Synthetic aperture sonar-the modern method of underwater remote sensing

Synthetic aperture processing-the coherent combination of data from multiple returns-has revolutionized radar imaging over the past 35 years. Sonar is in the early stages of the same revolution. Synthetic aperture sonar (SAS) data can be processed into images with resolution independent of range. Additionally, within diffraction limits, the resolution is frequency independent. Such characteristics make SAS a useful tool for applications such as bottom searching and mapping, mine hunting, or submarine detection, for near ranges (~50 m), mid-ranges (several hundreds of meters), and far ranges (several tens of kilometers) respectively. We have SAS-processed data collected by eleven different hardware suites at frequencies ranging from 240 kHz down to 600 Hz; physical array sizes ranging from half a meter to 256 meters; and with range-independent azimuthal resolutions ranging from 2.5 cm at 50 m for the high frequency systems to ~6 m resolution at 26 km for the lowest frequency system. Our results demonstrate that phase coherence can be maintained in spite of severe vehicle motion, medium fluctuations, and multipath propagation. For instance, we imaged the interior structure of a sunken airplane at 350 m and at 1,000 m with similar cross-range resolution, despite the rays having suffered a bottom bounce on both the transmit and return paths for the 1,000 m range data. The same imaging technology is applicable to cross-track interferometry (for resolving vertical features), bottom penetrating sonar, and wideband waveforms