[ntp:questions] Align PPS to UTC or GPS?

François Meyer fmeyer at obs-besancon.fr
Tue Feb 16 12:48:24 UTC 2016


On Wed, 10 Feb 2016, juergen perlinger wrote:

> On 02/10/2016 07:24 PM, Neil Green wrote:
>> I'm configuring a Skytraq Venus GPS module and have the 
>> option to align the PPS output to GPS or UTC. Which one is best for use over serial with NTP?
>
> IMHO that's a funny configuration option, as the difference between UTC
> and GPS time scale is an integral number of seconds, by definition. So
> it should not matter for a true PPS, that is, 1 pulse per second.

While it does not matter as far as ntp is concerned, the difference 
betweeen UTC and GPS consists in an integral number of seconds AND
a handful of ns, which is computed after some parameters transmitted
in the GPS navigation message ; so the option is not that funny for
applications below the 1 us range since the icd-gps-200 states :

" The OCS (control segment) shall control the GPS time scale to be 
" within one microsecond of UTC (Modulo one second).

That being said, a quick look at the BIPM circular T will show that in
practice the UTC-GPSTime offset has been almost continuously within the
10 ns range for at least the last 6 years :

ftp://ftp2.bipm.org/pub/tai/publication/utcgnss/utc-gnss

So really not a concern for ntp, but this is the rationale 
for the presence of this option.

(The january 26th incident was a bug affecting the nav parameters that
resulted in a 13 usec bogus estimation of this offset)

> This *might* be a difference if the 'PPS' is given every n-th second
> (say, every 5 or 10 seconds) and you want one pulse at second zero of
> the minute.
>
> Since NTPD syncs to UTC(*), I would suggest the UTC option, and don't be
> afraid to have the pulse every second, because it shouldn't make a
> difference anyway in that case.
>
> Unless I failed to grasp something?!?
>
> (*) The truth is more complicated, but for all practical purposes it's
> easiest to assume NTPD works with UTC.
>
> Cheers,
> 	Pearly
>
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-- 
François Meyer    Tel : (+33) 3 81 66 69 27   Mob : 6 27 28 56 83
Observatoire de Besancon - BP1615 - 25010 Besancon cedex - FRANCE
Institut UTINAM * Universite de Franche-Comte * CNRS UMR 6213 ***


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