least as an instantaneous 
offset value:

The GPS documentation states that each sat is maintained within 10 ns of 
UTC(USNO) and that UTC(USNO) is also kept within 10 ns of "real" UTC, 
for a total uncertainty of about 14 ns.

Sampling all visible sats at once, assuming their errors are 
independent, could get you close to the UTC(USNO) reference clocks, but 
you will always have an unknown offset between that value and the treu 
UTC time.

(Since UTC is a paper clock, determined a month or so after the fact, 
there isn't even theoretically a way to track it exactly, the best you 
can do is to have a local atomic clock which is at least as good as the 
best of source clocks for the UTC ensemble.)

OTOH, claiming that the long-term averaged offset between GPS and UTC 
would fall below 2 ns might well be true! :-)


- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

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