A system for determining the position and orientation of a moveable object in space relative to a stationary object has two transmitter units mounted in fixed, spaced relationship on a stationary console and two receiver units mounted on a headset or other moveable object. Each transmitter unit has a triplet of orthogonally arranged LEDs, each of which generates a hemispherical beam of radiation. Each receiver unit has six photodetectors (A-F), each of which has a plane of sensitivity such that the intensity of incident radiation is proportional to the cosine of the angle of incidence of the radiation. The six photodetectors are arranged such that they form four sets of three photodetectors with the photodetectors in each set having their planes of sensitivity arranged orthogonally. Each of the photodetectors generates a direction cosine signal proportional to the intensity of radiation received from each LED. The radiation is pulsed infrared radiation and each LED in turn is pulsed for a pre-set time period. Each photodetector is monitored sequentially during energisation of each LED to obtain a direction cosine signal for the radiation incident at each photodetector in turn from each LED in turn. The resulting signals are processed to determine the position and orientation of the headset in space relative to the LEDs.
