A miniature PMDC motor has a wound rotor 15 comprising a rotor core 16 and a commutator 17 mounted on a shaft 14. The method involves receiving terrain data representing a 3D terrain, receiving data defining parameters of the vehicle, and receiving data defining a path for movement of the vehicle from an initial position on the terrain to a displaced position on the terrain. The data is supplied to a circular memory device, which determines which data corresponds to which satellite.