I belive they were enhancing it with terrestrial base stations (Loran-C was mentioned in the discussion, which was 10+ years ago). John Deere has proprietary secret sauce and if I recall correctly, they can get sub 1" accuracy. That precision farming has enabled things like different amounts of fertilizer for different parts of the field, a substantial material savings and improvement for the environment.
u-blox's ZED-F9P chip is capable of 10 mm (0.39 inch) accuracy [0]. AFAIK, high accuracy requires being relatively close to a terrestrial correction station. Some states, such as Iowa, run a network of these stations and provide free access to the public [1].
