[time-nuts] TBolt: UTC PPS

[Arnold Tibus]

On Mon, 29 Mar 2010 08:24:00 +0200, Magnus Danielson wrote:

>Tom Van Baak wrote:
>>> 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. 
>The GPS time is steered towards UTC(USNO) to be maintained within 1 us. 
>The effective difference is in a few ns. I could dig up the webpage at 
>USNO giving the difference if needed to. (TvB already has a mail from me 
>on this topic)

>There should be no effective difference between them.


UTC(USNO)-UTC(BIPM) difference is actually swinging 
between +- 7ns:
http://tycho.usno.navy.mil/gif/tai.gif

A good summary showing the differences for the international laboratories 
UTC-UTC(k) for the past with last date 2010 March 12 ( and a lot more):
http://tycho.usno.navy.mil/latestcircT

Arnold