3.3 Acoustic Sensors

3.3.1 Warfighter Needs

Joint warfighting operational needs/capabilities in the areas of dominant battlespace knowledge, combat identification, joint readiness, and joint countermine are particularly dependent on acoustic, magnetic, and seismic sensor technology. These sensors provide reliable undersea and terrestrial surveillance against threat targets which is required to achieve and maintain battlespace dominance to enable timely execution of joint/combined operations in support of national security objectives. Undersea acoustic sensor efforts are Navy unique and Navy critical.

This Subarea develops surveillance and environmental science & technologies to acoustically and magnetically detect, classify, track and localize quiet threat targets in all operating environments across all missions and all platforms. Acoustic sensors are the primary sensor of choice to detect threat submarines operating below periscope depth. However, the increasingly quieter nuclear threat and the diesel-electric-on-battery threat limits traditional narrow-band processing, yielding shorter detection ranges and requires more array gain via more sensors and adaptive signal processing to counter the quieting trends. Acoustic sensors are highly dependent on environmental conditions so the shift in focus to the littorals presents a more difficult environment which is more familiar to the enemy and leads to increased clutter from biologics and other sources resulting in higher false alarm rates and greater weapon expenditures. To counter the environmental issues fusion of acoustic sensors with magnetic and other nonacoustic sensors are finding increased use. Effective multi-sensor data fusion offers more robust detection and classification performance and offers a greater range of adaptability. Larger array apertures requires emphasis on affordability if such systems are ever to be fielded. Navy applications include undersea surveillance in both open ocean and in highly variable, cluttered, shallow water areas by hull mounted, towed and deployed systems from surface ship, submarine, fixed-wing and helicopter platforms, while Army applications include shore area and battlefield surveillance to detect and classify ground and air targets with mobile and stationary sensor systems from fixed and mobile platforms as well as for short range mines.

3.3.2 Acoustic Sensors Overview

3.3.2.1 Goals and Timeframes. The worldwide proliferation of modern quiet diesel-electric submarines requires increased emphasis on the use of active sonar and full spectrum passive processing. Improved classification for existing active sonar systems is the short-term (<5 years) goal. Within 10 years, high gain passive systems and active sonar systems that can adapt to the highly variable littoral environment and accurately classify targets in high clutter environments with reduced false alarm rates are required. Modern Army battlefield acoustic systems have demonstrated the capability to detect, classify, and identify ground targets at ranges in excess of 1 kilometer and helicopters beyond 5 kilometers with meter-sized sensor arrays while netted arrays of sensors have been used to track and locate battalion sized armor movements over tens of square kilometers in non-line-of-sight conditions.

Far-term improvements will extend these capabilities to tactical ranges. Significant goals are:

Advances to meeting these goals depends on progress in Electro-optics Technology (3.7), understanding the Ocean Battlespace Environment (3.12) and Acoustic and Magnetic Materials.

3.3.2.2 Major Technical Challenges. In general, the major technical challenges are to provide:

• Highly effective USW shallow water ASW that can adapt to the environment; include high resolution environmental modeling and measurement; and incorporate "through the sensor" environmental characterization;



• Low cost options for USW systems (affordability); and



• Improved tactical decision aids through near real-time simulation coupled with more capable sensing & modeling of the tactical environment.

Specifically, major technical challenges included: (1) active sonar detection techniques for targets in clutter caused by non-target geologic features, biologic, man-made objects on the bottom and reverberation from surface, bottom and volume interactions, (for example, CST-5 sea trial data show "routine" deep water performance hampered by 2 to 12 false tracks per hour and in shallow water, performance is impeded by over 300 false tracks per hour); (2) passive sonar - algorithms capable of detecting targets in the midst of interference from local and distant shipping and (3) active and passive sensors - compact, high power, lower and broader frequency active acoustic sources and larger aperture receiving arrays in affordable applications on a diverse range of platforms.

3.3.2.3 Related Federal and Private Sector Efforts. COTS plays a significant and growing role in this subarea. Telecommunication technology, fiber optics and associated laser, coupler and splitter technology, polyvinylidene fluoride (PVDF) materials and computationally intensive hardware are applicable examples.

3.3.3 S&T Investment Strategy

3.3.3.1 Technology Demonstrations. The LBVDS will combine advances in high energy density transduction materials and in broad frequency bandwidth waveform generation and signal processing in a 1 to 6 kHz sonar system to provide shallow water environment USW capability to Naval surface ship platforms. Real-time clutter rejection, reverberation suppression, target highlighting, and classification will be evaluated through sea tests of the broadband waveforms. A compact, towable source projector and receive array with manageable ship design and operational impact will be developed and used as the test bed. The LBVDS payoff will be an estimated greater than 20 dB improvement in detection and classification, more rapid localization, a false alarm goal of less than one per hour against quiet, slow submarines and mines in shallow water. The technology is targeted for transition to SC21.

3.3.3.1.1 Lightweight, Broadband Variable Depth Sonar (LBVDS). This demonstration addresses DTO SE.14.02.N.

3.3.3.2 Technology Development

3.3.3.2.1 Sensor Signal Processing Technology. This technology development addresses DTO SE.15.01.ANE. Efforts develop active waveform designs; improve signal processing and displays to reduce clutter and false alarm rates encountered in cluttered environments; investigate bi-static/multi-static detection technologies; provide algorithms and data fusion techniques which increase probability of detection with reduced false alarms; demonstrate long range, ground and air target direction and identification at low cost. The signal processing approach is to develop algorithms and architectures to enable autonomous detection and classification, enabling reduced operator loading/manning. Target separation is addressed by improving on array bearing accuracy and beamforming. Fusion of multiple sensor modalities in a hierarchical neural net will be used to achieve a high probability of detection with a low false alert rate. Other techniques include passive processing utilizing the complete spectrum of target emitted signals, platform noise suppression, and wind noise reduction. A shallow water data base of target and non-target echoes in high cluttered environments will be collected to use in developing and demonstrating signal processing techniques to reduce clutter and improve classification.

3.3.3.2.2 Active/Passive Sensor Technology. This technology development addresses DTO SE.16.01.NE. Active efforts will develop innovative, high power transducers using new, high energy density transduction materials, e.g., the electrostrictive lead magnesium niobate (PMN) and the magnetostrictive Terfenol-D. Array element interactions will be modeled to aid in providing affordable, compact sources with minimal ship impact that can be towed at the optimum depth determined by the environmental conditions and the target's depth. Deployed or offboard sensors and distributed systems are needed to provide alerts and cueing to tactical platforms. Efforts include the development of affordable, lightweight, extended bandwidth optical sensors, velocity sensors, micromachined sensors, and rugged soldier mounted acoustic sensors for long-range and early warning threat detection. Sensor noise models and noise mechanism insights are required to optimize aperture designs.

3.3.3.3 Basic Research. This Subarea is interdisciplinary in nature, drawing on efforts in materials, mathematical, computer, information, cognitive, neural, surveillance and battlespace environmental sciences. Environmental effects play a major role in sensor performance and insight of the complexities offers a means to develop adaptable systems.