[ntp:questions] ntp pool servers disappear

William Unruh unruh at invalid.ca
Wed Jun 23 18:05:04 UTC 2021


On 2021-06-23, Jim Pennino <jimp at gonzo.specsol.net> wrote:
> William Unruh <unruh at invalid.ca> wrote:
>> On 2021-06-23, Jim Pennino <jimp at gonzo.specsol.net> wrote:
>> ...
>>>
>>> As for the USB GPS I was testing, it is called a VK-162 G-Mouse
>>> available from Amazon for $14, uses the Windows 10 native driver so it
>>> works with Meinberg ntp, and keeps the time within single digit
>>> milliseconds without any other servers.
>> 
>> Looks like a nice cheap receiver-- no PPS I assume so that accuracy,
>> after correction for the NMEA delay is probably in the 10s of ms, not
>> their claimed microsecond. But certainly for most people ms is more than
>> enough accuracy.
>> 
>
> Correct, no PPS.
>
> From short term (day or so) testing looking at peerstats data:
>
> Samples: 3914
> Avg offset: -0.00190137
> Std dev:    0.00921412

The problem is that that does not test the accuracy, just jitter. Ie,
the time could be off by a century, but it is repeatable, so ntp says
that the offset and standard deviation is small. 
You need to compare with another time source that is known to be more
accurate than yours. Typically nmea signals are delivered late and the
length of the signal is long delaying things still more, expecially if
you choose an nmea sentence which reports lots of information, not just
the time. 
>
> ntpq typically shows offset and jitter at about 1 and the satellites in
> view are usually 14 or more with the puck in a window sill.

And that means that the processor in the gps receiver has to work harder
and delays the report of the NMEA even longer. Now of course you may not
care -- 100ms may be perfectly acceptable (It is far more accurate than
a wrist watch for example) and then my comments are entirely irrelevant.
If you want higher accuracy however, then they start to become relevant. 
Hook it up to the network and use some of the low stratum sources to get
the time. That should be accurate probably to better than a ms. That
will allow you to calibrate your gps delay and tell ntp to subtract the
persistant offset from the gps signal, and improve your accuracy. 

Again that is only important if you have some reason to care. Again, if
accuracy to the nearest second is good enough, ignore this.

>
> That is from a linux PC where there are more tools for testing things.
>
> The reason I bought it is that it is advertised to work with the Windows
> native USB driver, which produces a virtual com port, which makes it
> usable with Meinberg ntp without any other drivers or software.
>
> I have yet to do any Windows testing other than to verify it does work,
> but I see no reason why Windows would be much different.

Probably not. But again, testing it against some good network ntp
sources should give you an idea of how well it is doing, if that is
important. 
Of course we all like our stuff to be better than others. My system,
using pps from a gps is probably goot to a few microseconds. Do I need a
few microsecond accuracy. No. Even ms accuracy would be way more than  I
need. But I like seeing how far I can push stuff. My only defence is
"its a hobby".

>
>



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