SAS Image Processing Software
Synthetic Aperture Sonar (SAS) systems use acoustic energy for creating ultra-high resolution images of the seabed. SAS image processing is very closely related to Synthetic Aperture Radar (SAR), seismic imaging and medical ultrasound. One of the fundamental challenges in SAS beamforming is processing speed. Large amounts of data needs to be quickly formed into images.
Synthetic Aperture Sonar offers significantly higher resolution imagery at longer ranges than conventional side-scan sonars. This is accomplished by replacing traditional sonar hardware with sophisticated signal processing software. The principle of SAS is that a long receiver array (much longer than a physical one) is “synthesized” in the along-track direction. This is achieved by coherent recombination of many pings that overlap the area of interest. Typically, a 300 kHz SAS would operate with 60 pings to form the synthetic aperture at 300m. This represents a major savings in sonar hardware and enables much higher resolution than is otherwise possible with conventional 300 kHz sonars.
In 1969, twenty Volvo bodies intended for the Volvo manufacturing plant in Halifax incurred severe water damage in transit across the Atlantic Ocean. With permission, the vehicles were dumped into the Bedford Basin, where they sit today in 65 metres water depth. The image above was produced by the AquaPix® InSAS during sea trials with Defence Research Development Canada in August, 2012. The AquaPix® system is capable of producing ultra-high resolution (3cm) seabed imagery across swath widths of 600m from very shallow water to full ocean depth.
In order to make SAS work, the relative position of the sensors must be known to within a fraction of a wavelength (typically 1/16 of a wavelength required) over the entire synthetic aperture. At 300 kHz, the wavelength is 5 mm so this requirement is nearly impossible to meet with current navigation sensors. As the array is formed from a moving platform with uncertainties in positional accuracy, the calibration and correction of element positions is very challenging. To overcome these challenges, Kraken has developed a number of micro-navigation techniques for this purpose that give extremely accurate positioning of the sonar array. In addition to being a requirement for SAS imagery, micro-navigation can also be used to aid high-accuracy positioning of both the sonar and the underwater vehicle itself.
Once the challenge of accurately computing the positions of the sonar array is overcome, the data must also be beam-formed into a SAS image. As the synthetic aperture array may contain many hundreds of synthetic elements, this beamforming often places a heavy demand on computational processing resources.
The central idea of SAS image processing is based upon matched filtering of the received signal in both the range and azimuth directions. Matched filtering is possible because the acquired SAS data are modulated in these directions with appropriate phase functions.
Kraken has developed a complete software application for signal processing of Interfermoteric Synthetic Aperture Sonar imagery called INSIGHT™ (INterferometric Sas Imaging Georeferenced High-fidelity Toolbox).
The following diagram shows the main building blocks of the INSIGHT™.
The INSIGHT™ toolbox implements advanced beamforming techniques to obtain SAS imagery using a unique algorithm called Widebeam Azimuth Compression that has the same accuracy as much slower backprojection or delay and sum beamformers while being much faster, allowing faster-than-real time processing rates using general purpose graphical processing units.
An interferometric sensor such as the AquaPix® system can be used to estimate the relative seafloor height (which can then be processed into bathymetry maps). The vertical direction to the seafloor is calculated from the estimated time difference of arrival from the two vertically displaced receiver arrays. Interferometry operates through noting that relative height differences give a phase shift at the carrier wavelength.
Interferometric height estimation can give height estimates at the same resolution as the original imagery. One of the biggest challenges is in resolving ambiguities caused by the cyclic phase shift measurements, i.e. un-wrapping the phase estimates. The number of phase wraps is dependent on the carrier wavelength and the interferometric baseline. However, the large bandwidth employed in the AquaPix® system is used to resolve these ambiguities. This allows rapid collection of large swathes of bathymetric information at low processing cost, while simultaneously allowing extremely detailed bathymetric imagery to be generated for the selected areas of interest.
Data is made available in standard file formats (XTF, GeoTIFF, etc). Depending on the required processing speed, a range of hardware options are available. Typical processing speed is 2 to 5 times faster than real-time for high resolution SAS imagery.
INSIGHT™ can be easily integrated with third party post-mission analysis and display software applications such as Triton Perspective™, Google Earth™, etc.
INSIGHT™ SAS mission displayed in Triton Perspective™
INSIGHT™ SAS image displayed in Google Earth™
Kraken is also developing an INSIGHT™ Embedded Real Time SAS Image Processor that will enable SAS image processing to be applied in real time inside the underwater vehicle. This will significantly reduce post mission analysis requirements while increasing operational effectiveness and efficiency in seabed imaging applications.