One optical distance sensor is installed in front of and one behind the large-scale storage system. The reflector is mounted on the RBG itself. The distance sensor now emits beams of light to the reflector which reflects these back to the distance sensor. This evaluates the time taken for the light to return and in this way calculates the distance between the distance sensor and reflector (RBG). For safety reasons, two distance sensors are installed: if one sends an incorrect signal then this is checked by the other one.