Has anybody tried to make a local differential GPS system using two Pi's?
It would need both Pi's to be fitted with similar GPS modules and both be on the same network so that they can communicate with each other. One of the Pi's (static Pi) will be in a fixed position and readings taken over a length time (minutes, hours?) until a good average position can be established which should be accurate to < 1m.
Having established its position the static Pi can now compare that accurate position with the current "live" position it is receiving and generate a "difference" error. This error is then passed to the mobile Pi and used to correct the "live" position it is receiving. As long as the two Pi's are in the same general locality then the mobile Pi's position will also be accurate to less than 1 metre. The initial averaging should only need to be carried out once as long as the static Pi is always placed in the same location.
I believe that land surveying instruments may use a similar system (I could be wrong!). It was certainly used for marine navigation years ago when selective availability was turned on and accuracy could be tens or even hundreds of metres out.
If anyone does take this further I would be grateful for copies of your code