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Products: Marport announces next generation synthetic aperture sonar

Posted on 01.06.2011 - 09:00 UTC in PRODUCT NEWS by Rons_ROV_Links

Marport announces next generation synthetic aperture sonarMarport Deep Sea Technologies Inc. continues to drive the latest advances in acoustic imaging science with today's announcement of AquaPix - a new interferometric Synthetic Aperture Sonar targeted to mine countermeasures, hydrography, seabed survey and other underwater imaging markets.

AquaPix is the latest product to be based on Marport's Software Defined Sonar, an underwater acoustics technology platform that enables advanced underwater sensing, communications and imaging products. The majority of Marport products - ranging from single channel acoustic sensors to sophisticated military sonars - are supported from a common electronics platform. Software Defined Sonar® technology simplifies design, lowers production costs and shortens the time-to-market for new sonar products. The software-centric architecture enables digital signal processing to be executed in multiple FPGA cores allowing dynamic reconfiguration and massively parallel processing performance.

Priced to compete with premium side scan sonars, AquaPix sets a new standard for high-speed, high-resolution underwater imaging. The sonar is frequency agile from 200 - 400 kHz and offers range independent resolution of 2.5cm by 2.5cm with co-registered 3D bathymetry. Effective swath widths are up to 12 times water depth in shallow water, to a maximum width of 600m. This swath width more than doubles that achieved by currently available high resolution bathymetric sonars. With a broad dynamic range and excellent imaging performance, the new sonar is ideal for applications where seabed image quality is critical. This step change in quality greatly facilitates the unambiguous detection of small objects, such as mines and underwater improvised explosive devices; and changes in seabed texture such as that caused by oil spills.

Today's medical imaging techniques often employ multi-aspect sensing to construct internal maps of patients. For example, a Computerized Axial Tomography (CAT) scan moves a probe around the patient and the multi-aspect data is processed to construct high resolution images of the areas of interest. In contrast, acoustic imaging of objects on the ocean floor often rely only on high resolution sensing from limited aspect angles using data gathered on straight-line trajectories.

"Underwater imaging markets present very demanding requirements not only for sonar performance but also overall image quality," said Karl Kenny, President & CEO of Marport. "With AquaPix, we are again leveraging our latest Software Defined Sonar technologies to bring critical improvements in quality and performance. AquaPix provides the underwater imaging markets with a price competitive solution to replace and overcome the performance shortfalls of conventional systems."

Synthetic Aperture Sonar (SAS) is a type of sonar in which sophisticated signal processing of successive acoustic pings is utilized to form an image with much higher resolution than conventional sonars. The image formation can require aligning the echoes to less than 0.1 millimetres. This is made possible through recent advances in broadband sonar technology combined with adaptive focusing techniques similar to those used in optics, but performed in software rather than in hardware. The practical realization of SAS, on surface ships and underwater vehicles, is one of the most significant advances in ocean systems engineering in recent times. Modern SAS systems provide an area coverage rate of several square kilometres per hour with centimetric resolution which enables an optical-like image quality, greatly facilitating the recognition of mine-like objects.

Synthetic Aperture Sonar has the potential for providing image quality that is unmatched by current sonars and is a key technology whenever high resolution is required. The technology can contribute towards the reduction of several capability shortfalls identified by NATO navies, such as difficulties in detecting and classifying maritime mines for reasons of size, shape, material or location. In addition, when combined with depth estimation techniques which exploit two SAS arrays with a large vertical separation between them for high accuracy interferometry, SAS can deliver high quality 3D images of the seabed which are sufficient to meet or exceed even the most demanding standards for hydrographic surveys today. Furthermore, a by-product of SAS is a highly accurate ground velocity estimate which is of great interest for underwater vehicle navigation applications, to further limit the drift of aided inertial navigation systems and to ensure the positional accuracy requirements of the IHO standards are met for surveys using underwater vehicles.

The results of recent modeling of sonar sensing requirements, undertaken by the NATO Undersea Research Centre, the UK Defence Science and Technology Laboratory, and other similar institutions in the USA, have shown that to achieve a given level of target recognition a combination of a minimum signal to noise ratio and along and across track resolution is required. This extends the Johnson criteria which provide the minimum resolution requirements for different tasks ranging from detection to identification using video imagery. Using the Johnson criteria, many predictive models for sensor technology have been developed that predict the performance of sensor systems under different environmental and operational conditions. Sonar imagery is very challenging, requiring even higher resolution than video, due to the presence of speckle noise which is not present in video. AquaPix has been designed to address these requirements and will therefore interface seamlessly with real-time Automatic Target Recognition (ATR) applications. It is expected that real-time ATR will ultimately replace the human operator to detect and classify mines.

The unique AquaPix sonar design exploits a dual row frequency multiplexed transducer array which allows selection of the vertical beamwidth of both the transmitter and the receiver on the fly in order to optimally suppress multipath. Two beams with different beam-widths are transmitted at the same time, at different frequencies, and the best beam is dynamically selected by frequency filtering, a technique which is directly inspired from Orthogonal Frequency Division Multiplexing (OFDM) used in radio communications for the same purpose. At the same time the multiplex provides the high level of spatial overlap required to robustly implement the adaptive focusing techniques referred to above.

The AquaPix sonar solution is modular in design, using broadband piezocomposite ceramic arrays which are fully encapsulated for robustness against harsh environments. The use of piezocomposite ceramics not only provides unsurpassed bandwidth and flexibility, but also delivers repeatability in manufacture of sonar arrays with precisely defined beam patterns to provide the spatial filtering crucial to achieving high performance. Transmit and receive electronics are fully encapsulated within the sonar array modules, with small, flexible cables for connectivity to a host pressure vessel.

The pre-amplifiers and sampling rates exploit many years of SAS science and engineering development and achieve true 24 bit sampling with a power consumption of 150mW per channel. The transmit electronics achieve 97% efficiency, with fine phase and amplitude control. The modular arrays are designed to be integrated into a range of platforms such as autonomous underwater vehicles, towed vehicles and hull mounts. The electronics design and wideband arrays facilitate operation on multiple frequencies simultaneously, for multipath suppression and adaptive focusing, but also for target classification purposes. The embedded computer provides increased data throughput for faster image processing, a reduced "click-to-capture" time for improved sonar imaging response, and lower power consumption for increased operational endurance when deployed on autonomous underwater vehicles.

Other features and capabilities of the AquaPix system include:

  • Superior Area Coverage Rate - meets IHO standards up to 12x water depth
  • Ultra High Resolution Seabed Imagery - 2.5cm x 2.5cm resolution cell across full swath
  • Precisely Co-Registered Seabed Imagery and 3D Bathymetric Data - real time geo-referenced data
  • Unsurpassed Shadow Depth - high image contrast due to multipath suppression
  • Modular and Flexible System Architecture - low power system designed for all water depths

Appointments for military test and evaluation sea trials are currently being booked. Commercial availability and volume production is planned for Q4 2011. For more information on the AquaPix Synthetic Aperture Sonar, please email aquapix[at]marport[dot]com

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