[time-nuts] TBolt: UTC PPS

Tom Van Baak tvb at LeapSecond.com
Mon Mar 29 01:28:42 UTC 2010


> It also has an option to sync the PPS with GPS or UTC.  I thought they were 
> off by an integral number of seconds so I don't expect any change.  Does 
> anybody know what's going on here?

The GPS broadcast message includes leap second and
a0 and a1 terms which are used to forward predict UTC
from GPS time, which is derived from UTC(USNO).

A0 and A1 are what you'd guess: a phase offset and a
frequency offset.

See page 157 of the GPS ICD:
<http://www.navcen.uscg.gov/gps/geninfo/ICD-GPS-200C%20with%20IRNs%2012345.pdf>

So yes, at the ns level the two 1PPS will differ since UTC
is not the same as UTC(USNO) or any other UTC(k) for
that matter. Have you measured it? I would be interested
to know what the delta is for your receiver today. You
can also dump out the a1 and a0 corrections with binary
commands on some GPS receivers.

/tvb





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