ntpd and GPS

Tom Storey tom at snnap.net
Sat Sep 20 03:10:36 UTC 2008

Thanks Bob.

I did a bit more reading, and it seems that I can turn on additional  
sentences in the driver. Ive been studying the NMEA output and there  
are two sentences which will give  time figures across two subsequent  
seconds, so I tried enabling those ("mode 6" in the server statement),  
but still no dice. I can see all of the correct sentences being picked  
up in ntpq using the "clocklist" command, but it just doesnt seem to  
want to work. Essentially my GPS reference just sits like this:

building# ntpq -p
      remote           refid      st t when poll reach   delay    
offset  jitter
  resolv.internod   2 u   72  256   17   19.343    
36.263  37.083
  sparky.services     2 u   64  256   17   24.882     
0.279  18.807
  GPS_NMEA(0)     .GPS.            0 l    -   64    0    0.000     
0.000 4000.00

Which I assume means "nope, not working".

Is PPS absolutely neccessary? The GPS12 doesnt have PPS, so perhaps  
this is my issue? The output of NMEA seems to happen every 1.5  
seconds, but there are 3 sentences which output a time figure, so I  
figured I'd enable the two furthest apart in the hope that they may  
coincide with different seconds, and hopefully ntpd would be able to  
work it out from that. I even tried enabling all 3 of them ("mode 7"),  
but still nothing.

Anyway, Im looking at grabbing a Garmin GPS18 LVC, they are only just  
over $100 so no biggie. People have reported wide success with this  
device, so I think I'll still with what is known to work and go from  


On 20/09/2008, at 8:09 AM, Bob Johnson wrote:

> 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  
>> 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:
> $PGRMO,,2
> and perhaps free up some CPU time (for faster position calculation) by
> (oddly enough) reducing the output data rate to 1200 bps:
> $PGRMC,,,,,,,,,,1,
> 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
> http://jensar.us/~bob/garmin/
>> 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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