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<[email protected]> 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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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.
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