From owner-freebsd-hackers Sun Mar 31 12:44: 8 2002 Delivered-To: freebsd-hackers@freebsd.org Received: from cessium.prosolve.com (gw.prosolve.com [63.225.188.140]) by hub.freebsd.org (Postfix) with ESMTP id EE75B37B405 for ; Sun, 31 Mar 2002 12:44:03 -0800 (PST) Received: from fs01.prosolve.com (fs01.prosolve.com [172.16.128.50]) by cessium.prosolve.com (8.11.4/8.11.4) with ESMTP id g2VILx127698; Sun, 31 Mar 2002 10:22:00 -0800 (PST) Received: by fs01.prosolve.com with Internet Mail Service (5.5.2655.55) id <2AZ8JCQ4>; Sun, 31 Mar 2002 10:21:58 -0800 Message-ID: From: Sean Mathias To: "'Erik Trulsson'" , Paul Halliday Cc: Mike Silbersack , Leo Bicknell , hackers@FreeBSD.ORG Subject: RE: GPS time. Date: Sun, 31 Mar 2002 10:21:57 -0800 MIME-Version: 1.0 X-Mailer: Internet Mail Service (5.5.2655.55) Content-Type: text/plain; charset="iso-8859-1" Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG 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. -- Erik Trulsson ertr1013@student.uu.se To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message