Stratum one autonomy and assumptions about GPS

[Gary E. Miller]

Yo Eric!

On Thu, 25 Aug 2016 00:19:46 -0400
"Eric S. Raymond" <esr at thyrsus.com> wrote:

> This was going to be a note to just Hal originally, but it will do the
> rest of the team no harm to know more about the scenarios and
> assumptions driving some of my design choices.
> 
> 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.
> 
> I shall now discuss three interlocking reasons this possibility does
> not loom as large in my mind as it does in Hal's.
> 
> 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.

> 2. The autonomy scenarios I think about are not hobbyist-budget
> productions

Yeah, and the big biys REALLY need to know their NTP is right.

> 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.

> 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.

> 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.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
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