I’m not sure if it is possible, but I’ll try to make a project with my own DGPS system…using a mobile station and a fixed station (previously knowing its location accurately).
I’ll implement a wireless communication system so both receivers - fixed and mobile - can share the required info. Basically, the fixed module sends the error offset to the mobile module to be taken in account when calculating its position.
The idea is based on the previously knowledge of the accurate position of the fixed module. Knowing this, I can compare the accurate coordinates with the ones calculated by the GPS module and extract the error caused by atmospheric interference for that area (assuming that atmospheric interference is the same for both modules within the operating area).
But (there’s always a but), the error can only be taken in account if both modules are using the same satellites for tracking their positions. Now, I need to know if it’s possible to choose which satellites to track or discard the ones that doesn’t matter.
There’s another way to work around, consisting in the fixed module calculating the error associated with each satellite available and somehow apply it to correct the location obtained by the mobile module. But for that I need to access the raw information provided by the GPS module and process it. As far as I know, that’s a very hard work that I want to avoid… :?
Ufortunately, extremely few GPS-receivers allow you choose which satellites to track. Even if you get the list of currently tracked satellites, there’s o way to know exactly which of those were used to calculate the position. This means that even if you have two identical receivers tracking the same set of satellites, they could infact (and probably are) using different data to calculate position.
You could probably improve absolute accuracy by some amount, but getting millimeter precision as from scientific laboraty GPS receivers is just not possible without access to the raw GPS data.
It isn’t hard to find [GPS receivers that will accept DGPS inputs. The trick is generating the DGPS data to start with. TAPR used to sell a kit for a DGPS reference station but it is not available because the receiver it uses hasn’t been made since 1999. I didn’t dig too deep but the [Coast Guard provides DGPS data. If you are within their coverage map you just need a receiver to feed DGPS data to your receiver. [CORS has data suitable for post processing.
I had to do basically what you want to do last summer for a farming operation. I can’t get into the real, hardcore coding details…basically 'cause I got paid for the job and I agreed not to share, which I thought was a crock because what I did for the guys wasn’t brain surgery, but I played their silly little game anyways, and I’m sticking by the agreement for at least a few years. Suffice to say, the math involved shouldn’t be too hard to figure out. The system was cheap and we ended up with good results. As always, YMMV…
Basically, I set a GPS receiver (the fixed unit) on the ground at point X, and let it sit for a few days continuously sampling position data, ran all this data thru a spreadsheet and averaged it all out and just assumed that point X was at lat-X/long-Y/alt-Z.
The ‘remote/mobile’ GPS was the exact same type of unit on an identical circuit board, powered the same way, and so on.
When it was all said and done, the fixed GPS unit was reprogrammed to compare it’s perceived position to it’s “known” position and transmit the difference over a simple serial radio link (using the cheap 434Mhz radio modules) to the mobile unit, which would compare the mobile GPS’s position output data, add/subtract the difference from the fixed GPS unit, and arrive at a solution which, in my case, was good enough for our work. We didn’t need 1 inch accuracy, or even 1 foot, but generally speaking, we were almost always (95% or so?) with 5 feet of where we wanted to be, and generally (>50% give or take) within about a foot or so of what we wanted.
In the end, we could’ve done the same work with 100% more accuracy using a tape measure, but these clowns wanted the latest/greatest/neatest method of doing things, or they could’ve just spent a little bit of extra money and got a commercial GPS/DGPS setup and run with that. Oh well. I made a few $$$ off it…and there I was…