A code-division, multiple beam sonar imaging system
In this thesis, a new active sonar imaging concept is explored using the principle of code-division and the simultaneous transmission of multiple coded signals. The signals are sixteen symbol, four-bit, non-linear, block Frequency-Shift Keyed (FSK) codes, each of which is projected into a different direction. Upon reception of the reflected waveform, each signal is separately detected and the results are inverted to yield an estimation of the spatial location of an object in three dimensions. The code-division sonar is particularly effective operating in situations where the phase of the transmitted signal is perturbed by the propagation media and the target Most imaging techniques presently used rely on preservation of the phase of the received signal over the dimension of the receiving array. In the code-division sonar, spatial resolution is obtained by using the combined effects of code-to-code rejection and the a-priori knowledge of which direction each code was transmitted. The coded signals are shown to be highly tolerable of phase distortion over the duration of the transmission. The result is a high-resolution, three-dimensional image, obtainable in a highly perturbative environment Additionally, the code-division sonar is capable of a high frame rate due to the simplicity of the processing required. Two algorithms are presented which estimate the spatial coordinates of an object in the ensonified aperture of the system, and the performance of the two is compared for different signal to noise levels. Finally, the concept of code-division imaging is employed in a series of experiments in which a code-division sonar was used to image objects under a variety of conditions. The results of the experiments are presented, showing the resolution capabilities of the system.
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Massachusetts Institute of Technology and Woods Hole Oceanographic Institution