Investigation of a Phase-Coded Pulse Compression Technique for Improved Unambiguous Sonar Transmission

Most transmitters are peak-power limited due to the allowable peak power to the transducer. A higher maximum average power can be achieved with these transducers by keeping the same peak power but increasing the pulse width. This allows the range of the sonar system to be increased, however, the range accuracy is degraded due to the larger pulse width. A common method of improving the range information is Pulse Compression. In effect, a wide pulse is transmitted with an FM component and, when received, is passed through a frequency-sensitive filter such that the higher frequencies will be delayed longer than the lower frequencies. The output of this filter exhibits a narrow pulse and thereby gives improved range information. The range information of this narrow pulse is improved but not to the accuracy of a conventional pulse system due to the filter sidelobe effect which adds range ambiguities. These time sidelobes can be reduced by the use of sidelobe weighting filters, but a loss in signal-to-noise ratio results. If the phase-coded pulse system proposed herein were used, the received decoded pulse would theoretically have unambiguous range information and the signal-to-noise ratio would depend only upon the total number of bits in the coded pulse. A brief description of FM pulse compression is given background reference. followed by the proposed Phase - Coded (PC) method. The logical development and mathematical, substantiation of the PC method is presented with various analytical treatments. The doppler dispersion effect, which is present whenever relative motion between target and source exists, is analyzed for various pulse code widths. A relationship between relative target speed and pulse amplitude is derived such that operational parameters may be determined. The sidelobe effect, which is present in both the FM and PC methods, is discussed and a method of elimination is developed for the PC system. Sidelobe elimination does have an effect on the signal-to-noise ratio, and a comparison is made between the two methods on this basis along with the standard basis of comparison