[ntp:questions] Accuracy of GPS device

Miguel Gonçalves mail at miguelgoncalves.com
Thu Sep 1 23:01:46 UTC 2011


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.

 > I have another machine (Linux, CentOS 5.6) that is a client to these
> stratum
> > 1 FreeBSD machines. Here's the relevant information:
> >
> > $ ntpq -pcrv 10.0.2.2
> >      remote           refid      st t when poll reach   delay   offset
> >  jitter
>
> >==============================================================================
> > +10.0.2.10       .GPS.            1 u  211  256  377    0.159   -0.139
> > 0.350
> > *10.0.2.9        .GPS.            1 u   71  256  377    0.166   -0.136
> > 0.468
>
> That is a huge offset for being on the same lan, and for being only
> .15ms away.


It's really strange... I am getting on another LAN connected to this one
these values...

$ 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?

Here in Portugal our Time Dissemination Authority has two stratum 2 servers.
One of them shows this:

$ ntpq -p ntp02.oal.ul.pt
     remote           refid      st t when poll reach   delay   offset
 jitter
==============================================================================
*ntp01.oal.ul.pt .GPS.            1 u  113  128  377    5.125   -0.263
0.320
 ntp03.oal.ul.pt .INIT.          16 u    - 1024    0    0.000    0.000
0.000
-ntp04.oal.ul.pt 194.117.9.138    2 u   57  128  377    0.377   -0.056
0.080
+ntp05.oal.ul.pt .GPS.            1 u   62  128  377    0.296    0.128
0.058
+ntp06.oal.ul.pt .IRIG.           1 u   56  128  377    0.310    0.104
0.045

Assuming ntp04, ntp05 and ntp06 are on the same LAN I see offsets higher
than 100 us. Is it possible to decrease these numbers?

> tick# ntpdate -p8 -q tock
> > server 10.0.2.9, stratum 1, offset -0.000004, delay 0.02577
> >  1 Sep 10:23:45 ntpdate[3537]: adjust time server 10.0.2.9 offset
> -0.000004
> > sec
>
> That probably says more about the symmetry of the path than the offset
> of the machine.


OK. But this is a 24 port Gigabit switch from Cisco. I wouldn't expect
asymmetry but it could be.

 > Are my GPS clocks OK? Does this happen due to the network latency? Are my
> > stratum 2 servers OK?
>
> Your GPS seems to be consistant with each other ( that is all one can
> say without another time source to compare them to). Your offsets are
> large compared with the delay times ( was the machine recently heated
> up due to working harder?)


All these machines sit in a room temperature controlled at 20 ºC. 10.0.2.2
is a backup server that just does some work every hour but nothing huge.

10.0.2.2 has been running for quite a while and it doesn't seem to get lower
offsets. Could it be because it's running Linux? I've heard Linux is not as
good as FreeBSD for time keeping.

Thanks!

Regards,
Miguel



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