Abstract
Synthetic aperture sonar (SAS) is an imaging technique to produce centimeter resolution over hundreds-of-meter range on the sea floor, by constructing a virtual aperture whose length automatically adjusts itself for a given focusing range. SAS is near-field acoustic imaging, and this implies that the sound velocity should be accurately estimated for well focused imaging. Otherwise there will be image quality loss. However, sound velocity in the ocean varies with space and time, and there might also be measuring error of CTD (Conductivity, Temperature, and Depth) sensor, so sound velocity error has become one of the limiting factors to improve SAS resolution further. To characterize the effect of sound velocity error quantificationally, the practice SAS resolution is mode as the convolution of ideal seafloor reflectivity function and a phase error function in frequency domain, where the phase error is caused by incorrect sound velocity. Then the SAS resolution parameterized is calculated as a function of the sound velocity measuring error, or sound velocity gradient. It is shown that SAS azimuthally (along track) resolution loss, caused by sound velocity measurement error, increases linearly with detection range. Meanwhile the loss caused by sound velocity gradient increases squarely. It is simulated by considering the synthetic aperture data collection for a particular pixel, and results show that the point scatter response will defocus when the sound velocity measuring error is up to 1% at 200m range, or the sound velocity changes up to 2% over a typical gradient at 200m range, and be worse at a longer range. Furthermore, we demonstrate the influence of sound velocity errors on SAS imagery using a sea trial data and real CTD measurements at South China Sea. We evaluate the degradation in image quality with respect to sound velocity errors by using two plastic balls and a variable seafloor scene, and results also support the accuracy of theoretical conclusions above.
Highlights
Synthetic aperture sonar (SAS) is an innovative and high technology in the 21st ocean century
Sound velocity in ocean varies with space and time, so sound velocity error has become one of the main limiting factors to further improve synthetic aperture sonar resolution, especially the low frequency and long detection range SAS
We have concluded from theoretical analysis that both sound velocity measuring error and sound velocity gradient can worsen SAS azimuth resolution
Summary
Synthetic aperture sonar (SAS) is an innovative and high technology in the 21st ocean century. Because of its key role in achieving long-distance high-resolution in low frequency detection, SAS becomes a powerful tool for underwater imaging and has broad application prospects in military and national economic fields[1,2]. Sound velocity in ocean varies with space and time, so sound velocity error has become one of the main limiting factors to further improve synthetic aperture sonar resolution, especially the low frequency and long detection range SAS. The basic principle of synthetic aperture sonar is introduced in this paper, and the effect of phase error on SAS resolution. It is calculated that extended value of SAS beam width, causes by sound velocity measurement error, increased linearly with detection range.
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