Date: Wed, 8 Oct 1997 12:39:16 -0600 (MDT) From: Nate Williams <nate@mt.sri.com> To: Mike Smith <mike@smith.net.au> Cc: Nate Williams <nate@mt.sri.com>, cliff ainsworth III <cliff@cliffsworld.com>, freebsd-hardware@freebsd.org Subject: Re: project truck.....ideas wanted Message-ID: <199710081839.MAA11594@rocky.mt.sri.com> In-Reply-To: <199710081825.DAA00927@word.smith.net.au> References: <199710081658.KAA10961@rocky.mt.sri.com> <199710081825.DAA00927@word.smith.net.au>
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> 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
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