Re: [chrony-users] Is my GPS receiver really less accurate than NTP servers?

[Steven Sommars <stevesommarsntp@xxxxxxxxx>]

Network delay asymmetry can also be introduced by local equipment.  The scatterplot below shows the round-trip delay between an NTP client and a nearby NTP server.

on my fiber internet service 1) at the local NTP client and 2) just above my home router.

If the local client and the remote NTP server both have accurate time it can be interesting to compare the request delay (client -> server) to the response delay (server -> client).

[Asymmetric delays are common]

This paper may be of interest.

On Sat, Apr 16, 2022 at 2:23 AM Rob Janssen <chrony-users@xxxxxxxxx> wrote:

When you have a systemic offset of external services vs your local clocks, it is often caused by asymmetric network delay.
E.g. when using VDSL or Cable, it may well be that your uplink and downlink rates are different, and also the error correction parameters are different.
In that case all your external servers get offset by some value, and it is the GPS time that is right, not those 20 external references.

I have seen that coming and going on my line, over the years.  The telecom operators sometimes decide it is better to have reliability than low roundtrip time and enable "long interleaving" and similar parameters, often only in one of the directions.  But then later they find themselves in a contest for low roundtrip times in the gaming world, and they change that again.
For quite some time I had an offset of a few ms in my VDSL line and even tweaked the config for it.  But at the moment it is quite symmetric.

But indeed, also check the pulse width of the PPS signal to see if it happens to be the same as your offset.  If so, change the PPS edge.

Rob

On 4/16/22 06:24, Dominick C. Pastore wrote:
>
> The one thing that does seem a little suspicious is that the offsets for all the servers fall pretty uniformly between about 0 and +5 milliseconds. That seems a little too coincidental, like maybe 5ms is the granularity of the timer used to timestamp the packets or something. But, admittedly, I have no other reason to believe the measurements aren't correct, and that sounds like an unlikely explanation. I suppose it's possible the PPS from the GPS receiver has a constant 2.5ms delay from somewhere, but I'd be a little surprised (it's not a timing GPS, but the datasheet still quotes a PPS accuracy within 60ns).


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