Stratum one autonomy and assumptions about GPS

[Achim Gratz]

Dan Drown writes:
> The GPS module I'm using still outputs PPS even when it loses lock.
> That won't be true for every GPS module.

I also have a NavSpark mini attached at the moment and I was surprised
it kept outputting the PPS when losing 3D lock (it stops blinking the
LED).  It seems semi-documented to keep the PPS on if it at least sees
one satellite, although the corresponding settings can't be read or
altered (that seems resereved for the timing firmware, on the mini I
just get an NACK response).  Losing 3D lock happens about once or twice
a day for me at the moment, probably because of a somewhat too agressive
mask settings.  It usually lasts less than half an hour with negligible
effect on ntp.

> My results were a long term average of 250us-500us lost per day
> (~3-6ppb).  This surprised me because of how low it was, I assume I
> got lucky with the long term average temperature of the GPS module.

Recently I've had an extended loss of lock and it was in fact drifting
by about 1ms/hour.  It may have been the result of some sort of power
fault, since I've needed to reboot the Pi and powercycle the mini to
recover.

> Of course, this data doesn't apply to all GPS outage situations.  But
> it does suggest that holdover within 30ms is easy for the "can't hear
> the satellites for a few hours" case.

You might want to play with the elevation and CNR mask settings and
check what happens with just one or two satellites considered in view.
I think the degradation there is actually larger than when it is in
complete holdover.


Regards,
Achim.
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