A CTFM synthetic aperture sonar
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis describes the theory and operation of a seafloor imaging Synthetic Aperture Sonar (SAS), based on broadband Continuous Tone Frequency Modulation (CTFM). Narrowband synthetic aperture techniques are reviewed, and some of the limitations of using such techniques in sonar applications are described. Aperture undersampling is a particular problem when towing single-beam sonars at realistic speeds. However, the mapping rate constraints may be relaxed by using broadband signals, but at the expense of increased self-clutter (or background). One such broadband signal that is suitable for synthetic aperture operation is CTFM. The signal processing requirements for the CTFM signal are investigated, and are shown to be considerably simplified by decomposing each echo sweep into an ensemble of narrowband components. Images reconstructed from each of these components may be combined in a variety of ways. The relative merits of these differing methods are examined using a computer simulation of a side-scan CTFM sonar. The temporal phase stability of the acoustic environment is vital to the formation of a synthetic aperture. An experiment was performed which indicates that the phase stability is much better than anticipated, and certainly adequate for the formation of undersea synthetic apertures. This prediction was confirmed by another experiment in which the prototype CTFM sonar was moved along a fixed cableway under realistic operating conditions. Images of a test target (an air-filled steel buoy) were successfully reconstructed using data measured from this experiment. Reconstructions obtained from a number of different algorithms are presented, for differing values of the various operating parameters. It is demonstrated that artefacts resulting from aperture undersampling are reduced by using broadband CTFM.