AN/UQQ-2 Surveillance Towed Array Sensor System
The AN/UQQ-2 Surveillance Towed Array Sensor System (SURTASS) is a passive sonar system consisting of a long linear array of hydrophones towed by United States Navy ocean surveillance ships to detect and track submarines through acoustic surveillance.[1][2]
Deployed primarily from T-AGOS class vessels such as the Victorious and Impeccable classes, the system operates at low towing speeds of around 3 knots to maintain array stability and maximize detection range against quiet nuclear and diesel-electric submarines.[2][3] It provides real-time passive acoustic data for integration into the Navy's anti-submarine warfare operations, supporting the detection of submerged threats in open-ocean environments.[4]
Key enhancements include the SURTASS Block Upgrade, which improves signal processing for better performance against advanced submarine threats, and variants like the Low Frequency Active (LFA) mode for active transmission in low-signal conditions.[1] The system's design emphasizes long-duration deployments, with arrays extending thousands of feet to achieve superior sensitivity over fixed bottom-mounted sensors.[3] Ongoing international interest, such as Australia's acquisition of a modular SURTASS variant, underscores its enduring role in undersea surveillance.[5]
Development and History
Origins in Cold War Submarine Detection Needs
The United States Navy's imperative for enhanced submarine detection during the Cold War stemmed from the escalating Soviet submarine threat, which included over 250 nuclear-powered submarines by the late 1970s, comprising ballistic missile submarines (SSBNs) like the Delta class and attack submarines (SSNs) such as the Victor class, designed to challenge American sea control and second-strike capabilities.[1] These vessels, increasingly equipped with noise-reduction technologies, diminished the effectiveness of earlier detection methods against quieter propulsion systems, necessitating passive, long-range acoustic surveillance to monitor deep-ocean transits and maintain strategic warning without compromising fixed-array positions.[6] The Surveillance Towed Array Sensor System (SURTASS), designated AN/UQQ-2, emerged as a mobile extension of the Integrated Undersea Surveillance System (IUSS), enabling ship-based deployment to fill coverage gaps in remote or contested areas where stationary Sound Surveillance System (SOSUS) hydrophone arrays proved insufficient for tactical responsiveness.[7] Development of SURTASS originated in the early 1970s amid these detection challenges, with initial efforts focusing on hybrid towed-array prototypes to achieve superior signal gain through extended hydrophone lengths for low-frequency passive sonar.[6] Full-scale engineering development was authorized on November 26, 1974, prioritizing countermeasures to Soviet nuclear submarine quieting while integrating advanced signal processing for real-time analysis.[6] By leveraging lessons from prior towed-array experiments, the system was engineered for deployment from non-combatant ocean surveillance vessels, ensuring covert operations aligned with Cold War emphasis on persistent, low-signature monitoring of adversary undersea forces.[8] This initiative directly addressed the causal vulnerabilities exposed by Soviet advancements in submarine stealth, such as pump-jet propulsors and anechoic coatings, which reduced self-noise and demanded arrays capable of detecting faint acoustic signatures at ranges exceeding 100 nautical miles in favorable conditions.[1] SURTASS's passive architecture avoided active sonar emissions that could alert targets, prioritizing empirical acoustic propagation models derived from oceanographic data to optimize performance against the deep-ocean threat environment.[9]Initial Deployment and Early Upgrades
