Nothing in the fields. I'm no GPS expert but I do have an embedded systems background. It's my understanding that this generation of GPS receivers, specifically the uBlox stuff is like 90% as good as "Survey grade" gear from Trimble/etc at like 1/10th to 1/20th the cost. For a farm this is more than enough. Below 10km between the base and the rovers you're looking at about a 2-5cm minimum absolute error or basically 0.75" to 2"
RTK is siginifcantly more accurate than dGPS

For the RTK base we set up the same model receiver plugged into that Pi running RTKBase, you let it collect 1hz data for ~24 hours and then you send that data to one of several worldwide government PPP processing services, we used NRCan's PPP (Canada). They take all your data and basically give you back the exact location of the base (I think the accuracy on the position we got back was quoted at 0.02 meters for X/Y and 0.05M for Z and you program this into the base station.
Then you can either send this data to the rover directly with Tailscale or some overlay, or use a free service like RTK2Go (which we did), that allows anyone else in the area to benefit from it should they want.
Inside the tractor there's a windows tablet running the AgOpen software that talks to the PCB and part of that is a NTRIP client that gets the correction data from the base station, that sends the correction data down into the receivers, which do their own magic and pop out with a standard GPS message the software can consume. Oh, right, these receivers are also 10hz capable at full performance and 50hz capable at lower accuracy. They're pretty nuts.
My understanding is that because the base knows exactly where it is, any atmospheric interference will affect receivers the same in a given region. So the base says "Hey you're getting this data from these GPS sats (it can only correct for data from shared visible birds) but you actually need to adjust what you think about the data from those by XXX because I know where I am and I can see the interference"
The rover (technical name for the moving thing) has two receivers that it uses for extremely accurate roll/heading calculations. Basically the right GPS receiver does position and the left receiver is used to calculate direction/roll. You can use 1 receiver and an IMU instead but it's not as good to save $200 bux on the F9P.
tl;dr we should get roughly 1-2" of repeatable year over year row over row accuracy.