Just thinking here... GPS "receivers" typically have an internal "patch" antenna. It would make sense to me that the antenna was in the top of the receiver package, to better "see" the satellites above. If the receiver is mounted inverted, the internal circuit board would be between the patch antenna and the sky. I would think that would adversely affect the signal strength picked up from the satellites. You can try it for yourself, right side up, and up side down, by monitoring the satellite signal strength using the instructions on pages 8-13 through 8-15 of the SkyView System Installation Guide - Revision AE.