This paper presents a new ultrasonic imaging system operating in air. A capacitive ultrasonic transducer array with a non-linear amplitude-detection system is used to get a two-dimensional (2-D) picture of targets. The time-of-flight determines the distance and the time delay between the receiver elements gives the direction of the target. The prototype ultrasonic transducer array consists of one transmitter and four capacitive receiver elements; however, the number of receiver channels can easily be increased. The transducers are designed to cover a wide horizontal but a rather narrow vertical beam pattern to cancel erroneous echoes from the floor when the array is used in a mobile robot. A new beam-forming method based on non-linear order statistic filtering is presented. The proposed robust method determines the time-of-flight and the angle of incidence by exploiting the amplitude domain information of measured signals. In the prototype system a field programmable gate array (FPGA) board preprocesses array signals and a digital signal processing (DSP) board carries out additional tasks and controls the whole system. The distance and direction of targets within the sector-shaped imaging region are shown at the host PC screen either as a mesh picture or as a vector map.