[ntp:questions] PPS versus serial offset

unruh unruh at invalid.ca
Wed Aug 28 14:50:37 UTC 2013

On 2013-08-28, David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> On 28/08/2013 10:10, Rob 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.
> Indeed, or use a very high baud rate for the NMEA.
> I would be more worried about the 11 ms step in the PPS at about -155 
> hours....

If you look closely that is an 11ms step in the external stratum 1 servers, not in
PPS. The lgiht blue line goes straight through at 0 at that time. (hard
to see since the background is almost the same shade of grey)

> Nice graph, though....

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