[time-nuts] Sawtooth correction: next or previous PPS

Hal Murray hmurray at megapathdsl.net
Mon May 21 21:13:45 EDT 2018


holrum at hotmail.com said:
> One thing to look out for when messing with sawtooth messages is the
> question of does the message come out before or after the PPS pulse...  good
> look finding the answer in the receiver documentation... 

Has anybody asked the manufacturers?

This should be easy to see if you record the PPS offset referenced to a good 
clock and compare that to the reported offset.  If the frequency is stable 
and you are getting a sawtooth (rather than a bridge) then a point on a 
corrected graph next to the jump in the sawtooth will look good if you have 
it right or be off by a clock cycle if you have it wrong.


holrum at hotmail.com said:
> "After" seems to be the most common answer.  That makes hardware/delay line
> compensation rather tricky.  ...

The slides from Tom Clark and Rick Hambly's VLBI talk (page 29) show a before 
setup.
  http://www.gpstime.com/files/TOW/tow-time2015.pdf

You said "most common".  That implies there are both types.  (or 
documentation errors)  We should make a list of which GPS modules do it which 
way.


-- 
These are my opinions.  I hate spam.






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