Civilian GPS is not accurate enough for precision farming which is why these tractor manufacturers in my above link were using Inmarsat L-band to transmit correction information in addition to using GPS.
It can also be done locally with a fixed base station.
I'm even less worried about the security of military GPS.
I don't know what that has to do with this topic. Starfire (and similar) farm tractor guidance systems are not that. Zero farm tractors use military GPS.