[ntp:questions] Accuracy of GPS device

Miguel Gonçalves mail at miguelgoncalves.com
Fri Sep 2 00:13:05 UTC 2011


Comments bellow...

On 2 September 2011 00:47, unruh <unruh at wormhole.physics.ubc.ca> wrote:

> On 2011-09-01, Miguel Gon?alves <mail at miguelgoncalves.com> wrote:
> > Hi!
> >
> > Thanks for your reply. My comments bellow.
> >
> > On 1 September 2011 18:24, unruh <unruh at wormhole.physics.ubc.ca> wrote:
> >
> >> On 2011-09-01, Miguel Gon?alves <mail at miguelgoncalves.com> wrote:
> >> > Hi all!
> >> >
> >> > I have two internal FreeBSD with GPS receivers attached (Garmin 18
> LVC:
> >> > 10.0.2.10 / Sure Evaluation Board:10.0.2.9). Both machines are on the
> >> same
> >> > LAN segment (VLAN).
> >> >
> >> > For redundancy, I've configured a Cisco switch as a stratum 2 server.
> >> Here's
> >> > the relevant information:
> >> >
> >> > $ ntpq -pcrv 10.0.2.254
> >> >      remote           refid      st t when poll reach   delay   offset
> >> >  jitter
> >>
> >>
> >==============================================================================
> >> > +ntp0.as34288.ne .PPS.            1 u  814 1024  377   72.750
> -1.084
> >> > 0.780
> >> > +canon.inria.fr  .GPSi.           1 u  399 1024  377   55.110
>  0.218
> >> > 0.400
> >>
> >> What are those machines? You have names rather than IP addresses.
> >> Are they your pps machines?
> >
> >
> > No. This is a stratum 2 server and it gets the time from stratum 1
> servers
> > thus the names and not IP addresses.
>
> What I am asking is what the mapping is between these names and the
> numbers you have later. I assume that some of those names are the same
> machine as the IP addresses you list below but We do not have that
> infomation.
>

Which numbers?

10.0.2.254 is a stratum 2 server that gets time information from outside
servers (I've included the information). canon.inria.fr is a server in
France that has a GPS clock receiver AFAIK.

10.0.2.254 is just a Cisco switch that I set up internally for comparison to
the the stratum 1 servers (10.0.2.9 and 10.0.2.10).

10.0.2.2 is another server (Linux, CentOS) that gets time from 10.0.2.9,
10.0.2.10 and 10.0.2.254. I've written all this before. I thought I was
clear enough.


> > $ ntpq -p 10.0.99.99
> >      remote           refid      st t when poll reach   delay   offset
> >  jitter
>
> >==============================================================================
> > *10.0.2.10       .GPS.            1 u   21  256  377    0.173    0.196
> > 0.008
> > +10.0.2.9        .GPS.            1 u   93  256  377    0.175    0.191
> > 0.014
> > +10.0.2.254      81.94.123.16     2 u  149  256  377    0.583   -6.884
> > 0.152
> >
> > This is a FreeBSD embedded PBX machine running Asterisk. The machine is
> > mostly idle. What kind of offsets should I get with local machines?
>
> in the 10s of usec range max. Certainly less than the delay.
>

tens of usec is good... Anyone here which can explain why NTP isn't getting
that?


>  > Assuming ntp04, ntp05 and ntp06 are on the same LAN I see offsets higher
> > than 100 us. Is it possible to decrease these numbers?
>
> Sure. all my systems have offsets in the 10us range--  on the same lan
> as my time server.
> Mind you I do use chrony, not ntpd but even ntpd should be in a few 10s
> of usec.


Can ntpd really get there? I'll try to query some good public servers to see
what others are getting...

All my machines are Linux machines. Linux is fine for timekeeping.
>

OK. No criticism intended. I'm just a BSD guy.

Thanks!

Miguel



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