Don, I'm green as grass at this so there's lot I don't understand. I do know a little about antennas though and it makes sense to me that a good GPS antenna would have a very sharp roll-off at angles less than 0°, with straight up being 90°. So I did a quick search and came up with the notion that the "Gold Standard" for GPS antennas is something called a "Choke Ring ground plane". Looking at radiation plots for that animal confirms a uniform response starting at about 10°, continuing through 90° and back down to about 170° of elevation and a drop off the cliff at 0° and 180°. Looks like signals have no way to get to the antenna if they come in from the bottom. The engineers seem to be happy with that too, as the stuff that bounces off things is a major source of multi-path interference.

I can only make a guess as to what happens when you drive on a slope, but I'd be surprised if you didn't loose the birds that appeared below the plane of the antenna due to the tractor tipping to follow the slope. Mounting the antenna 10° or more from level would have to exacerbate the situation when you were traveling on the slope in a way that would further tip the antenna. Tipping it in a northerly direction should hide viewable satellites and tipping it southward should increase chances of multi-path problems.

Lastly, a question? I downloaded the Trimble planning software and the latest ephemeris files. They still have the AOR-W bird parked off the eastern coast of Brazil?? I wonder what's the deal there.