1. A position detection system for detecting the three dimensional position of at least one target, including: at least one target, wherein the or each target is configured to act as a retro-reflector for light incident from any direction; at least one light emitter for illuminating the at least one target; at least one detector for detecting and taking measurements of light retro-reflected from a target; and a processor for processing measurements taken by each detector to determine the three dimensional position of the at least one target.