[ntp:questions] Timing GPS recommendations

Brian Utterback brian.utterback at oracle.com
Tue Aug 21 14:33:54 UTC 2012


You seemed to have missed the point that unruh was making. Sure you can 
find the mean of the round trip time, but the accuracy you get setting 
the clock can never be better than the jitter you get on those times. 
Say the mean is 50 seconds, but the jitter is +/- 3 seconds. It doesn't 
help to buy a clock that measures in tenths of a second instead of whole 
seconds, your accuracy is still going to be +/- 3 seconds.

If the distribution of that jitter is normal, you might be able to 
compensate by doing many samples, but you can't know what the 
distribution is.

On 08/21/12 02:30, Chris Albertson wrote:
> Actually can CAN set a remote clock very accurately.  This is how NTP works
> and what it idoes best.    NTP uses the Internet to synchronise clocks to
> MUCH higher precision than the delay in Internet messages.
>
> Try this:  You try to adjust the rate of an old mechanical clock by
> adjusting a screw that says "faster/slower".  But your "reference clock" is
> across the street and you have to walk back and forth.   This can actually
> work very well if you can wait a few months.   First you walk across the
> street and back and measure the time it takes.  Do this many times and keep
> the mean and sigma.  Now you can set your clock to what you read on the
> reference clock but add in 1/2 the mean round trip time.   Now wait 100
> days.   Go and read the reference clock and come back add 1/2 the mean and
> note the error.  Let's say your clock gained one minute.   Now you know the
> rate of your clock relative to the standard to better then one part in
> 100,000.  We adjust the screww to remove some fraction of the observed
> error.  Then we wait aother 100 days and do it again.  Eventually the error
> in the rate gets small.    NTP uses this same method but repeats it
> thousands of times.
>
> The accuracy with which you can set your remote clock by walking down the
> street depends on measuring the mean time to wak down the street.  But we
> are using a computer so measure many tests
>
> On Mon, Aug 20, 2012 at 12:17 PM, unruh<unruh at invalid.ca>  wrote:
>
>>   Imagine that you have a wristwatch
>> that is accurate to the second but your send out messangers by foot to
>> deliver the time to your friend across town. Do you really think that
>> buying a new wrist watch is the way to improve the time your friend
>> gets?
>
>
> Chris Albertson
> Redondo Beach, California
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> questions mailing list
> questions at lists.ntp.org
> http://lists.ntp.org/listinfo/questions


-- 
blu

Always code as if the guy who ends up maintaining your code will be a
violent psychopath who knows where you live. - Martin Golding
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Brian Utterback - Solaris RPE, Oracle Corporation.
Ph:603-262-3916, Em:brian.utterback at oracle.com



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