Hi there
There could be several options, as Razor Concepts said, a compass sensor might work, such as the Dinsmore 1490. Another possibility is GPS, a reciever on the helicopter and on the 4 wheeled thing, with a radio link between them, so that the 'copter can tell the truck where it is, and the truck can go there. What exactly is the 'attaching device'?
Thanks,
WaterPig Master