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).

Dan Schaefer