Mike, Chat - GPS antennas don't require an external ground plane, whatever
they need is internal to the antenna module. Mike, what you're calling a
ground plane in the submitted photo is probably nothing more than a mounting
shelf. Mine is mounted just behind my head aft of the roll-bar on the
center-line on a fiberglass shelf (no metal other than the connector and
cable) and does just fine (very high S/N ratio on a bunch of satellites).

Mike,  Chat, Dan, et al,

 

Yes, it appears that the GPS antennae are sufficiently self-contained.  Mine, like Dan's, is mounted just behind the roll-over and above the forward edge of a giant top mounted circular ADF ground plane (ADF now defunct along with my sock garters). The Garmin 430 has had no trouble finding all the satellites it needs above the horizon for RAIM approved approaches.  Years ago an older GPS with a radio stack top mounted antenna had trouble in steep turns - but that was so long ago that there just weren't enough satellites, and certainly not enough in view during the turn.