Performance evaluation of the Golay code pulse compression technique

The Center for NDE, Iowa State University, has developed a laboratory prototype Golay code pulse compression system and tested it on a variety of materials. The performance of the system was evaluated in terms of signal-to-noise ratio enhancement (SNRE), resolution, and computation speed. The system's error sources also were discussed. The Golay code pulse compression was simulated on a computer and demonstrated the effective noise suppression. In addition, an equivalent pulse of the Golay code (delta-like pulse) was derived theoretically using a simple ultrasonic inspection model, which demonstrated its equivalence on the output correlated signal. Overall, the pulse compression technique extended the detection range for a given peak power and considerably reduced the system'swhite noise, hence providing enhanced signal-to-noise ratios (SNRs). An average of 30 dB improvement in SNR was obtained from highly energy-absorbent materials such as rubber, plastics, corks (insulation materials), and thick composites using the Golay codes of up to 512 bits. However, the technique did not effectively reduce coherent scattering noises from the coarse grain boundaries in cast stainless steels, Inconel weld metal, and material lay-ups in thin composites. Furthermore, it was found that, depending upon the system's hardware capabilities, the overall performance could be degraded considerably.