ntpd and GPS

Bob Johnson fbsdlists at gmail.com
Fri Sep 19 22:39:59 UTC 2008

On 9/19/08, Tom Storey <tom at snnap.net> wrote:
> Hi all,
> Ive been toying with setting up my old Garmin GPS12 as a reference for a
> server (FreeBSD 6.2) running ntpd, but Ive run into an issue.

Is it possible the issue isn't what you think it is?

> Ive searched around a bit and cant find an answer, perhaps because there
> isnt one.

I once (years ago) had a Garmin GPS working with ntpd, so it's
reasonable to believe it can be done again, unless support for that
capability was dropped (which I doubt). Unfortunately, it was long
enough ago that I don't remember what I did. It's possible I used the
1 PPS output without NMEA sentences, but that's not my recollection.

> Is there any way I can set ntpd to expect a $GPRMC string every 2 seconds,
> which is the frequency at which the GPS12 transmits them?

> Alternatively, does anyone know how to make the GPS12 transmit a $GPRMC
> string every second?

I'm almost certain you can't. The complete set of all NMEA sentences
takes more than one second at the default 4800 baud, so IIRC it
outputs sentences only on odd seconds, and perhaps the older units are
too slow to compute a fix once per second. Two things that may work
around this are to turn off everything except the GPRMC sentence:


and perhaps free up some CPU time (for faster position calculation) by
(oddly enough) reducing the output data rate to 1200 bps:


but I don't think that will actually work. To go back to 4800 bps, use
3 instead of 1. I think there are 11 commas after the "C" in that
command, but my eyes aren't so sharp any more.

There is a Linux driver for the Garmin proprietary protocol. Don't
know if it is distributed in a FreeBSD version. Try

> If there is a better place I can post this, please let me know.

This is probably a good place for this question, but if you don't get
a better answer, try the archives of the "time nuts" mailing list
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts which
unfortunately appears to be down right now. General info about that
group is at http://www.leapsecond.com/time-nuts.htm

If that yields nothing, you might post your question to the Time Nuts
list, time-nuts @ febo.com. It is probably a FAQ for them, but they
will be polite about it. And I had hoped to once again stick an old
Garmin on an NTP server, so I'll be curious to know if this turns out
to be insurmountable.

Good luck,

-- Bob Johnson
   fbsdlists at gmail.com

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