SOund Navigation And Ranging or SONAR is a technique of distance measuring between detector and an object base on sound reflection. The distance can be calculated from propagation time and speed of sound in specific mediums. For military, Sonars are the eyes and ears of ships or submarines in water used to detect, locate and identify objects in water. They are used for underwater navigation, especially by submarines and surveillance. Sonar systems can also be used to realign inertial navigation systems by identifying known ocean floor features.
One of the evolutions of modern sonar is synthetic aperture sonar (SAS), which allows for much higher resolution images than traditional sonar by combining different acoustic pings. 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.
SAS technology has potential in many underwater imaging applications including offshore energy, seabed surveying, marine archaeology, debris mapping and search and salvage operations. Synthetic aperture sonar mounted on an autonomous underwater vehicle can be efficient tool for deep ocean mapping of seafloor massive sulfide deposits generated by hydrothermal systemssuch deposits. The fine resolution and
large areal coverage possible with SAS enables highly efficient mapping of the seabed with a resolution sufficient to determine the location of active and extinct hydrothermal systems.
Mine hunting in particular is an area where SAS promises to solve the inherent problem of covering a large area in a short time – with sufficiently high resolution to allow detection and classification of small, low-signature bottom mines. We have capable adversaries with equally capable mines, which can be planted very deep or in such a way that they’re camouflaged. Raytheon is really focused on, in particular, advancing the synthetic aperture sonars that we produce in order to be able to find these very difficult targets.
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. Synthetic aperture sonar technology is currently commercially available from multiple vendors including Kongsberg Maritime, Kraken Sonar Systems and Raytheon.
Northrop Grumman demonstrated its seabed warfare capabilities at U.S. Navy’s Advanced Naval Technology Exercise (ANTX) in August 2018 in Newport, Rhode Island. A central part of this demonstration was Northrop Grumman’s µSAS and real-time automated target recognition technologies. The µSAS is a low size, weight and power, high-performance payload that can operate off a man-portable autonomous underwater vehicle for real-time classification of mine-like objects. The demonstration showed how we can operate that level of μSAS from a man-portable AUV, company officials said. This data was then relayed to a ground control station comprising five displays, EO cameras, situation awareness tools, and advanced human-machine control software to manipulate the various unmanned systems.
“This technology is an example of leveraging digital transformation to rapidly provide software defined, hardware enabled capabilities to the fleet,” said Alan Lytle, vice president, undersea systems, Northrop Grumman. “Integration of unmanned and autonomous capabilities into the battle space reduces staffing required to conduct operations and risk to personnel, while supporting our nation’s continued undersea superiority.”
Synthetic aperture sonar
When autonomous underwater vehicles (AUVs) started to become commonplace in the 1990s, the side-scan sonar soon became a sensor of choice. However, when high-resolution imaging is required, the area coverage rate of such a system is severely limited. The far-field side-scan sonar along-track (azimuth) resolution is inversely proportional to frequency and antenna length. Antennas longer than 1–2 metres are difficult to fit on AUVs and high-frequency signals have limited range. For a resolution of say 10cm, the maximum range is limited to around 50m even with high-end side-scan sonars.
Synthetic aperture sonar (SAS) is a technology that allows resolution higher than the most advanced side-scan sonars, with a range up to hundreds of metres. SAS is analogous to synthetic aperture radar (SAR) and shares the same basic principle: the forward motion of the platform is utilised to synthesise a long antenna (and hence improve the azimuth resolution). For this to work properly, the platform needs to move less than one-half the physical antenna length between pings. By moving slightly less, overlapping data can be used to estimate the motion of the platform with extreme accuracy. The maximum achievable range with SAS is thus inversely proportional to platform speed. This means that the area coverage rate is independent of speed and directly coupled to the receiver array length.
Synthetic aperture sonar (SAS) has been under active development for decades. The technique is particularly well suited for autonomous underwater vehicles, and it is expected that SAS will replace traditional side-scan sonars for many ‘high-end’ AUV applications in the years to come. 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.
SASs can be mounted on remotely operated vehicles (ROVs), they can be towed or even hull-mounted, but AUVs have some distinct advantages as platforms for SAS systems. Their size, speed and stability can be made to match the requirements of a SAS system. AUVs also usually operate close to the sea floor, away from the difficult near-surface layers. Finally, AUVs tend to be equipped with a highly accurate navigation system, which is a requirement for the optimal use of a SAS.
Mine hunting in particular is an area where SAS promises to solve the inherent problem of covering a large area in a short time – with sufficiently high resolution to allow detection and classification of small, low-signature bottom mines. Modern mine countermeasure (MCM) sonar systems need to have the capability to locate small objects at a range of depths. At least 5cm of target pixel resolution is required for effective minehunting. This can be achieved with synthetic aperture sonar (SAS) to ranges of over 250 metres. The SAS can provide up to 10 times the range and area coverage rates of conventional side scan sonar.
Probably the first commercial delivery of an AUV-based SAS was the EdgeTech 4400-SAS on the HUGIN 1000 AUV, delivered to the Royal Norwegian Navy (RNoN) in January 2004. While the system eventually functioned well in benign environments, the main lesson learned from this and other early ‘real-world’ SAS deployments was that further development (in sonar design, hardware and software) was needed to reach the level of robustness and performance required for deployment under challenging conditions.
