Active synthetic aperture sonar (SAS) is a powerful imaging technique that coherently combines
echoes from multiple pings along the trajectory of a survey path to construct a long virtual array of
hydrophones, which are microphones designed to be used underwater for recording or listening to underwater sound.
When synthetic aperture techniques are applied at sufficiently low acoustic frequencies, where sound absorption in the ocean medium is minimized, a modest-sized side-scan sonar can generate imagery with a constant azimuth resolution comparable to that of higher frequency sonar systems,
but with a longer range potential, as shown in Figure 1.
Side-scan sonar is a category of sonar system that is used to efficiently create an image of large areas of the seafloor. It may be used to conduct surveys for maritime archaeology; in conjunction with seafloor samples it is able to provide an understanding of the differences in material and texture type of the seabed. Side-scan sonar imagery is also a commonly used tool to detect debris and other obstructions on the seafloor that may be hazardous to navigation, as illustrated in Figure 2. Side-scan data are frequently acquired along with bathymetric soundings and sub-bottom profiler data, thus providing a glimpse of the shallow structure of the seabed. Bathymetry is the study of underwater depth of lake or ocean floors. (See Figure 3.)
Synthetic aperture technology originated in the radar community in the mid-20th century, and was adapted by the sonar community approximately 20 years later. For some time, SAS was not practical because of the limitations associated with enabling technologies, such as underwater platforms, suitable motion measurement instrumentation, accurate motion estimation techniques, and the storage and processing components needed to meet the computational requirements
associated with SAS beamforming. This has changed over recent years and SAS systems are now being fielded in a wide range of military and commercial applications such as geological mapping,
telegraph and pipeline surveys, environmental remediation, marine salvage and archeology and mine
countermeasures. Beamforming is a general signal processing technique used to control the directionality of the reception or transmission of a signal on a transducer array. A transducer is a device that converts a signal in one form of energy to another form of energy. Energy types include electrical, mechanical, electromagnetic (including light), chemical, acoustic or thermal energy.
Small Synthetic Aperture Minehunter
Since the 1980s, the U.S. Office of Naval Research (ONR) has developed advanced synthetic aperture sonars for detection, localization, and classification (DLC) of mines, for protection of sea lines of communication and naval operating areas, and for support of amphibious operations. The range of activities required by these sensors includes: intelligence preparation of the operational environment (IPOE), search-classify-map (SCM) operations, and reacquisition-identification (RI) of mine-like objects for subsequent neutralization.
Recently developed SAS systems have been designed to operate over a wide range of wavelengths and aspects. Centimeter-scale wavelengths (with acoustic frequency typically greater than 100 kilohertz) are used for fine-detail imaging of seabed texture and of small man-made objects.
Longer wavelengths, which propagate deeper into the sediment volume, are used for imaging and spectroscopic analysis of buried objects that lay proud on the seabed. Spectroscopic analysis refers to the measurement of electromagnetic radiation intensity as a function of wavelength.
The Small Synthetic Aperture Minehunter (SSAM), developed by the Naval Surface Warfare Center Panama City Division (NSWC PCD) and the Applied Research Laboratory, Penn State University (ARL-PSU), is a multiscale frequency design that exploits all of these advantages. It consists of two SAS systems: a high frequency (HF) synthetic aperture sonar and a long-wavelength broadband (BB)
synthetic aperture sonar, wherein two separate projectors share a common hydrophone array, as shown in Figure 1. The SSAM is deployed on a Woods Hole Oceanographic Institution Remote Environmental Monitoring UnitS 600 (REMUS 600), commonly referred to as the 12.75, because of its
hull diameter. It may be operated to a depth of 600 meters.
