Stratum one autonomy and assumptions about GPS

[Eric S. Raymond]

Dan Drown <dan-ntp at drown.org>:
> >We may already be at a technological place where GPS outages don't bust the
> >tolerable-error budget, even with cheap hardware. If we aren't, we'll
> >probably be there soon.  One of my medium-term agenda items is to measure
> >and see.
> 
> I had setup a test along these lines a week ago:
> https://blog.dan.drown.org/gps-pps-drift-when-it-has-no-signal/
> 
> The GPS module I'm using still outputs PPS even when it loses lock.
> That won't be true for every GPS module.
> 
> 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.
> 
> ntpsec finally rejected the PPS at a 2ms offset.  I think I could
> change that with the "tos mindist" setting (which was at its default
> of 1ms), but I haven't verified that.
> 
> 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.

Very interesting.

I have a USB thermometer on order, they're cheap.  Might I suggest you
get one and repeat this experiment, actually plotting your temperature
variation?
-- 
		<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>