On Sun, 13 Feb 2011 12:01:19 +0100 Stefan Keller <[email protected]> wrote: > I'm just wondering about a fundamental configuration decision. > My configuration idea is to have two low cost GPS receivers, where one > of them is on a known position. Both issue just GPS position and log > time data
So the point of your base station is twofold. First, to have a known reference point with which to measure tropospheric delays for gross adjustment to the beginning of the signal "chip" (a message bit, not called a bit because there's bits that go into decoding that bit), the second to do the same thing for relative carrier phase (the signal on which the chips are broadcast). For the first part, you're not going to get better than a two or three meters anyway, so you may as well use reference stations that other people are running. This is what an SBAS, like WAAS, does, and these days that second tier of consumer devices receive SBAS signals anyway and do their corrections in real-time. If you have a device that doesn't, and you want to apply that correction as a post-process, then NOAA CORS publishes reference data that you can use. To get better than a few meters, you need a receiver that can do carrier-phase positioning and, as Brian Russo points out, log that data. Consumer grade receivers don't do this. Furthermore, you need to know the location of your base station sub-millimeter. In my area the density of known base stations is high enough that people use CORS or commercial providers for their reference stations for centimeter level positining. In your case, since your receivers can't do carrier-phase positioning, you don't care if you're a couple of hundred miles from your reference stations. Conceivably yes, you could put two consumer grade receivers down, one at a known position, one mobile, and do your own augmentation. But other people already publish free data from their reference stations such that unless you need centimeter accuracy (in which case you need to pay for carrier-phase capable receivers), there is a reference station near you that someone else is already running that's better than whatever you'll hack together. And in most cases, if you're using the second tier of consumer devices or better, they're already receiving and applying SBAS corrections. Maybe if you shared your application we could suggest cheaper ways to accomplish what you're trying to do? Dan _______________________________________________ Geowanking mailing list [email protected] http://geowanking.org/mailman/listinfo/geowanking_geowanking.org
