I've heard of farmers going so far as to make their own GPS guidance systems, enhanced before they were widely available and reasonably priced. I'm not sure how accurate and repeatable they were.
Going to any farm that's been around for 20+ years, you'd see so many custom solutions that are really functional.
Farmer one-off solutions are kind of like homebrew Excel applications - they're infinitely customizable, the end user knows exactly how it should work and can reasonably make it happen, and it's a small fraction of the cost of a commercial/IT SOLUTION. It may not be as reliable, but it's good enough.
Trimble GPS receivers go for around $10k (USD) [0], so I'm not surprised farmers try to build their own devices. Here's one who built an RTK GPS system out of a Raspberry Pi (and various other parts) for £250: https://www.youtube.com/watch?v=Kwxpo04AC5Q
Not by itself, in most situations. Maybe if you're automating a combine, where you're doing repeating patterns in a big flat field with no obstacles and where 1 meter deviations from the desired trajectory isn't an issue (not an uncommon situation, but for a lot of use cases you really do care about that 1 meter deviation, e.g. for plowing or cultivating).
Otherwise you have to augment, eg with relative GPS or machine vision.
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].
Going to any farm that's been around for 20+ years, you'd see so many custom solutions that are really functional.
Farmer one-off solutions are kind of like homebrew Excel applications - they're infinitely customizable, the end user knows exactly how it should work and can reasonably make it happen, and it's a small fraction of the cost of a commercial/IT SOLUTION. It may not be as reliable, but it's good enough.