[ntp:questions] Linux, Garmin GPX-18X LVM & PPS

Paul Duncan pdu at noc.ac.uk
Wed Dec 7 16:58:15 UTC 2011


On Tue, 2011-12-06 at 19:31 +0000, unruh wrote:
> On 2011-12-05, Duncan, Paul A. <pdu at noc.ac.uk> wrote:
> > Hello,
> >
> > I am trying to set up an NTP server using the Garmin GPS-18 as the reference clock. I think I'm most of the way there, but I have a couple of questions.
> >
> > Firstly, here is the output from ntpq:
> >
> >      remote           refid      st t when poll reach   delay   offset  jitter
> >==============================================================================
> >  LOCAL(0)        .LOCL.          10 l 2028   64    0    0.000    0.000   0.000
> > *GPS_NMEA(0)     .GPS.            0 l   43   64  377    0.000  -10.969  11.575
> 
> GET RID OF LOCAL. Sorry to shout but so many distributions have totally
> incompetent instructions. LOCAL is the clock itself. It it is totally
> and utterly useless for keeping time. Setting a clock with itself as the
> reference will always say that the clock is exactly right, and will
> always be totally wrong. 

Okay, how do I get rid of local? Is it a configure option?

> You do not have PPS 

This much I now know :-(

> > I'm fairly sure PPS is working, because ppstest shows:
> >
> > trying PPS source "/dev/gpspps0"
> > found PPS source "/dev/gpspps0"
> > ok, found 1 source(s), now start fetching data...
> > source 0 - assert 1323090421.923486039, sequence: 8369 - clear  1323083494.562494878, sequence: 133
> 
> Notice your system  is out by 76 ms. 

How did you work that out?

> 
> > source 0 - assert 1323090422.923554857, sequence: 8370 - clear  1323083494.562494878, sequence: 133
> > source 0 - assert 1323090423.923625515, sequence: 8371 - clear  1323083494.562494878, sequence: 133
> > source 0 - assert 1323090424.923692379, sequence: 8372 - clear  1323083494.562494878, sequence: 133
> > source 0 - assert 1323090425.923762947, sequence: 8373 - clear  1323083494.562494878, sequence: 133
> >
> > So, I'm guessing it must be a problem with my ntp.conf. The top part of this is shown below:
> >
> > server	127.127.1.0	# local clock
> > fudge	127.127.1.0 stratum 10	
> 
> Why do you have that line there? Please get rid of it. It is useless.

I'm trying pretty much everything right now, since I'm desperate! The
current ntp.conf is:

server 127.127.20.0 prefer mode 2 minpoll 4 maxpoll 4
fudge 127.127.20.0 refid PPS0 flag1 1 flag2 0 flag3 1 flag4 1 time2
0.195

peer ntp.my-inbox.co.uk

tos mindist 0.250

statistics loopstats
statsdir /var/log/ntp/
filegen loopstats file loop type delay enable

restrict 127.0.0.1
restrict 139.166.0.0 mask 255.255.0.0
restrict default nomodify notrap nopeer


> >
> > #
> > # NTP server (list one or more) to synchronize with:
> > #server pool.ntp.org iburst
> 
> No harm in leaving the pool server in as a backup.

Okay, well I'll put that back in.

> >
> > # LinuxPPS: Garmin GPS-18
> > server 127.127.20.0 mode 0 prefer
> > fudge 127.127.20.0 stratum 0 flag1 1 flag2 0 time2 0.600
> 
> You have told it to use the nmea source. 
> Not the PPS. 
> Either use gpsd and set it up to use the shm refclock, or use the Atom
> PPS refclock as well. 

I'm a little confused here. According to:
http://support.ntp.org/bin/view/Support/ConfiguringNMEARefclocks
the NMEA driver looks after PPS0 without the need for a seperate
PPS/Atom driver... Are you saying this is wrong?

> > This message (and any attachments) is for the recipient only. NERC
> > is subject to the Freedom of Information Act 2000 and the contents
> > of this email and any reply you make may be disclosed by NERC unless
> > it is exempt from release under the Act. Any material supplied to
> > NERC may be stored in an electronic records management system.
> 
> And another of those stupid boilerplates which in fact destroy any
> usefulness then have by use in totally inappropriate situations. Any
> court would throw it out since it is obvious that the sender does not
> really believe it. 

Sorry. That is actually done at the server - I can't turn it off :-( I
do appreciate that it breaks the signature rules as well.

Best Regards,

Paul.


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