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

Chris Albertson albertson.chris at gmail.com
Wed Aug 31 18:42:35 UTC 2011


positional accuracy is

On Wed, Aug 31, 2011 at 9:08 AM, 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.


positional accuracy is better than 5M.  The Motorola Oncore series of GPS
timing receivers will perform an automatic  survey.  The reciever runs for
about 30 minutes or longer and computes it's possition.  You can watch it's
estimated possition error slowly go down until it is about a foot.

Then after the automatic survey the timing error is reduced.  remember that
for time keeping we assume the antenna remains bolted down and never moves,
 So after a long period of time the uncertaintly of the antenna location can
be removed.     Surveyor use GPS and can get cenimeter level accuracy but it
takes all day or longer to collect enough data.  For nanosecond level timing
you only need about 1/4 meter and that be be done with  an hour of so of GPS
data.

Then along those same lines some people are gets MUCH better timing then 1nS
by maintaining a very stable local oscillator and phase locking it to the
GPS' PPS using a very long time constant.   This has the effect
of averaging out errors in the PPS.     I suspect the GPS itself might be
long time constant to clean up the PPS.

You 5M figure, I think might by an instantaneous error circle.   That
applies to a navigation GPS receiver that can't assume a fixed location.
 A timing receiver has the ability to average many tens of thousands
of positional fixes.

And if (BIG if here) you don't care about the phase but only care that the
PPS happens at exactly one second intervals you don't NEED to know the
antenna location but only that it is not moving

One last point.  Almost all timing recivers allow the operator in INPUT a
surveyed location.  I could hire a survey company to tell my the location of
the antenn and then remove all uncertainty.   I actually did this, The city
required me to have a survey before I could pour concrete for a
house remodel job.  (I had to keep 5 feet clearance from the property line
and there was question about were the line was)  So I had a survey crew out
to the house and they worded to the 1/100 of a foot.  So by chance I have
means to verify the GPS. (I wouud never have paid for the surveyors if not
for the city building department insisting on it)  The answer is that my
cheap $18 Motorola Oncore UT+ can find it's location to about 1/3rd of a
meter using about 60 minutes of GP data and after that position does not get
better even after days of averaging.






Chris Albertson
Redondo Beach, California



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