[ntp:questions] PPS versus serial offset

unruh unruh at invalid.ca
Wed Aug 28 11:30:57 UTC 2013

On 2013-08-28, Rob <nomail at example.com> wrote:
> detha <detha at foad.co.za> wrote:
>> I have a test setup with a RaspberryPi and a SiRF/gpsd module. All
>> working quite well, but one thing bugs me. Looking at the ntpq -p output
>> the serial port readings seem to drift away slowly but steadily from the
>> PPS, see <http://detha.co.za/ntp/ntpmon.20130828.jpg>.
> It is normal.  The offset between serial data and PPS changes all the time.
> It is often a bit better when you put the GPS in binary mode instead
> of NMEA.

You havee adjusted the nmea offset. It is actually probably something
like 100 ms off from pps. (It certainly can never be before the pps). 
And then it can wander. If the offset doubles, I would start worrying. 

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