From owner-freebsd-hardware Wed Oct 8 11:30:12 1997 Return-Path: Received: (from root@localhost) by hub.freebsd.org (8.8.7/8.8.7) id LAA06087 for hardware-outgoing; Wed, 8 Oct 1997 11:30:12 -0700 (PDT) (envelope-from owner-freebsd-hardware) Received: from word.smith.net.au (ppp20.portal.net.au [202.12.71.120]) by hub.freebsd.org (8.8.7/8.8.7) with ESMTP id LAA06063 for ; Wed, 8 Oct 1997 11:30:07 -0700 (PDT) (envelope-from mike@word.smith.net.au) Received: from word.smith.net.au (localhost [127.0.0.1]) by word.smith.net.au (8.8.7/8.8.5) with ESMTP id DAA00927; Thu, 9 Oct 1997 03:55:47 +0930 (CST) Message-Id: <199710081825.DAA00927@word.smith.net.au> X-Mailer: exmh version 2.0zeta 7/24/97 To: Nate Williams cc: Mike Smith , cliff ainsworth III , freebsd-hardware@FreeBSD.ORG Subject: Re: project truck.....ideas wanted In-reply-to: Your message of "Wed, 08 Oct 1997 10:58:11 CST." <199710081658.KAA10961@rocky.mt.sri.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Date: Thu, 09 Oct 1997 03:55:40 +0930 From: Mike Smith Sender: owner-freebsd-hardware@FreeBSD.ORG X-Loop: FreeBSD.org Precedence: bulk > > > 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