[time-nuts] Long period variation of GPS PPS timing?

Bob Camp lists at rtty.us
Fri Jul 2 07:17:30 EDT 2010


Hi

For the $50 to $100 cost it's tough to beat a Thunderbolt. It's overkill for the application, but easy to find. 

Bob


On Jul 2, 2010, at 6:45 AM, Paul Nicholson wrote:

> David Partridge wrote:
> > Is this a timing GPS receiver, or a generic fast start
> > navigation receiver?
> 
> Just an ordinary nav GPS, Garmin model GPS16HVS intended
> I think for marine/vehicle applications.  Spec says cold
> boot 45 seconds, warm boot (position unchanged) 38 seconds.
> Measured warm boot is about 25 seconds.
> 
> I haven't sent any setup commands to the thing, it is just
> running with out-of-the-box defaults.
> 
> The 1PPS is specified at +/-1uS accuracy, which is fine for
> this application if the error averaged to zero (which it
> probably does) and was randomly distributed (which it
> appears not to be).
> 
> My target for VLF phase error is +/- a few degrees at 77.5kHz,
> ie about +/- 0.1uS accuracy, and naively I thought I could
> reach this by a moving average of 100 seconds to smooth out
> the quoted 1uS jitter of the PPS.   But that assumes random
> jitter!
> 
> But this 50 minute cycle of about +/- 1.5uS is a bit of a
> show stopper.   Pity, the GPS16 is a nice unit in every
> other respect.
> 
> I'm trying to avoid using any specialised, hard to obtain,
> GPS.  Will have to order some other types and hope they
> have different, better behaved, chip sets.  Recommendations
> welcome.
> --
> Paul Nicholson
> http://abelian.org
> --
> 
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