Synthetic-aperture sonar (SAS) is a form of sonar in which sophisticated post-processing of sonar data is used in ways closely analogous to synthetic-aperture radar. Synthetic-aperture sonars combine a number of acoustic pings to form an image with much higher along-track resolution than conventional sonars. The along-track resolution can approach half the length of one sonar element, though is downward limited by 1/4 wavelength. The principle of synthetic-aperture sonar is to move the sonar while illuminating the same spot on the sea floor with several pings. When moving along a straight line, those pings that have the image position within the beamwidth constitute the synthetic array. By coherent reorganization of the data from all the pings, a synthetic-aperture image is produced with improved along-track resolution. In contrast to conventional side-scan sonar (SSS), SAS processing provides range-independent along-track resolution. At maximum range the resolution can be magnitudes better than that of side-scan sonars. A 2013 technology review with examples and future trends is also available. For academics, the IEEE Journal of Oceanic Engineering article: Synthetic Aperture Sonar, A Review of Current Status gives an overview of the history and an extensive list of references for the community achievements up to 2009. The length of the synthetic aperture is
L s a ≈ R η λ d {\displaystyle L_{sa}\approx R\eta {\frac {\lambda }{d}}}
Where R is the range, λ {\displaystyle \lambda } is the wavelength at center frequency and d is the along-track element size in the array. η {\displaystyle \eta } is a programmable parameter which controls the process beamwidth—the beamwidth actually processed.
Challenges The SAS system relies on a stable sensor platform, being able to determine to a high accuracy where the sensors are over several meters of travel distance—all the pings captured will be used in the formation of a synthetic aperture. Due to currents, heave or sway, a sensor platform may undergo lateral movement known as "crabbing", which have the potential to heavily impact SAS image formation. SAS arrays may not be the best choice for a sensor platform in rough terrain nor areas where one can expect currents from the sides. Mission planning and selection of sensor platform can alleviate some of these challenges. When operating a SAS system in shallow waters, multiple reflections may come back to the sensor from the sea surface, impacting the quality of the data. This also depends on the seafloor conditions, sound velocity profile as well as how rough the sea surface is. One way to alleviate this issue is to angle the beams up slightly—to reduce reflections from the nearest bottom.
Comparison between SSS and SAS Traditional side scan sonars (SSS) have along-track resolution, along-track sampling and range closely coupled. This means that the maximum range and resolution depends primarily on the transmit frequency. A higher transmit frequency gives increased along-track resolution but reduced range. Synthetic-aperture sonars (SAS) on the other hand, limited by cost and complexity, allows free selection of these parameters, providing the potential for long range as well as high resolution.
Along-track resolution Along-track resolution δ x {\displaystyle \delta _{x}} in a traditional side-scan sonar will deteriorate with range in the far field, an object will be imaged with a higher resolution when closer to the sensor, and less when further away. Along-track resolution δ x {\displaystyle \delta _{x}} is constant at all ranges for a synthetic-aperture sonar system, this means an object should be equally visible at most ranges from the sensor.
δ x = R λ L {\displaystyle \delta _{x}={\frac {R\lambda }{L}}}
Where R {\displaystyle R} is the range to target, L {\displaystyle L} is the array length, and λ {\displaystyle \lambda } is the acoustic wavelength, a function of frequency. This means that a traditional side-scan sonar with high along-track resolution will require a very long array length for a distant target. Attenuation of the acoustic energy as frequency is increased and wavelength thus decreased, reduces the effective range. A synthetic-aperture sonar creates a synthetic array of a long length, moving preferably in a straight line, providing a theoretical along-track resolution of a few centimeters. In practice, resolution will be somewhat worse, but still much better than an equivalent sized traditional side-scan sonar.
Across-track resolution The across-track (range) resolution δ y {\displaystyle \delta _{y}} of a SAS, with a broadband FM signal, is given by:
δ y = C 2 B {\displaystyle \delta _{y}={\frac {C}{2B}}}
Where C ≈ 1500 m s − 1 {\displaystyle C\approx 1500ms^{-1}} is the speed of sound in water and B {\displaystyle B} is the bandwidth of the transmitted pulse.
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