[ntp:questions] garmin 18x and linux
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
>> 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?
>> questions mailing list
>> questions at lists.ntp.org
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