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Re: [gpsd-dev] PPS over USB


From: Ed W
Subject: Re: [gpsd-dev] PPS over USB
Date: Mon, 07 May 2012 15:52:07 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:12.0) Gecko/20120428 Thunderbird/12.0.1

On 07/05/2012 14:35, Terje Mathisen wrote:
Ed W wrote:
On 06/05/2012 22:56, Terje Mathisen wrote:
BTW, I just picked up a NetGear 3800 for the BufferBloat firmware, I
intend to point that router at the local S1, which should result in
better timing than a PPS over USB solution.


Big problem... If you look on the chrony mailing list you can find a
post plotting the divergence of worldwide stratum 1 clocks. I'm afraid
the answer is that stratum 1 is not a definition of accuracy, just an
observation on the clock source. In particular many have fixed offsets,
poor stability, synced to radio clocks rather than GPS, etc.

You misunderstood my post.:-(

The S1 is my personal, at-home, FreeBSD+Garmin18 ntpd server.

Aha! Then in that case I definitely agree you are sorted! I would presume you can achieve low few 100s micro second sync (or better?) quite easily?

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). Problem seems to be that unless you all have the same reference (eg gps) then you could all be microsecond "accurate", just all at an offset to each other?

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? So I'm still a touch unsure what the ultimate reason is for this? (Note in case that sounds like an attack - it's not, just that I can't immediately see what the parameters for "success" for the thumbgps project - I'm sure there are several, perhaps there is a web page with a list?)

Good luck - fun project!

Ed W



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