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

[Paul Nicholson]

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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