Date: Thu, 4 May 2000 08:08:44 -0600 (MDT) From: Nate Williams <nate@yogotech.com> To: Mike Nowlin <mike@argos.org> Cc: Wes Peters <wes@softweyr.com>, Warner Losh <imp@village.org>, Matthew Dillon <dillon@apollo.backplane.com>, hackers@FreeBSD.ORG Subject: Re: GPS heads up Message-ID: <200005041408.IAA17349@nomad.yogotech.com> In-Reply-To: <Pine.LNX.4.05.10005040321200.4046-100000@jason.argos.org> References: <39112085.DD0E6502@softweyr.com> <Pine.LNX.4.05.10005040321200.4046-100000@jason.argos.org>
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> > This is what "Differential GPS" provides: a standard time source that > > can be used to remove the SA meanderings from the GPS fix. > > If I'm understanding this correctly (with very little actual research into > it) is that a DGPS station essentially transmits the difference between > what it "hears" as it's location and what it's actual measured location > is So far so good. The DGPS transmitter transmits the 'error' offsets for each satellite, along with timestamps so that each GPS receiver can apply the differences to the signals it receives to correct for SA. However, it means that you must been close enough to the DGPS receiver so that both of you are receiving the same offsets. Another way of doing DGPS correction can be done 'after the fact' with certain receivers. Basically, instead of saving the measured location, these save the location received from each satellite into a logfile, and then at some point in the future, this information along with the same information from a DGPS unit is fed into a program to post-process the received information to determine where a particular GPS unit was. This is how the State of Montana maps it's highways. They don't need real-time information, but they can't afford to keep a DGPS unit near the van driving down the road that is mapping the roads. However, the one problem with this approach is that if the 'receiver' is far enough away from the DGPS receiver, the satellites it picks up are different, and in some cases there is not enough overlap in satellites to get a good enough fix. to the clients, which apply this change to their local "heard" > position. (Assuming that both sides are within the appropriate range of > each other that allows them to hear the same satellites.) > > Do I have it about right? On the money. Nate To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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