a method of calculating a time difference is disclosed . the method includes receiving a first ultrasonic signal having a first frequency from a first transducer at a first time and receiving a second ultrasonic signal having the first frequency from a second ultrasonic transducer at a second time . the first ultrasonic signal and the second ultrasonic signal are sampled at a second frequency . the first sampled ultrasonic frequency is interpolated . the difference in travel time between the first and second ultrasonic signals is calculated in response to the interpolated first sampled ultrasonic signal and the sampled second ultrasonic signal ).