A method and apparatus for retrofitting a backlit sign with an LED module. The sensing system for a self-service terminal comprises an aerial comprising at least one receive circuit and a processor arranged in use to determine the position of a first resonant circuit mounted in a media cassette inserted into the terminal based on a signal received via the aerial. A translational potential of the mobile body and a rotational potential of the mobile body for avoiding interference with the obstacle are generated, based on the distance from the autonomous mobile body to the obstacle at each directional angle.