[ntp:questions] garmin 18x and linux

David Lord snews at lordynet.org
Fri Sep 2 13:13:32 UTC 2011

Greg Hennessy wrote:
> On 2011-09-02, Chris Albertson <albertson.chris at gmail.com> wrote:
>> As I said,  Without a PPS signal you will not get reasonable
>> results.
> I would appreciate advice on solving the problem I have, not buying
> the equipment you wished I have. I don't need microsecond timing. I
> don't need millisecond timing. I have a need to have my clock set to
> about a second, which a NMEA only gps should be able to solve.
>> The problem with NMEA only setups is that (I think) the firmware takes
>> non-determinic time to convert its internal data to ASCII.  This means the
>> sentence is never output at the exact second "tick".   This error is enough
>> to cause the GPS to fail NTPs clock selection algorithm.
> With only one source, how do I verify that your suspicion is correct? 
> ntpq -p shows reach never changing from zero.

You need to check the gps on a pc with ntp working correctly
and having a low offset.

I have four GPS, BR304, Garmin 18x-LVC, Sure and Motorola
Oncore. I've not yet coupled up the Oncore. The NMEA output
of the BR304 has too much variation of >> +/- 100ms and I
gave up on use as ntp timesource. The Garmin and Sure are
probably around +/- 50ms and could be used by ntpd. The
3.70 firmware for the Garmin would be a big improvement but
but it has PPS which gets me within a few us same as with
the Sure.

If you can't couple it to an internet connected pc, the next
best is to use the computers local clock as reference with
the gps nmea marked as 'no select' and running loopstats,
peerstats and summary utilities from your ntpd distribution.


>> OK, no Internet at the observatory.  But try using pool servers while you
>> are setting up and debugging.  You will need additional ref clocks to verify
>> your GPS. 
> My problem is not that my GPS reported time is bad. My problem is that
> ntp is not showing it is processing any of the data. Running ntpd with
> the -n and -d flags shows that input from the GPS goes into ntp, but
> reach never changes from zero.
>>> On 2011-09-02, Chris Albertson <albertson.chris at gmail.com> wrote:
>>>> Also, don't expect any reasonable result until you connect the pulse per
>>>> second to NTP.
>>> There is no PPS for the GPS18x PC.
>>>> You only listed one line from the config file.  Post more of it.   And
>>>> finally it helps a lot if you can add some pool servers to the config
>>> file.
>>> The only other line is the driftfile line. The computer is meant for
>>> control of a telescope and observatory, and isn't on the
>>> internet. Adding pool servers won't help for that reason.
>>> Given that ntp in debug mode shows gpsread showing NTP at least sees
>>> the GPRMC messages, can anyone sugguest why NTP never syncs? Is there
>>> additional debug flags that would be useful?
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