Synthetic aperture sonar-the modern method of underwater remote sensing

Abstract

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 difhction limits, the resolution is frequency independent. Such characteristics make SAS a usell 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.Over the past ten years, we have shown to be false the longheld notion that medium-induced signal fluctuations render SAS unfeasible. Through a careful importation and extension of key synthetic aperture radar (SAR) technologies, such as "autofocusing," we are successfully able to process SAS data. SAS is by no means the same as SAR, but is very similar. Among the differences are the types of medium-induced signal fluctuations, e.g., the velocity of sound changes with temperature, and the amplitude of the motions to which the vehicle is subject. Autofocusing and other data driven compensation techniques are the key to overcoming these problems. For instance, our algorithms have routinely estimated and compensated for 40,000 degrees of phase error that arose from uncompensated vehicle motion and medium induced fluctuations.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 26km 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 .

Spectral Normalized CycleGAN with Application in Semisupervised Semantic Segmentation of Sonar Images

“…Compared with real aperture side-scan sonar, synthetic aperture sonar (SAS) has higher hydrographic surveying and charting speed and can produce higher resolution images [ 1 – 3 ]. The accurate detection and identification of underwater targets in synthetic aperture sonar continue as a significant issue [ 4 – 6 ].…”

Coordinate transformation and three-dimensional backprojection algorithm for multiple receiver synthetic aperture sonar motion compensation

“…Synthetic aperture sonar (SAS) [1][2][3][4] is a novel imaging technology that can provide centimeter resolution over hundreds-ofmeters range on the seafloor. Generally, SAS is carried on the unpowered towed fish or the autonomous underwater vehicle (AUV).…”

Azimuth-Invariant Motion Compensation and Imaging Chirp Scaling Algorithm for Multiple-Receiver Synthetic Aperture Sonar

“…Synthetic aperture sonar (SAS) is an imaging technology that can provide centimeter resolution over a range of hundreds of meters on the seafloor [1][2][3][4][5][6][7][8]. In recent years, it has been widely used in underwater topographic mapping, archaeological search and rescue, sunken-object salvage, submarine cable and oil pipeline laying, and mine hunting and high-resolution reconnaissance in important sea areas.…”