Project has been untouched since we finished harvesting soybeans. Performance was down to an average cross track error of about 2 inches. I was running a 7 inch overlap between passes without missing anything.
In terms of accuracy, there are two variables. GPS position accuracy and vehicle control accuracy. The GPS accuracy part can be resolved by using a RTK system, which will get you to approximately 1 inch position data. The vehicle control part is my problem. 4 MPH may sound slow, but a lot of things have to happen in order to keep that machine within an inch of some imaginary line, especially when you're on rough ground.
I've used DGPS when using the WAAS correction signal, but it can drift a few feet in an hour. RTK is much more stable.
-Lance