Date: Wed, 8 Oct 1997 10:58:11 -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: <199710081658.KAA10961@rocky.mt.sri.com> In-Reply-To: <199710081650.CAA00519@word.smith.net.au> References: <199710081433.IAA10497@rocky.mt.sri.com> <199710081650.CAA00519@word.smith.net.au>
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> > > Damn, I forgot DGPS. You will want a decent wireless radio link > > > between the truck and your base station to run the DGPS data (from > > > memory the Rockwell modules want 9600 bps) over. A relatively cheap > > > shorthaul wireless modem set should give you enough to run PPP over. > > > You could use this for realtime telemetry. 8) > > > > 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. 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.) 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*. Nate 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.
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