On another forum I had a PM response from a MG engineer. Whether he was or not, I have no idea. He told me it was a compass in the head, or possibly 2, I can't remember, that made the unit more accurate. The GPS can only hold as good as the govt accuracy allows. By using a compass or 2? the unit can tell if it is moving off the spot before the GPS. Made sense to me. Not sure how 2 GPS receivers would help if they are both getting the same info?