Abstract: A mobile robot which has a guidance and navigation system (sometimes called a "vision" system) employing one or more arrays of ultrasonic transducers and, preferably, one or more infrared detectors, as well. Each of the ultrasonic transducers is arranged to monitor its own sector of the robot's workspace. Sector monitoring is facilitated by providing for each such transducer a horn fronting the transducer; at least part of the horn's surface is covered by ultrasonic energy absorptive material. According to a second aspect of the invention, the robot is used in conjunction with an infrared beacon system including one or more (preferably ceiling-mounted) beacons each of which emits a uniquely coded infrared signal. The robot is equipped with an infrared detection means affixed to the robot and arranged to allow light from the beacon to impinge on an infrared sensor only when the detector is in precise alignment with the beacon.