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GPS Slowly Changing How Things Are Done
Hemos forwarded me a link to a story at Fast Company about how GPS is changing the way people do business. Several good examples are used, from farmers in Alabama to anti-theft devices. Some notes on GPS' military origins as well. Also worth noting is how GPS, like computers, wasn't adopted overnight, but rather over time as applications were found.
so he can drive over the same tracks in his wheat field every year (I'm not kidding, read the fine article) and compress as little of his soil as possible
everything in moderation
I think you are refering to DGPS?
Conventional civillian GPS (which is not the same as Military GPS, even with SA turned off as it is now) is accurate to typically ~10m. You can enhance that a long way by doing phase matching as well as code matching - survey GPS devices can get down to a few cms (for a price!).
DGPS works on the basis that for each satellite in the area the error arriving at two units within a couple of hundred miles is roughly the same. (Extra delay is caused by things like atmospheric conditions.) You put one reciever on a known point, and calculate the error for each satellite you can see. You then send all of the calculated corrections to the roaming reciever so it can remove the error in the signals it's getting before it calculates it's position. This is considerably cheaper than using a survey grade GPS, as well as faster, but unlike a survey grade GPS you need to have set up a nearby DGPS transmitter first. The (FAA?) have done this around US airports I believe, to allow autolanding systems to double check against DGPS data as well as ILS beacons.
It's worth noting that to be able to use DGPS it's _not_ enough to calculate the error in your _position_ and transmit the correction to that as the roaming unit may be using different satellites to you - you have to transmit the error on each satellite signal. Some Garmin units let you extract this data using an undocumented API.
Beep beep.
Not to shoot you down or anything but I work specifially with GPS. The GPS C/A code broadcast on L1 (1.57542 GHz) has never been encrypted. The military simply encoded ephemerides for the GPS satellites that were inprecise (this was called "Selective Ability") onto the L1 signal. This led to a user range error of ~30 meters. After this was turned off in 2001 the error went down to ~3 meters. There has always been the PPS ("Precise Positioning Service") P-code signal on the L2 frequency (1.22760 GHz). This is actually encrypted, and is what the military uses in its. Acurracy with this service can be in the range of centimeters (low dynamics case). Working with the L2 signal requires a security clearance and a bunch of goverment red-tape. In the next 10 years there is going to be an explosion of GPS tech. First off the EU is putting up Galileo, which will double the number of SV's orbiting the earth (more satellites in view = better positioning accuracy). Althought the signal structures are not the exact same, because they will be broadcasting at similar carrier frequencies designing a dual use receiver will be a piece of cake. Also GPS is being heavily upgraded. They are adding a third signal with M-code(L3), and adding C/A code on L2. There is also talk about increasing the signal strength, which is a great boon to indoor GPS and using the GPS signal for remote sensing applications. All in all it is a great industry to be in.
You laugh, but in England there's already a service that lets you locate mobile ("cell" in your 'Merkin lingo) phones without using GPS: http://www.fleetonline.net/