Abstract: A position sensing apparatus 1 having two base stations spaced-apart a known distance on a datum line L. The base stations each comprise generators 2(a) and 2(b) which emit a rotating laser beam. The rotating laser beams are detected by a movable sensor 6 and datum sensors 7(a), 7(b) which are associated with the generators 2(a), 2(b) and are located on the datum line L. The time difference between detection of each laser beam by the movable and datum sensors 7(a), 7(b) is determined and is used to calculate positional angles .alpha. and .beta. between position lines between the movable sensor and each base station and the datum line L. As there are only two laser beams, they are differentiated by contra-rotation in the same horizontal plane. Vertical position is determined according to the vertical position on the movable sensor 6 at which the beams are detected.