unruh wrote:
On 2011-08-31, Uwe Klein<[email protected]> 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
--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"
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