An ultrasonic sensor assembly for testing a pipe includes a first and second transducer rings attached to the pipe and spaced apart along a length of the pipe. The first transducer ring includes a plurality of transmitters for transmitting a wave, such as a non-dispersive guided wave. The first transducer ring transmits the wave along the pipe. The second transducer ring includes a plurality of receivers for receiving the wave. A relative position of the first transducer ring with respect to a circumferential position of the second transducer ring is determined based on characteristics of the wave received by the second transducer ring. A method of positioning the ultrasonic sensor assembly on the pipe is also provided.