[lug] NTP with a GPS

[George Sexton]

I hate to say it, but you really have the wrong unit. If you want time from
a GPS unit you needs PPS output. Without PPS, you're only going to get +/-
several hundred milliseconds. There's just too much jitter in the arrival of
the timing message without PPS. This is why your NTP daemon is throwing out
both units. 

You should use a Garmin 16X because it has PPS output. Here's the output of
ntpq -p for mine:

     remote           refid      st t when poll reach   delay   offset
jitter
============================================================================
==
+time-B.timefreq .ACTS.           1 u   21  512  377   35.680   -0.237
0.545
+time-A.timefreq .ACTS.           1 u  480 1024  377   36.306   -0.555
0.648
-utcnist2.colora .ACTS.           1 u   75  512  377   36.542    1.298
1.180
-masql.mailarmor 192.43.244.18    2 u   31   64  377   14.540    6.851
2.345
*SHM(1)          .PPS.            0 l   16   16  377    0.000    0.002
0.003


The offset and Jitter for PPS is .002 milliseconds, or 2 microseconds.
Maximum offset and jitter is generally around 10 microseconds.

If it were me, I would use one unit with PPS.

Here's a pretty good reference. 

http://time.qnan.org/


The author used a 18LVC. I chose to use a 16x so that I could use a higher
voltage power supply and not worry about voltage drop on a longer cable run.


George Sexton
MH Software, Inc.
303 438-9585
www.mhsoftware.com


> -----Original Message-----
> From: lug-bounces at lug.boulder.co.us [mailto:lug-
> bounces at lug.boulder.co.us] On Behalf Of Gary Hodges
> Sent: Monday, April 25, 2011 12:45 PM
> To: Boulder (Colorado) Linux Users Group -- General Mailing List
> Subject: [lug] NTP with a GPS
> 
> I've been doing some testing, and have actually deployed at a couple
> sites, a Garmin GPS 18x to keep the time set for one computer.
> Everything was going OK, until the most recent case.  At this site I
> have the computer set up on the network, and had been keeping time set
> with NTP to another machine that uses GPS.  I don't know the physical
> location of that machine, or anything about it actually.  Due to
> network
> rules, that was the only time server available to me.  I hooked up a
> GPS
> to my machine, figuring two is better than one, but I may have invoked
> the adage "One clock is correct.  With two both are wrong."
> 
> I logged on today to see how my new set up was working, and I found
> that
> both time servers had an "x" preceding them when querying with  ntpq -
> p.
> 
>       remote           refid      st t when poll reach   delay   offset
>   jitter
> =======================================================================
> =======
> xSHM(0)          .GPSe.           0 l    8   16  377    0.000  -65.986
> 29.232
> x192.168.241.135 .GPS.            1 u  255  256  377    3.979  -11.009
> 18.461
> 
> My GPS is noted by GPSe.  If I comment either one out in the ntp.conf
> file, ntp works as expected.  That is I get an "*" preceding the time
> server.  My assumption is that with a stratum 0 and 1 server available,
> but with the difference in the offsets too great, it concludes neither
> can be trusted and both are stamped with x.
> 
> I have played around with the time1 parameter in the ntp.conf file to
> bring the offsets closer together, and that seems to work.
> 
>       remote           refid      st t when poll reach   delay   offset
>   jitter
> =======================================================================
> =======
> *SHM(0)          .GPSe.           0 l    2   16  377    0.000  -23.996
> 14.344
>   SHM(1)          .GPSe.           0 l    -   16    0    0.000    0.000
>    0.001
> +192.168.241.135 .GPS.            1 u  252  256  177    4.132  -27.314
> 12.543
> 
> Here it has my GPS as the preferred time server, and the other is
> indicated by the + as a high quality replacement candidate.  Even
> though
> I seem to have made it work, going forward I'm considering commenting
> one out as there appears to be potential issues with using two time
> servers.
> 
> Is anyone here able to verify or refute my assumption about using two
> time servers?  Accuracy to within one second is more than sufficient
> for
> my needs.
> 
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