Synthetic aperture sonar technology is currently commercially available from multiple vendors including Kongsberg Maritime, Kraken Sonar Systems and Raytheon. The Currently SAS systems include the Kraken Aquapix™, Kongsberg HISAS 1030, and Raytheon ProSAS 60. The
Aquapix™ is a dual-frequency SAS with a swath width of ~600 m, which was recently used in the search for the HMS Erebus, a ship. from the lost Franklin expedition. The Kongsberg HISAS 1030 has a 600 m swath width and a theoretical resolution of 3 x 3 cm, and is typically mobilized on the HUGIN AUV. The ProSAS 60 is a lower frequency SAS with an effective swath of 3,000 m and a theoretical resolution of 10 x 10 cm; this system was deployed as part of the flight MH370 search.
Hydroid has integrated the Kongsberg High-resolution Interferometric Synthetic Aperture Sonar (HISAS) 2040 system onto a REMUS 600 unmanned underwater vehicle (UUV), Hydroid announced in early December. The addition of the HISAS 2040 module, when combined with an associated in-mission processing capability, provides up to 2 cm by 2 cm underwater surveillance resolution “across a 300 m swath”, the company said in a statement. “Synthetic aperture sonar uses algorithms to synthetically lengthen the aperture, providing consistent resolution across the entire swath, both along and across track, as opposed to traditional real aperture side-scan sonars,” said Hydroid.
The 300 m swath “is the maximum width of the sonar image that can be delivered at 2 cm by 2 cm resolution”, Sandor Becz, Hydroid’s Vice President of Engineering, told Jane’s on 11 December. “There are two transducers, one on each side of the UUV, [with] the EM2040 multibeam sonar that acts as a gap filler.” The EM2040 fills what is known as the ‘nadir gap’ that occurs between two outward-looking sonars, Becz explained. “The sonar software stitches together data from these [sensors,] which results in the total swath of 300 m,” he added. “With synthetic aperture sonar you get consistent resolution across the entire swath. This is a benefit over real aperture sonar, where the data resolution will degrade with range as you move to the outer edge of the swath.”
SAS Challenges and HISAS design
With the next generation sonar system HUGIN AUV can perform detection, classification and identification during a single mission, substantially saving time in mine countermeasure missions (MCM). In order to allow in-mission adaptation, the new HUGIN system includes hardware and software for real-time SAS processing, real-time automated target recognition (ATR) based on the SAS data, and in-mission re-planning for identification of automatically classified mine-like targets using an optical camera and/or other sensors. The upgraded HISAS system also computes a performance measure in real time, which can be used to change track spacing automatically to ensure optimum coverage.
Part of the trade-off analysis of integrating a high-resolution SAS in an AUV is deciding what parts of the processing to do in real-time in the vehicle and what to do in post-processing after AUV recovery. As SAS processing is computationally expensive, this will increase the power consumption and consequently reduce AUV endurance. Thus, the amount of processing to perform in the vehicle is a trade-off between mission endurance and post-processing efficiency.
Long-range sonar imaging in shallow water will always be limited by multi-path and direct surface returns. The phased-array transmitter of HISAS 1030 allows optimisation of the beam pattern for long-range shallow-water performance. This substantially reduces the direct surface return. In addition, SAS processing inherently increases the signal to multi-path level due to the coherent integration in time.
Coherent reconstruction of SAS images requires knowledge about both the vehicle position and sea-floor bathymetry. Norwegian waters often have rough bathymetry, making it almost impossible to run AUVs on straight lines and at the same time have favourable operational conditions for other payload sensors such as multi-beam echosounders. Being an interferometric system, HISAS 1030 collects data for both imagery and bathymetry in the same swath. This improves the robustness for SAS in rough terrain.
Synthetic aperture imaging is near-field imaging. To produce high-quality SAS images, an accurate estimate of the sound speed profile is required. In the littorals, the sound speed can vary rapidly. The HISAS 1030 with the FOCUS toolbox uses two different strategies to combat this problem. First, the vehicle can adaptively collect the environmental data needed for an accurate map of the sound speed. Second, the FOCUS toolbox uses adaptive image-correction techniques that estimate and correct for inaccurate environmental data.
Kraken Kits Out US Navy AUV with Real-Time Sonar
At least 5cm of target pixel resolution is required for effective minehunting. This can be achieved with synthetic aperture sonar (SAS) to ranges of over 250 metres. The SAS can provide up to 10 times the range and area coverage rates of conventional side scan sonar. According to Kraken, it is particularly well suited for AUVs and industry experts predict that SAS will replace conventional side scan sonars for many military applications.
Kenny added: “The major concern of mine countermeasures is to keep sailors far away from the mine threat while having a high level of confidence in the detection and classification of possible threats. Real-time SAS signal processing onboard an AUV is a significant step forward from the current approach. Kraken’s real-time SAS can increase safety, improve operational efficiency, reduce the mission risk of missing mines and significantly reduce and/or eliminate post mission analysis time.”
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.
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.