An ultrasonic obstacle detection system for mounting on the front bumper of an automobile was designed. Using an omnibus ultrasonic transducer array and ultrasonic pulse coding techniques, an algorithm for collision identification was formulated based on a synthetic aperture to discriminate between echoes constituting real threats and those that do not. Theoretical calculation and test results show that the detectable diameter of flat boards is 15mm, and the active range is between 0.2～25m. Furthermore, the multiple obstacles in front of the automobile can be located simultaneously.