# [ntp:questions] Timing GPS recommendations

Chris Albertson albertson.chris at gmail.com
Tue Aug 21 16:41:32 UTC 2012

On Tue, Aug 21, 2012 at 7:33 AM, Brian Utterback <brian.utterback at oracle.com
> wrote:

> 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.
>
>
I think it comes to +/-3 times the square root of the number of
measurements.  Or something proportional to that.  You are correct for the
case were you only do it once.

An example is you want to measure the location of a hole 500 feet above the
ground where someone is droping bird feathers.  You could look straight up
from where one fell but they flutter randomly as they fall.   But if you
waut until 1,000 have fallen you can compute a centroid of the scatter.
You r estimate of the centriod gets better the more feathers you can
measure.  Accuracy is pproportional to the square root of the number of
feathers

With a clock once you get the RATE set.  Then we can measure the phase many
times and find a "mean phase".   You are right in that there is a limit.
This is because you can't get the rate set perfect so the number of
samples in your mean is limited.

I think in the analog world Phase Locked Loops do about the same thing.  A
PLL can sync two oscillators very well even even with a noisy channel
between them it's used for "everything" mostly clock recovery in serial
data.  It in effect figures out the phase by averaging over a great many
cycles.  I think a PLL is proff that we CAN synchronize phase to much
better than the jitter in the communicant path.

Chris Albertson
Redondo Beach, California