Several of the handheld GPSs, including most of the Garmin offerings, have external antennae as optional extras. One friend of mine mounts the antenna on the top of the external air intake of his vehicle, a diesel 4WD Toyota with the type of dust-avoiding air intake common in northern Australia's outback. These diesels have the air intake at the same level as the roof of the cabin so an external antenna mounted on top of it is out of harm's way and would rarely have any obstruction to all the satellites above the horizon. He originally got it when using his vehicle as a support vehicle for a participant in the Simpson Desert Cycle Race; navigation by map and compass out there can be a bit tricky.

Cheers, Rowan