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Date:      Sun, 31 Mar 2002 10:21:57 -0800
From:      Sean Mathias <SeanM@prosolve.com>
To:        "'Erik Trulsson'" <ertr1013@student.uu.se>, Paul Halliday <dp@penix.org>
Cc:        Mike Silbersack <silby@silby.com>, Leo Bicknell <bicknell@ufp.org>, hackers@FreeBSD.ORG
Subject:   RE: GPS time.
Message-ID:  <CD855A91DC3CD411BEB20050DA2CB7D107676A@fs01.prosolve.com>

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Why wouldn't time from a local GPS receiver constitute good time?  Provided
the receiver is configured properly and acquires reference satellites fairly
regularly, this should provide almost the best possible time.  For LBS, the
norm is to acquire three primary satellites and an additional satellite if
possible for reference.  As each GPS satellite has 2 or 3 onboard atomic
clocks, this would seem like the best possible reference available and given
the availability of receivers in a PCI form factor, inexpensive and broadly
available to all.

SM

-----Original Message-----
From: Erik Trulsson [mailto:ertr1013@student.uu.se]
Sent: Sunday, March 31, 2002 9:32 AM
To: Paul Halliday
Cc: Mike Silbersack; Leo Bicknell; hackers@FreeBSD.ORG
Subject: Re: GPS time.


On Sun, Mar 31, 2002 at 10:44:55AM -0600, Paul Halliday wrote:
> On Sun, 31 Mar 2002, Mike Silbersack wrote:
> 
> >
> > On Sat, 30 Mar 2002, Leo Bicknell wrote:
> >
> > > Your NTP servers are better.
> > >
> > > I tested a III Plus, and without a 1 PPS source (which that model
> > > doesn't provide) it's accurate to about 100ms, give or take.  Since
> > > real NTP servers are < 1ms, they really aren't that good.  It's
> > > not that the time isn't accurate, it's that they were not designed
> > > to communicate with that accuracy to an external device.
> >
> > OTOH, 100ms is pretty close; I doubt many people need time better than
> > that.  The one big advantage I can see with using a GPS receiver vs NTP
> > servers is security & reliability; I've always worried that my clock
> > might start to drift to a misconfigured NTP server.  Taken to a paranoid
> > level, you could worry that someone was faking NTP replies to throw your
> > clocks off. :)
> 
> 	This is the answer I was kinda hoping for. I think that accuracy
> to ~100ms from a known source is a little more comforting than <1ms from a
> server that I have no control over. I am not maintaining a space program,
> just a dozen machines in my room that really serve no other purpose than
> personal entertainment.

Yes, but that is why one shouldn't rely on *a* server.  When using NTP
it is a good idea to get the time from several NTP servers.
The chance that all of them are misconfigured at the same time is
fairly small.

OTOH, taking the time from a local GPS receiver doesn't sound like a
bad idea either if one doesn't need extremely good timekeeping.


-- 
<Insert your favourite quote here.>
Erik Trulsson
ertr1013@student.uu.se

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