Incoherent processing of synthetic aperture sonar
The technique of synthetic aperture imaging is well applied in radar to produce images with high along-track resolution. However, when applied in sonar imaging, maintaining phase coherence across the length of the synthetic aperture had proven to be the greatest challenge due to the relatively much slower speed of sound waves and the instability inherent in an underwater towed platform. This work aims to study, analytically and experimentally, the potential of reconstructing synthetic aperture images, by the exact reconstruction algorithm in the time-domain, without the use of phase information. The absence of phase information results in images with a relatively lower along-track resolution. However, the benefits of hardware simplicity and higher mapping rate, not present with the use of side-scan sonar, are still enjoyed. In addition, greater flexibility and robustness is achieved as compared to a coherent synthetic aperture system. This is manifested in its resilience to path deviations, tolerance to sparse arrangement of elements in the aperture, and consequently higher achievable mapping rate. Software simulations were performed to study the impact of key parameters on the performance of an incoherent synthetic aperture system. Concepts such as the point-target energy response and signal to self-noise ratio were formed, which led to the formulation of design methodologies and tools for such a system. A novel algorithm is introduced to address the problems of false targets and long processing time. A new bulk motion compensation method is presented for situations where large path deviations cause severe image degradation and compensation is necessary. This technique, together with the algorithms developed in this work, was implemented with data from a sea-survey of the Aeolian Sky wreck. Results gave a positive indication of the theoretical frameworks established. This technique was then extended to achieve 3-D imaging capability. Results from laboratory tank trials that were carried out were encouraging, and led to the development of an original concept of incoherent synthetic aperture synthesis that is practical, robust, economical, and has 3-D imaging capability.