Date: Thu, 09 Oct 1997 03:55:40 +0930 From: Mike Smith <mike@smith.net.au> 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: <199710081825.DAA00927@word.smith.net.au> In-Reply-To: Your message of "Wed, 08 Oct 1997 10:58:11 CST." <199710081658.KAA10961@rocky.mt.sri.com>
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> > > You don't even *need* a wireless radio link, since the GPS unit will > > > allow you to synchronize your clocks with the satellites, thus allowing > > > you the ability to use time-stamps for your readings that you can > > > 'differentialize' after the run. > > > > Uh, hang on a second. You want to use DGPS to remove the SA jitter, > > correct? SA jitter is by definition random, and DGPS uses the fact > > that the reference is known to be stationary to calculate the SA > > jitter. > > Shh, don't tell anyone, but let me let you in on a little secret. The > 'jitter' in GPS that makes most of the difference *isn't* random. Most > of it is introduced, and that's what makes the accuracy < 100 M. If you > remove the accuracy, you're down to ~1M accuracy, and by taking out the > rest of the 'jitter', you can do better than .1M accuracy. However, for > the above application, I suspect 1M accuracy is probably good enough. 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? > We call this 'fake' DGPS, and use it for many projects at SRI which > don't need *really* accurate measurements. (SRI helped develop GPS, and > continues to do alot of GPS research.) Heck, I'm not telling you off here; I'm just trying to get a handle on this. For at least one of our applications, this would make a *very* significant difference (we need to get < 1/2 wavelength real-space measurements for portable gear over 5-50km, with wavelengths in the few-metres range) - so pony up, how do you do it? > Again, I'm talking about removing the 'introduced' jitter, and not > removing the jitter related to moving satellites/moving cars, but I > don't think that kind of accuracy is *necessary*. No, not at all. I was merely concerned about SA jitter which is bad enough to make solution-to-solution measurements unreliable for something like a moving vehicle. > ps. However, I'm not sure how the 'static shield' talked about earlier > would affect the GPS receiver. If it does, then GPS is a non-starter. Given that it appeared at around 200mph, there's a good 50+mph of clear air left 8) mike
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