[ntp:questions] How to determin hardware latency for PPS offset given simple tools.

unruh unruh at invalid.ca
Sat May 25 05:06:48 UTC 2013

On 2013-05-24, Paul G <bodosom at gmail.com> wrote:
> I have four GPS receivers into four different computers.  I've assumed that I

GPS receivers are not particularly good time sources UNLESS you use PPS. 
If you use PPS they should all agree to the 1micro second level. 
You should NOT need time1. If youjust use the serial data, you will be
lucky to get millisecond agreement, and that only with a lot of work.
Ie, if you are going to set up gps, set up pps. 

Since you seem to be using it, you should get us accuracy from them
without any offset. grabbing times via the net from the those computers
will be dominated by network issues. 

> should be able to get small offsets between them but to do so I need
> to set time1.  If that's the correct approach how do I pick the "right"
> system to be the benchmark.  So far I've just been assuming that
> the purpose built M. Tharp server should have the lowest latency but I'm not sure -- in any case, at present, I can't adjust it.
> Each machine in the truncated table below (except 210) has a time1
> value derived in an ad hoc fashion.
Are they a secret?

> l     r    refid       st   delay   offset  jitter
> 1   o22.0  .PPS.        0   0.000   -0.001   0.001
> 2   +244   .PPS.        1   0.068    0.000   0.006
> 2   *210   .GPS.        1   0.514   -0.008   0.136
> 2   +192   .PPS.        1   0.455    0.008   0.032
> I feel as if this topic should be clearly explained somewhere but I've been
> unable to find it.

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