a transmitter arranged to transmit signals of coded information about a position close to the unit in a space so as to provide a receiver arranged on a moving object, present in said space with position information for determining the position of said object, said transmitter being adapted to normally be in an inactive non-transmitting state; and

a detector adapted to detect movement of said object within said space and to activate said transmitter to transmit said signals when a movement of said object in said space is detected, wherein said detector is adapted to detect movement of said object only within a restricted predefined volume of said space; and wherein said transmitter is activated only as long as movement within said predefined volume is detected.