From owner-freebsd-hackers Sun Mar 31 12:30:46 2002 Delivered-To: freebsd-hackers@freebsd.org Received: from rover.village.org (rover.bsdimp.com [204.144.255.66]) by hub.freebsd.org (Postfix) with ESMTP id 02B7037B419 for ; Sun, 31 Mar 2002 12:30:35 -0800 (PST) Received: from harmony.village.org (harmony.village.org [10.0.0.6]) by rover.village.org (8.11.3/8.11.3) with ESMTP id g2VKUXi46426; Sun, 31 Mar 2002 13:30:34 -0700 (MST) (envelope-from imp@village.org) Received: from localhost (warner@rover2.village.org [10.0.0.1]) by harmony.village.org (8.11.6/8.11.6) with ESMTP id g2VKUPf51571; Sun, 31 Mar 2002 13:30:33 -0700 (MST) (envelope-from imp@village.org) Date: Sun, 31 Mar 2002 13:29:40 -0700 (MST) Message-Id: <20020331.132940.91795402.imp@village.org> To: bicknell@ufp.org Cc: dp@penix.org, hackers@FreeBSD.ORG Subject: Re: GPS time. From: "M. Warner Losh" In-Reply-To: <20020330142859.GA19243@ussenterprise.ufp.org> References: <20020330142859.GA19243@ussenterprise.ufp.org> X-Mailer: Mew version 2.1 on Emacs 21.1 / Mule 5.0 (SAKAKI) Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit 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 In message: <20020330142859.GA19243@ussenterprise.ufp.org> Leo Bicknell writes: : In a message written on Fri, Mar 29, 2002 at 06:04:11PM -0600, Paul Halliday wrote: : > I just connected my gps (garmin gps III plus) to my serial port : > and realized that simply cat'ing cua0 displays date/time/position of the : > unit. (neato). Anyway, how accurate would it be to use the time from this : > output for ntp as opposed to my current setup using ntp servers. : : 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. Also, the reference clocks that ntp servers use generally are in the +- 1us or better rather than 1ms sync that you can get over a wan. Of course, don't look too closely under the covers of ntp. it has well documented startup transient conditions that some applications may be senentive to. Warner To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message