[ntp:questions] Odd offset for PPS DCD w/ Garmin GPS 18x LVC

lellis larry.ellis at gmail.com
Sun Apr 17 15:03:58 UTC 2011

On Mar 22, 11:51 am, Dave Hart <daveh... at gmail.com> wrote:
> I think your PPS is probably right on, while your NMEA is off by
> nearly a second.  Make sure either the GPS is generating only the
> sentence you want each second, or use the mode option to the NMEA
> driver to select a sentence.  Try using the same value for fudge time1
> and fudge time2 for NMEA (one is used to offset NMEA end-of-line
> times, the other to offset PPS time.  Thanks to the user PPS hack in
> Windows ntpd, the "end of line" times should actually be PPS times
> with tens of microseconds offset and a bit more jitter than PPSAPI
> timestamps.

At the moment, and likely with the help of some suggested
configuration changes, the oddness has gone away.

With the help of several radio-controlled clocks in my office, I
noticed that the local clock on my machine was occasionally off by
exactly one second (sometimes slow, sometimes fast).

While I'm not sure this is the definitive answer, the following
changes seem to have helped:

1. I configured the Garmin to output both an RMC and a GGA sentence.

2. Using a small com port test program I wrote, I determined that the
GGA end-of-line is received about 850 ms after the leading edge of the
DCD pulse

3. I then modified my configuration file to include a time2 value of
850 ms:

        server mode 2 minpoll
4                            #mode 2, use GPGGA
        fudge flag1 1  time2
0.850                         #time2 compensates for GGA offset

I also deleted my drift file before restarting.

I am now using a very recent build (4.2.7.p150) without apparent

Larry Ellis

More information about the questions mailing list