Date: Thu, 09 Oct 1997 02:19:58 +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: <199710081650.CAA00519@word.smith.net.au> In-Reply-To: Your message of "Wed, 08 Oct 1997 08:33:17 CST." <199710081433.IAA10497@rocky.mt.sri.com>
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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. (If you're known to be stationary, any movement you detect must be bogus.) Once you have this data, you feed it in _real_time_ to your mobile; it removes the jitter and everything else must be "real" movement. You can't achieve this by "synchronising with the satellite clocks"; if you could predict the SA sequence from a given point it'd be worthless. Ergo you need a realtime link between the base and the mobile. A couple of Rockwell Jupiter modules (we pay ~AUD$300 each) and support parts will do this for you, or you could spring for a Garmin or similar handheld/console mount unit. These cost more but have pretty displays and lots of buttons. Less robust of course. mike
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