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Re: [gpsd-dev] PPS over USB

From: Eric S. Raymond
Subject: Re: [gpsd-dev] PPS over USB
Date: Mon, 7 May 2012 16:58:43 -0400
User-agent: Mutt/1.5.21 (2010-09-15)

Ed W <address@hidden>:
> I guess I still don't understand the bufferbloat / thumbgps
> requirements, but either you need a very stable clock local, or you
> need a bunch of clocks which are sync'ed to each other (across the
> world).

The latter.  What we're really after is accurate packet transit times.
So it matters less whether the clocks are accurate than whether
they're highly stable to a common timebase. Which in this case is GPS
atomic clock time.

> I might guess that this is the goal of thumbgps - get the offset to
> zero?  However, it appears in principle that you might achieve
> almost the same simply syncing to a carefully chosen pool of stratum
> 1 servers?

But we're not sure we can trust NTP.  That's the whole problem; if bufferbloat
really is inducing very large, very short-period latency spikes, it may be
screwing with the symmetry and statistical-smoothness assumptions that
NTP synchronization relies on.

One of the explicit goals of the Cosmic Backround Bufferbloat Detector is to
sanity-check NTP.
                <a href="";>Eric S. Raymond</a>

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