[ntp:questions] Accuracy of GPS device

unruh unruh at wormhole.physics.ubc.ca
Thu Sep 1 17:24:30 UTC 2011


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?




> *ntp1.inrim.it   .CTD.            1 u  677 1024  377   64.540   -0.284
> 0.080
> +ntp-p1.obspm.fr .TS-3.           1 u  205 1024  377   57.040    1.004
> 1.450
> -metasweb01.admi .HBGi.           1 u  841 1024  377   74.600   -4.075
> 0.200
> associd=0 status=0600 leap_none, sync_ntp, no events, unspecified,
> system="cisco", leap=00, stratum=2, rootdelay=64.540,
> rootdispersion=1.370, peer=7253, refid=193.204.114.232,
> reftime=d209d7f4.6500bcf0  Thu, Sep  1 2011 11:07:48.394, poll=10,
> clock=d209da99.b57d6483  Thu, Sep  1 2011 11:19:05.708, phase=-0.628,
> freq=9.60, error=0.95
>
> 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. 


> +10.0.2.254      193.204.114.232  2 u   22  256  377    0.560    1.557
> 0.082
>  li298-159.membe 217.20.44.6      2 u   69  256  377   51.718   -1.200
> 0.919
> associd=0 status=06f4 leap_none, sync_ntp, 15 events, freq_mode,
> version="ntpd 4.2.2p1 at 1.1570-o Sat Dec 19 00:58:16 UTC 2009 (1)",
> processor="i686", system="Linux/2.6.18-238.19.1.el5", leap=00, stratum=2,
> precision=-20, rootdelay=0.166, rootdispersion=11.880, peer=24216,
> refid=10.0.2.9, reftime=d209da8a.8b5bd2c6  Thu, Sep  1 2011 11:18:50.544,
> poll=8, clock=d209dad1.3849eae8  Thu, Sep  1 2011 11:20:01.219, state=4,
> offset=-0.033, frequency=44.545, jitter=0.653, noise=0.030,
> stability=0.007, tai=0
>
> I've checked 10.0.2.9 and 10.0.2.10 and they are usually less than 5 us
> apart:

Fine. That is what your results from 10.0.2.2 suggest as well. 

>
> 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. 


> tick# ntpdate -p8 -q tock
> server 10.0.2.9, stratum 1, offset -0.000004, delay 0.02577
>  1 Sep 10:23:47 ntpdate[3538]: adjust time server 10.0.2.9 offset -0.000004
> sec
> tick# ntpdate -p8 -q tock
> server 10.0.2.9, stratum 1, offset -0.000003, delay 0.02577
>  1 Sep 10:23:49 ntpdate[3539]: adjust time server 10.0.2.9 offset -0.000003
> sec
> tick# ntpdate -p8 -q tock
> server 10.0.2.9, stratum 1, offset -0.000005, delay 0.02577
>  1 Sep 10:23:50 ntpdate[3540]: adjust time server 10.0.2.9 offset -0.000005
> sec
>
> What I find strange is the offset of the stratum 2 server above (10.0.2.2)
> to the other stratum 2 machines (Cisco and external one that is 50 ms away).
>
> 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?)


>
> Can anyone help?
>
> Thanks!
>
> Miguel




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