Presently, two generations of the SSAM concept exist, both of which operate in strip-map mode: monostatic and utilizing broadside beams. A conventional SAS strip mapping mode assumes a fixed pointing direction of the hydrophone array broadside to the platform track. A strip map is an image formed in width by the swath of the SAS and follows the length contour of the flight line of the platform itself. The first generation SSAM system was fielded from 2005 through 2009 and participated in 11 events surveying more than 23 square nautical miles of seabed. The second
generation system (SSAMII) has been fielded since 2010, and is designed for hunting proud and heavily scoured objects in shallow water and nearshore environments. New features include an improved hydrophone array and projectors that effectively reduce interference from surface multipath reflections, thus extending the range of the system in shallow water environments. To accomplish this, the hydrophone array has a multichannel vertical aperture that allows beam
steering to reject energy scattered from the sea surface. This new design houses the receiver electronics in an oil-filled cavity behind the array, and is used for enhanced motion estimation and generation of high resolution bathymetry maps.
The HF projector was redesigned in an asymmetric curve to reduce surface ensonification further improving signal to reverberation ratio. As in the previous generation, the SSAMII can
accommodate storage and processing components for real-time SAS image formation and implementation of automatic target recognition (ATR) for initial generation of a sortie report that can be transmitted by a RF link or acoustic communications.
Tomographic and Interferometric SAS Processing
A recently developed modality exploiting tomographic processing (taking measurements around the periphery of an object) has been demonstrated with the SSAM system. This modality, referred to as “circular synthetic aperture imaging” (CSAS) in technical literature, is capable of very high fidelity image generation. CSAS is similar to conventional strip-map SAS in the sense that the sonar trajectory is exploited to synthesize a much larger array than that of the physical sonar.
Unlike strip-map SAS systems operating on a linear trajectory, CSAS, as implied by the name, circumnavigates and repeatedly ensonifies the area to be imaged. Signal processing techniques
similar to those applied by medical computerized tomography (CT) scanners are used to reconstruct
a very high resolution image from the back-scattered acoustic information. Though neither strip-map nor circular SAS need to operate on their ideal linear and circular paths to form high resolution imagery, the platform position must be precisely known. Position estimation has historically been a primary cause of SAS image degradation and the major handicap preventing field usage of the tomographic imaging modality. Powerful motion estimation and data-driven focusing
techniques are now capable of making high quality linear and tomographic SAS images in a consistently robust manner.
The photographic quality of circular scans provides images that an operator could use to identify objects with high confidence. The resulting information content in the digital data is
extremely rich, appropriate for use by a variety of scene analysis and target recognition algorithms. In undersea warfare, a canonical minehunting procedure comprises target detection and classification, with a wide-swath seafloor imaging sonar (SCM phase), followed by confirmation using divers or a short-range identification sensor (the RI phase). The processing techniques described in this article demonstrate the possibility of combining the SCM and RI phases within a single sortie; where the AUV first maps an area using strip-map SAS processing, produces a contact list via in-vehicle beamforming and automatic target recognition; and then returns to circle the object for target identification.
The Small Synthetic Aperture Minehunter system contains vertically spaced rows of hydrophones for interferometric (technique to extract arrival angle of acoustic waves) data
processing. Interferometric processing exploits timing differences in received signals to estimate bathymetry.
The interferometric data channels on the SSAMII can be used to generate bathymetric estimates that are co-registered with the output SAS images. The capability of generating centimeter-scale resolution in all three spatial domains should provide significant performance improvements in the classification and identification of small objects. Additionally, interferometric data can be used to aid the coherent beamforming process making a more reliable and robust system.
Transition and Future Innovations
SSAM technology is transitioning into the acquisition phase for use in autonomous search-classify-map operations and intelligence preparation of the operational environment missions. It is also being used for site inspection and detection of unexploded ordnance in active and formerly used military test ranges, with successful 2012 deployments in the waters off Naval Air Station Patuxent River, Md., and Naval Support Activity, Panama City, Fla. Further deployments are being planned for a variety of sites along the Gulf Coast and Eastern Seaboard.
The next generation of the SSAM, in the early stages of development, is being designed to improve detection, localization, and classification capabilities against fully buried objects. Here,
spectroscopic techniques (multi-aspect, broadband measures of target strength) for the broadband synthetic aperture sonar will be combined with image-based processing from the high frequency synthetic aperture sonar to substantially reduce false alarm rates.