The AN/UQQ-2 Surveillance Towed Array Sensor System achieved initial operating capability in November 1984 aboard the USNS Stalwart (T-AGOS-1), the first dedicated SURTASS vessel, which had been launched in July 1983 and delivered earlier that year following self-noise trials.[6] Deployed as the mobile component of the U.S. Navy's Integrated Undersea Surveillance System (IUSS), the passive towed array enabled long-range submarine detection by ocean surveillance ships operating in the Atlantic, Pacific, Mediterranean, and North Sea, providing acoustic data collection and analysis to cue other antisubmarine warfare assets.[1][8] Early enhancements focused on improving array versatility and signal processing amid evolving submarine threats. In 1989, the Navy awarded a contract for the SURTASS Reduced Diameter Array (RDA), designed for both passive surveillance and integration with low-frequency active sonar capabilities, addressing limitations in the original AN/UQQ-2's diameter for active transmission.[6] By the mid-1990s, the Block Upgrade introduced the Enhanced Modular Signal Processor (AN/UYS-2), tested aboard the USNS Able (T-AGOS-20) and completed in 1994, which expanded data processing capacity and improved detection against quieter targets through advanced algorithms and modular hardware.[6] Platform advancements complemented these sensor upgrades; the introduction of Small Waterplane Area Twin Hull (SWATH) designs, starting with the USNS Victorious (T-AGOS-19) delivered in August 1991, enhanced stability for towing the array in higher sea states, reducing noise and improving overall system performance over monohull predecessors like the Stalwart class.[6][8] These modifications maintained the AN/UQQ-2's core passive hydrophone architecture while incrementally boosting sensitivity and operational endurance in response to post-Cold War acoustic challenges.[7]Post-Cold War Evolutions and Modern Enhancements
![USNS Able (T-AGOS-20), upgraded for AN/UQQ-2 SURTASS(V)7 in 2007][float-right][10] Following the end of the Cold War in 1991, the AN/UQQ-2 SURTASS system evolved to address emerging threats from quieter diesel-electric submarines and shifting operational priorities toward littoral environments, while maintaining its role in deep-water surveillance. The SURTASS Block Upgrade, implemented in the 1990s, enhanced passive acoustic data collection, analysis, and shipboard processing capabilities, including improved data display and integration with the Range Data Analysis (RDA) subsystem for superior performance over baseline systems.[1][7] This upgrade was backfitted to vessels like T-AGOS 19 and incorporated into new platforms such as T-AGOS 20, enabling better detection in contested acoustic environments.[11] Subsequent modernization efforts in the 2000s focused on version-specific enhancements, such as the SURTASS(V)7 configuration, which supported reactivation and conversion of surveillance ships like USNS Able (T-AGOS-20) in 2007 to extend operational life and adapt to advanced signal processing needs. These upgrades improved reliability and sensitivity to counter post-Cold War advancements in submarine quieting technologies, with embedded nonacoustic sensors for precise array orientation and noise reduction.[10][12] Into the 21st century, the AN/UQQ-2 system has received sustained funding for production, upgrades, and maintenance, reflecting its enduring relevance against evolving underwater threats, including those from peer competitors. As of 2024, ongoing enhancements emphasize modular scalability and integration with broader undersea surveillance networks, ensuring long-range passive detection capabilities amid reduced T-AGOS fleet sizes.[13][1]Technical Design and Components
Core Towed Array Architecture
The core towed array of the AN/UQQ-2 Surveillance Towed Array Sensor System (SURTASS) is a passive horizontal line array (HLA) comprising a flexible, oil-filled hose-like structure housing multiple hydrophone elements for detecting low-frequency underwater acoustic signals from distant sources, such as submarine propulsion noise.[3][1] This design prioritizes a large physical aperture to enable narrow beamforming for bearing estimation and signal gain, with the array towed in a stable horizontal orientation to preserve phase coherence across elements.[3] The HLA incorporates distributed electronic modules for preamplification and initial signal conditioning, minimizing noise introduction prior to data transmission.[14] In its baseline configuration, the array measures approximately 2,600 feet (792 meters) in length and contains numerous hydrophones spaced at regular intervals to form sub-arrays for processing.[3] It is deployed via a 6,000-foot (1,829-meter) electro-optical tow cable that conveys analog or digitized acoustic data, power, and depth-control commands from the towing vessel, while withstanding hydrodynamic forces and maintaining electrical integrity at operational depths of 500 to 1,500 feet (152 to 457 meters).