1. In a mobile robot of the type having (a) a vision system, (b) memory means for storing data derived from the robot vision system, and (c) a computer for processing data derived from the robot's vision system, the improvement wherein the robot's vision system comprises (i) a first array of ranging transducers for obtaining data on the position and distance of far objects in a volume of space, the transducers of the first array being symmetrically disposed on the mobile robot with respect to an axis of symmetry within the mobile robot, each transducer of the first array being fixed in position with respect to that axis of symmetry and seeing a portion of the volume of space seen by its entire array; (ii) a second array of ranging transducers for obtaining data of the position and distance of near objects in the same or an overlapping volume of space, the transducers of the second array being symmetrically disposed on the mobile robot with respect to said axis of symmetry, each transducer of the second array being fixed in position with respect to said axis of symmetry and seeing a portion of the volume of space seen by its entire array, the angle of view of the transducers of the second array being different from the angle of view of the transducers of the first array with respect to the same object in space; and (iii) means for polling said ranging transducers in sequences determined by the computer.