Since the 1950s, Naval Surface Warfare Center Panama City Division (NSWC PCD), then called the Mine Defense Laboratory, has been a leader in the development of side-scan sonar systems for the U.S. Navy. Over the last three decades NSWC PCD, working with its partners in government, industry and academia, has pioneered development of synthetic aperture sonars, with system designs, signal and information processing, platform innovation, and concepts of operation that have had a transformational effect on mine countermeasures and undersea warfare, with commensurate effect in the commercial marketplace.
The mission of NSWC PCD is to conduct research, development, test and evaluation, and in-service support of mine warfare systems, naval special warfare systems, diving and life support
systems, amphibious/expeditionary maneuver warfare systems, and other systems that operate primarily in coastal regions.
Today, NSWC PCD employs more than 1,300 employees of whom more than 880 are scientists and engineers serving at the only U.S. Navy RDT&E laboratory located on the Gulf of Mexico.
The authors gratefully acknowledge the support of the Office of Naval Research, Ocean
Engineering and Maritime Systems (Code 321), Dr. Jason Stack and Dr. Thomas Swean for development of the sensor, vehicle, and signal and information processing associated with the SSAM system.
For More Information
Naval Surface Warfare Center Panama City Division (NSWC PCD): www.navsea.navy.mil/nswc/panamacity/default.aspx
NSWC PCD Public Affairs Office: firstname.lastname@example.org
Office of Naval Research: www.onr.navy.mil
ONR Ocean Engineering and Marine Systems: http://www.onr.navy.mil/Science-Technology/Departments/Code-32/All-Programs/Ocean-Systems-321/Ocean-Engineering-Marine-Systems.aspx
Dr. Daniel Sternlicht is the head of the sensing sciences division at NSWC PCD, which
specializes in development of advanced sensors and processing for Navy and Marine Corps missions. He received a Ph.D. in electrical engineering and applied ocean science from the University of California, San Diego, and Scripps Institution of Oceanography.
Jose Fernandez is the senior sonar engineer for the sensing sciences division at NSWC PCD. He has worked in the design, testing and data analysis of several sonar systems. Most of his recent work has been related to the development of synthetic aperture sonar (SAS) technology.
Dr. Timothy Marston, a research scientist in the field of signal processing at NSWC PCD,
received a Ph.D. in acoustics from Penn State University in 2009. Since 2010, his primary focus has been the development of robust algorithms for synthetic aperture data processing.
For Further Reading
D. Sternlicht and J. F. Pesaturo, Synthetic Aperture Sonar: Frontiers in Underwater Imaging, Sea
Technology, 45(11), pp. 27-32. November 2004.
D. Brown, D. Cook, and J. Fernandez, Results from a Small Synthetic Aperture Sonar, Proceedings
MTS/IEEE OCEANS, September, 2006.
D. D. Sternlicht, J. E. Fernandez, R. Holtzapple, D. P. Kucik, T. C. Montgomery, C. M. Loeffler.
Advanced Sonar Technologies for Autonomous Mine Countermeasures, Proceedings MTS/IEEE
OCEANS, September 2011.
T. M. Marston, J. L. Kennedy, P. L. Marston, Coherent and Semi-Coherent Processing of Limited-
Aperture Circular Synthetic Aperture (CSAS) Data - Applications for Target Field Analysis and Object
Classification, Proceedings MTS/IEEE OCEANS, September 2011.
T. Marston, A Correlation Based Autofocusing Algorithm for Coherent Circular Synthetic Aperture
Sonar, Proceedings of the European Conference on Synthetic Aperture Radar (EUSAR), April 2012.
T. R. Clem, D. D. Sternlicht, J. E. Fernandez, J. L. Prater, R. Holtzapple, R. P. Gibson, J. P. Klose, T.M. Marston. Demonstration of Advanced Sensors for Underwater Unexploded Ordnance (UXO)
Detection, Proceedings MTS/IEEE OCEANS, October 2012.