Apparatus for use in one of a plurality of sensors each having a respective transmitter which transmits pulses for sensing, a respective clock which controls timing of the pulses transmitted from the respective transmitter, and a respective receiver which receives echoed instances of the pulses. The apparatus comprises: sensing logic configured to sense a being or object in dependence on the echoed pulses received back by the respective receiver from the respective transmitter, and timing logic configured to compensate for a clock discrepancy between the respective clock and that of one or more others of the sensors. The timing logic does this by using the respective receiver to listen for instances of the pulses from the one or more other sensors, and adjusting the timing of the pulses from the respective transmitter based thereon.