[3] The array achieves neutral buoyancy at depth through syntactic foam and weighting elements, allowing towing speeds of about 3 knots without significant deformation.[3] Integrated environmental sensors along the array measure parameters like sound velocity, temperature, and pressure to support adaptive beamforming and propagation modeling, enhancing detection accuracy in varying ocean conditions.[14] The modular construction facilitates maintenance and upgrades, with hydrophone clusters typically arranged in vertical stacks within each section to provide limited vertical aperture for ambiguity resolution in bearing estimates.[15] This architecture supports real-time passive surveillance by relaying pre-processed signals to shipboard processors via the tow cable's fiber-optic or coaxial pathways.[1]Hydrophone Arrays and Signal Processing
The hydrophone arrays in the AN/UQQ-2 SURTASS consist of a long, flexible, oil-filled tubular structure housing numerous passive acoustic sensors spaced at precise intervals to capture low-frequency underwater sounds.[3] These hydrophones, typically piezoelectric transducers, convert acoustic pressure waves into electrical signals optimized for detecting submarine-generated noise in the 10-100 Hz band, enabling long-range passive surveillance over hundreds of kilometers in deep ocean environments.[1] Early configurations featured arrays approximately 800 meters (2,600 feet) in length towed by a 1,800-meter (6,000-foot) non-conducting cable to minimize ship noise interference, while later variants extended to around 1,600 meters for improved aperture and resolution.[3] The linear, horizontal array design provides a broad azimuthal field of view, with hydrophone spacing determined by wavelength considerations to avoid grating lobes in beam patterns at operational frequencies.[16] Signal processing begins with analog-to-digital conversion of hydrophone outputs aboard the host vessel, followed by digital beamforming to synthesize multiple narrow beams across the horizontal plane and suppress ambient noise.[6] This onboard processing employs adaptive algorithms to enhance signal-to-noise ratios, including time-delay summation and spectral analysis via fast Fourier transforms to identify tonal signatures and broadband noise characteristic of submarine propulsion or machinery.[3] Beamformed data, reduced in volume by a factor of approximately 10 through selective filtering and compression, is transmitted in real-time via satellite links at rates around 32 kilobits per second to shore stations for advanced correlation, tracking, and integration with other undersea surveillance assets.[3][1] The system supports bistatic modes when paired with active sources, leveraging pre-formed beams for echo detection, though primary operation remains passive to maintain stealth and long-range cueing capabilities.[1] In variants like SURTASS TwinLine, additional synchronization processing resolves left-right ambiguities by comparing outputs from parallel sub-arrays towed side-by-side.[17]Integration with Shipboard Systems
The AN/UQQ-2 SURTASS integrates with shipboard systems on T-AGOS-class vessels via dedicated mechanical, electrical, and data interfaces designed for seamless deployment and real-time processing of acoustic signals. The towed array connects to the host ship through a multi-conductor towed cable, which supplies electrical power from the vessel's generators, transmits control commands, and relays raw hydrophone data back to onboard electronics. Deployment and retrieval are managed by stern-mounted hydraulic winches and tension control systems, enabling operations at towing speeds of approximately 3 knots and array depths exceeding 1,500 feet to optimize signal reception while minimizing self-noise from the platform.[1][2] Onboard signal processing occurs within shipboard consoles and rack-mounted hardware that perform analog-to-digital conversion, beamforming, and automated detection algorithms to identify, classify, and track submarine contacts over long ranges. This processing suite interfaces directly with the ship's navigation systems for correlating acoustic bearings with GPS-derived positions and feeds outputs into the Integrated Undersea Surveillance System (IUSS) network for broader data fusion.[4][7] Operators monitor results via dedicated displays in the surveillance control center, which are linked to the vessel's combat information center for tactical decision-making.