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.
I imagine a mostly autonomous land vehicle relying on GPS would also require RTK GPS, which does require an external source of the data. Some GPS receivers can receive that from satellites, but the data is encrypted and requires expensive commercial licenses or military grade equipment.
You can generate your own correction data for GPS, using a fixed base station, and relaying the data over a radio link or through the cell network. Look up ArduSimple if interested how. Something like $400 IIRC (for the base station).
Source: we are doing it, using AgOpenGPS (OSS autosteer) with RTK
Oh I know. I've plumbed RTCM3 streams over radios and cell networks aplenty as well. There's a real cost to setting up and maintaining a base station properly, though. Subscribing to a service is attractive if you're only operating a small fleet, which is sort of the point?