I'm thinking about building a gps system for a tractor. The gps receiver can be placed on the roof of the tractor to minimize interference. Regular GPS receivers are accurate upto "a few meters". (https://www.adafruit.com/products/746) I found you can correct your position with a base station. (RTK)
I have seen the Starfire gps receiver getting a good precision (a few cm i think) without basestation. I wonder how they work.
Are there other ways to get a more accurate position?