Stratum one autonomy and assumptions about GPS

[Eric S. Raymond]

Gary E. Miller <gem at rellim.com>:
> > 1. GPS outage length and frequencies are decreasing
> 
> Don't care.  If you need your NTP to work, you need to know it is working.
> Otherwise failure are not noticed.

OK, the test for "know it is working" is: you have lock, or you had lock
less than x seconds ago where x is a worst-case of your drift model to
whatever confidence interfal you want to fix.

> > 3. There's a lower bound below which outages don't matter; we may be
> > there.
> 
> I don't agree.  I monitor all my services 24x7, and I do get NTP
> problems in my logs.

And you also said in recent mail that you don't work with the kind of
hardware a serious autonomy-seeker would use.  So *your* NTP problems
are not determinative, though they could be useful input data for
improving error-estimation techniques.

> > Any given fixed accuracy target for deviation from UTC, combined with
> > a maximum crystal drift rate, defines a longest tolerable GPS outage. 
> 
> Not the majority failure mode.

That's an interesting statement.  What *is*, in your experience, the
dominant failure mode.

> > 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. 
> 
> We can't define a single tolerable error budget.  We can provide some
> ranges of options for the user.

And that's exactly what I've been pushing towards - to develop some
statistical modeling on the basis of which we can make estimates to whatever
confidence bound the user wants to set as a parameter.
-- 
		<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>
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