[ntp:questions] garmin 18x and linux

Chris Albertson albertson.chris at gmail.com
Fri Sep 2 04:50:47 UTC 2011

As I said,  Without a PPS signal you will not get reasonable results.  If
the GPS lacks the PPS get another GPS.  They are cheap.   Really nice units
with timing specs literally 100X better than the 18X can be bought on eBay
for $20 to $30.

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.

There are two classes of GPS.  The first is the more common one.  These are
"navigation receivers".  The second class are "timing receivers" these are
special units that can take advantage of the fact that they KNOW their
antenna is not moving.    Knowing your exact position allows better timing
solutions.  The best timing GPS have nanosecond level error.   You NMEA only
unit likely has millisecond level error which is about 6 orders of magnitude
worse.  To use an expression.  NTP has likely "voted it off the island".

I said there were two classes of GPS, but really as long as a GPS has a PPS
output it can be used for timing.  These is considerable crossover.    The
one thing all dedicated timing receivers will have is the ability to tell it
the antenna location and that it is not moving.

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.  the "one second off" problem is common as are delays in the PPS
being out of phase with the true UTC second "tick".

I did this exact something a few years back.  We built an astronomical
camera specialized for photometry.  I put NTP into the firmware so when the
camera wrote out the FITS image files the time would be accurate.

On Thu, Sep 1, 2011 at 7:24 PM, Greg Hennessy <greg.hennessy at cox.net> wrote:

> 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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Chris Albertson
Redondo Beach, California

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