How to get sub-meter or centimeter GPS accuracy?
There are few commercial products designed to do exactly what you are planning. They usually use RTK, with a known local reference station, up to 30km radius, and rovers installed on the tractors. Correction data is transferred via cellular or local ISM band radio. 
Multi constellation (GPS + GLONASS etc) receivers can provide improved performance even without RTK, but not to sub meter precision.
And, since you don't have access to mil spec receivers, that option is also moot.