[18] Data reporting and external integration leverage the ship's communication infrastructure, including SHF satellite links for high-bandwidth ship-to-shore transmission of processed tracks, UHF SATCOM for voice and low-data needs, INMARSAT for backup connectivity, and SIPRNET-secured interfaces compatible with GCCS-M version 3.1.1 for automated cueing to other naval assets. Maintenance integration involves shipboard teams handling array handling, calibration, and fault isolation, supported by field engineering for upgrades to ensure 365-day operational readiness.[1][18] This modular design allows for variants like TwinLine arrays to plug into the same shipboard framework with minimal reconfiguration, enhancing flexibility across surveillance missions.[19]Variants and Capabilities
Standard Passive SURTASS Configuration
The standard passive configuration of the AN/UQQ-2 Surveillance Towed Array Sensor System (SURTASS) utilizes a long, flexible hydrophone array towed behind specialized ocean surveillance vessels to detect and track underwater acoustic signals from submarines and surface ships. This setup relies exclusively on passive sonar principles, listening for radiated noise without emitting signals, enabling covert long-range surveillance in deep ocean environments. The system provides automated processing for rapid target detection, classification, and tracking of both nuclear and diesel-electric submarines, even those operating quietly, with real-time data reporting to anti-submarine warfare commanders via satellite links.[1][4] The core component is a tube-like towed array approximately 2,600 feet (792 meters) in length, containing numerous hydrophones spaced along its length to form a linear aperture for beamforming and signal analysis. Towed at speeds of about 3 knots by a 6,000-foot neutrally buoyant cable, the array operates at depths between 500 and 1,500 feet (150 to 457 meters), optimizing sensitivity to low-frequency sounds propagating efficiently in the deep sound channel. Onboard shipboard processors handle beamforming, noise reduction, and automated classification algorithms, preprocessing acoustic data before transmission at rates up to 32 kilobits per second for further analysis ashore.[3][3][3] This configuration excels in open-ocean scenarios where ambient noise is low, offering detection ranges extending tens to hundreds of kilometers depending on target self-noise, oceanographic conditions, and array performance. It supports strategic intelligence collection by identifying vessel signatures through spectral analysis of propeller cavitation, machinery, and biological noise rejection. Unlike active variants, the passive mode minimizes self-disclosure risk, prioritizing stealth in contested waters, though it depends on target acoustic emissions for cueing.[1][7]Low Frequency Active Sonar (SURTASS LFA)
The Surveillance Towed Array Sensor System Low Frequency Active (SURTASS LFA) sonar serves as the active sonar augmentation to the primarily passive AN/UQQ-2 SURTASS, enabling detection and tracking of submarines operating in quiet modes that evade passive acoustic surveillance.[20] It transmits low-frequency acoustic pulses that propagate long distances in the ocean, reflecting off submerged targets to provide range, bearing, and classification data when received by the SURTASS hydrophone array.[15] Operating in the 100–500 Hz band, LFA exploits the superior propagation characteristics of low frequencies for extended-range anti-submarine warfare (ASW), particularly against diesel-electric or advanced quiet nuclear submarines.[21] The system consists of a vertical projector array with 18 acoustic transducers suspended on a tow cable, deployed at a nominal depth of 122 meters and requiring a minimum tow speed of 3 knots (5.6 km/h) for stability.[21] Each projector has a source level of approximately 215 dB re 1 μPa at 1 meter, yielding an effective array source level of 230–240 dB re 1 μPa at 1 meter after beamforming and array gain.[15] The receive function integrates directly with the SURTASS towed array of hydrophones, which processes echoes for target localization; signal processing includes adaptive beamforming to suppress reverberation and ambient noise.[1] Transmissions consist of continuous-wave or frequency-modulated pulses lasting 6–100 seconds, repeated at intervals of 6–15 minutes, with a low operational duty cycle of 10–20% to minimize energy use and environmental exposure.