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

jimp at specsol.spam.sux.com jimp at specsol.spam.sux.com
Wed Aug 31 16:44:14 UTC 2011

unruh <unruh at wormhole.physics.ubc.ca> 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. 

Goto http://en.wikipedia.org/wiki/Global_Positioning_System

Look at References #81 and follow the links to the paper.

On page 31:

"Since USNO has been successful in predicting UTC to within about 10 ns,
combining these two independent error sources yields a real-time potential
uncertainty for UTC available from GPS at about the 14-ns level."

There is much detail on how he gets to that number.

This is from 1997, so some of the numbers may be better by now, in particular
the ability of USNO in predicting UTC.

However, only 19 of the 31 satellites in orbit as of May 2010 are post 1997.

Jim Pennino

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