From owner-freebsd-hackers Thu May 4 0: 2: 4 2000 Delivered-To: freebsd-hackers@freebsd.org Received: from obie.softweyr.com (obie.softweyr.com [204.68.178.33]) by hub.freebsd.org (Postfix) with ESMTP id A3B2837BFDD for ; Thu, 4 May 2000 00:01:43 -0700 (PDT) (envelope-from wes@softweyr.com) Received: from softweyr.com (Foolstrustident!@homer.softweyr.com [204.68.178.39]) by obie.softweyr.com (8.8.8/8.8.8) with ESMTP id BAA04219; Thu, 4 May 2000 01:01:27 -0600 (MDT) (envelope-from wes@softweyr.com) Message-ID: <39112085.DD0E6502@softweyr.com> Date: Thu, 04 May 2000 01:02:29 -0600 From: Wes Peters Organization: Softweyr LLC X-Mailer: Mozilla 4.7 [en] (X11; U; FreeBSD 4.0-STABLE i386) X-Accept-Language: en MIME-Version: 1.0 To: Warner Losh Cc: Matthew Dillon , hackers@FreeBSD.ORG Subject: Re: GPS heads up References: <200005040303.UAA66590@apollo.backplane.com> <200005032313.QAA65552@apollo.backplane.com> <200005031744.KAA63550@apollo.backplane.com> <20000503200006.A35116@cichlids.cichlids.com> <200005031957.NAA01354@nomad.yogotech.com> <20000503130759.A15403@orion.ac.hmc.edu> <20000503221528.A37472@cichlids.cichlids.com> <20000503151513.D337@beastie.localdomain> <200005040230.UAA36034@harmony.village.org> <200005040321.VAA36306@harmony.village.org> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG Warner Losh wrote: > > What I was trying to say was that SA is caused by the satellites > reporting times that have a small offset added to or subtracted from > them. Knowing where you are requires that you know what time it is to > a very precise degree. Once you know what time it is, you can know > where you are. That's why SA injects a pseudo random noise factor > into the timing information that the satellites report. If you have > an atomic clock and a GPS clock, you can measure the offset between > the two fairly easily and graph the results. That is what I mean when > I say you can compensate for the SA if you have a good atomic clock. This is what "Differential GPS" provides: a standard time source that can be used to remove the SA meanderings from the GPS fix. -- "Where am I, and what am I doing in this handbasket?" Wes Peters Softweyr LLC wes@softweyr.com http://softweyr.com/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message