Date: Wed, 8 Oct 1997 16:43:40 -0400 (EDT) From: "Christopher R. Bowman" <crb@Glue.umd.edu> To: Nate Williams <nate@mt.sri.com> Cc: Mike Smith <mike@smith.net.au>, cliff ainsworth III <cliff@cliffsworld.com>, freebsd-hardware@FreeBSD.ORG Subject: Re: project truck.....ideas wanted Message-ID: <Pine.SOL.3.95q.971008163831.4216A-100000@modulation.eng.umd.edu> In-Reply-To: <199710081839.MAA11594@rocky.mt.sri.com>
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On Wed, 8 Oct 1997, Nate Williams wrote: > > Um. So you are saying that the "introduced" jitter, ie. SA (Selective > > Availability), is actually predictable? And that this prediction is > > available to a commercial consumer? What's the point of it then? > > It's not predictable (well, it is, but it's a fairly complex algorithm > that is not published, essentially making it unpredictable). The point > is that the jitter introduced is the same across all of the recievers in > the same general area, so if one of the receivers isn't moving, you can > take the 'difference' of the changes made to it and apply it to the > values on the other receivers taken at the same time. > > (In case anyone is concerned, my brother-in-law at the State of Montana > is also doing this, so it's not like it's a state secret or > anything. *grin*) > > Another rumor I've heard about is that the introduced jitter may go > away. Interestingly enough, during the most recent 'war' (Desert > Storm), there weren't enough GPS receivers that could read the > 'encrypted' channel which has no introduced jitter, so they ended up > turning off SA and using standard commercial receivers. It kind of > defeats the purpose. That, and DGPS has made it virtually useless for > anything 'sensitive', so there is serious consideration being made to > kill it. > > But, many of the GPS manufacturers are fighting it, because it makes > alot of their 'new generation' products useless, which rely on the > introduced jitter. *sigh* Capitalism at it's worst. > > > > Nate > Scientific Americans Feb '96 page 44 has really good article on GPS and techniques for improving acuracy by Thomas A. Herring of MIT --------- Christopher R. Bowman crb@Glue.umd.edu <A HREF="http://www.glue.umd.edu/~crb">My home page</A>
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