Stratum one autonomy and assumptions about GPS

[Dan Drown]

Quoting "Eric S. Raymond" <esr at thyrsus.com>:
:
> Hal objected (off list) to me drawing a conclusion from today's
> offset multiplot that check servers aren't necessary when you have
> a local GPS - a Stratum 1 really can run autonomously. He said,
> correctly of course, that the check servers aren't there to improve
> time accuracy when the GPS has sat lock, but to backstop the GPS when
> it flakes out.
:
> 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.