If it is consistantly in the same region of a field then it is likely some interference or signal reflection.

I have seen the same issue in a area around Greenville, IL. Turned out to be both Raven and Trimble recievers doing it at the same time. Then either wait an hour or move to another location and it goes away.

Raven helix antennas are pretty good about tracking the satilite positions in the sky relative to the others being recieved. But if there is a reflective signal from somewhere it could throw the whole thing off.

The creators of WAAS designed it for 3 meter position accuracy. Everything better than that is being done by your reciever. So you can imagine it won't take much to screw the whole system up. Why do you think the whole Lightsquared has been such a big issue for ag.