A method for determining a position of a device in a reference coordinate system. The method including: receiving, at the device, less than all of GPS signals necessary to determine the position of the device in the reference coordinate system; transmitting a signal from aν illuminating source defined in the reference coordinate system; receiving the signal at a cavity waveguide disposed on the device; and determining the position of the device in the reference coordinate system based on the GPS signals and the signal received in the cavity waveguide. The signal received in the cavity waveguide can also be used to confirm a position determined by the GPS signals.