Ah yes, we've spent a lot of time getting GPS's to work in all attitudes in a turbine-powered plane. The only definitive thing you can say is that what you need to do to make it work varies from airframe-to-airframe, and GPS unit-to-GPS unit.

There are two fairly easy things you can do to try and make this work. First, if you're using the hockey-puck style antenna, try putting a small ground plane under it. It only needs to be about 2" bigger all around than the antenna, and you can use aluminum tape to make it, but this helps most of those antennas ALOT. Of course, this may not help getting GPS in unusual attitudes (like inverted), but sometimes it even does help that. The second thing you can do is try the RCATS Geohelix antenna:

http://www.rcatsystems.com/cube/inde...d&productId=17

This has provided us with GPS reception that is very (but sometimes not completely) attitude independent in several of our vehicles.

Bob