Determining the distance of reflectors to an automated guided vehicle

A method for finding the distance of reflectors to an automated guided vehicle (AGV), wherein the reflectors are arranged in positions in a transport area (210, fig. 2). The method comprises emitting a beam B from the vehicle (200) over a search sector within the transport area (210) and receiving at the vehicle reflected signals Br from at least one of the reflecting objects. The beam B is modulated at two modulation frequencies MF1,2 so that a reflection is received from the at least one reflecting object R1 at each modulation frequency. The phase difference between the beam B and the corresponding reflected signal Br is measured to determine a distance between the at least one reflecting object R1 and a reference point on the vehicle. Range ambiguity may be solved by comparing the range of measured distances from each modulation frequency. The method may be carried out by a navigation module 100. The beam B may be modulated at each of the two modulation frequencies separately. The beam may be caused to revolve, and the modulation frequency switched after every revolution. After positional initialisation, the beam may be modulated at a single modulation frequency.