[ntp:questions] garmin 18x and linux

unruh unruh at wormhole.physics.ubc.ca
Fri Sep 2 07:06:03 UTC 2011

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.  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.

No. ntp can use the GPS nmea sentences to time the clock to a few 10s of
ms. -- you may need to fudge the time offset to make up for the delay of
the nmea sentence. 

> 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".

It would only vote it off the island if it had a better source. Since
this is the only source the vote is rigged. It is always on 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.

A gps with just nmea can be used for timing. Not as accurate, but then
10ms is only .1 seconds of arc on the telescope and I doubt he needs
better accuracy than that. 

> 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.

Accurate to what accuracy?

> 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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