Stratum one autonomy and assumptions about GPS
dan-ntp at drown.org
Thu Aug 25 15:23:50 UTC 2016
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:
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.
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