[ntp:questions] Accuracy of GPS device

Miguel Gonçalves mail at miguelgoncalves.com
Thu Sep 1 10:30:23 UTC 2011


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
*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
+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:

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
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?

Can anyone help?

Thanks!

Miguel



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