An object is detected by generating a m-ary primary signal having an irregular sequence of states. Each transition results in the transmission of a pulse encoded according to the type of transition. Reflected pulses are processed with a delayed, reference version of the primary signal. The presence of an object at a range corresponding to the delay is determined from the extent to which the reflected pulses coincide with transitions in the reference signal. In one aspect, transitions between states of the primary signal occur at varying time offsets with respect to nominal regular clock pulses. In another aspect, the object-detection system is operated while inhibiting the transmission of pulses, and if a significant output is obtained, the parameters of the transmitted signal are altered.