[21] LFA capabilities emphasize long-range acquisition, with effective detection ranges extending beyond typical submarine weapon engagement distances (e.g., torpedo or missile firing ranges), allowing surface forces to maintain standoff protection.[15] It complements passive SURTASS by enabling precise ranging and reacquisition of contacts lost in low-signal environments, enhancing cueing for other ASW assets like attack submarines or aircraft.[20] The system achieves this through high-power, narrow-beam transmission patterns that focus energy horizontally, reducing vertical and surface/bottom reverberation; however, acoustic signals attenuate rapidly within the first kilometer due to projector spacing and near-field effects.[21] Up to four LFA-equipped SURTASS variants are deployable on Victorious-class (T-AGOS 19) ocean surveillance ships, with integration involving shipboard power amplifiers, winch handling systems, and real-time data links to command centers.[22] A compact variant, Compact LFA (CLFA), has been backfitted on select platforms for improved maneuverability without sacrificing core performance.[8]Advanced Configurations like TwinLine and Modular SURTASS-E
The TwinLine configuration of the AN/UQQ-2 SURTASS, designated TL-29A, employs two shorter passive acoustic line arrays towed side-by-side at low frequencies to detect and track submarines, resolving bearing ambiguities that single-line arrays require ship maneuvers to address.[23][1] The parallel arrays, separated by a variable distance of 2.2 to 8.8 meters, enhance aperture size and suppress horizontal noise interference from the sea surface, improving performance in shallow-water environments where long single arrays face deployment liabilities due to variable ocean conditions.[1][24] This setup achieved initial operational capability with the U.S. Navy in November 2005, integrated on Stalwart-class (T-AGOS) ocean surveillance ships for mobile undersea surveillance.[23] Modular SURTASS-E represents an export-oriented, fully containerized adaptation of the AN/UQQ-2 system, enabling rapid deployment on non-specialized vessels or auxiliary platforms without permanent ship modifications.[5] This variant maintains core passive towed array capabilities for long-range submarine detection while prioritizing modularity for allied interoperability, as evidenced by its basis in U.S. Navy SURTASS deployments on Victorious-class (T-AGOS 19) ships.[5] In May 2023, the U.S. State Department approved a $207 million Foreign Military Sale of Modular SURTASS-E to Australia, including handling equipment, mission systems, and training, to bolster Indo-Pacific undersea surveillance amid regional submarine threats.[5] Similar containerized systems have supported operations for the Japan Maritime Self-Defense Force, demonstrating adaptability for partner nations' ocean surveillance needs.[25]Operational Employment
Primary Platforms and Deployment Ships
![USNS Able (T-AGOS-20)][float-right]The AN/UQQ-2 Surveillance Towed Array Sensor System (SURTASS) is primarily deployed from U.S. Navy Auxiliary General Ocean Surveillance (T-AGOS) ships operated by the Military Sealift Command.[26] These vessels, crewed by civilian mariners supplemented by Navy technical specialists, tow the SURTASS arrays during undersea surveillance missions as part of the Integrated Undersea Surveillance System (IUSS).[27] The T-AGOS fleet provides stable platforms for long-duration deployments, enabling passive acoustic detection over extended ranges.[13] Current active deployment ships include the Victorious-class SWATH (small waterplane area twin hull) vessels USNS Victorious (T-AGOS-19), USNS Able (T-AGOS-20), and USNS Effective (T-AGOS-21), designed specifically for SURTASS operations with enhanced stability for towing in varied sea states.[28] [29] USNS Impeccable (T-AGOS-23), another SWATH-configured ship, also supports SURTASS missions, contributing to the Navy's four operational surveillance platforms equipped for low-frequency active sonar variants where applicable.[30] These ships conduct missions in support of anti-submarine warfare, gathering acoustic data on submarine threats.[26] The Navy is recapitalizing the T-AGOS fleet with the Explorer-class (T-AGOS-25 and follow-ons), SWATH catamaran designs procured starting in fiscal year 2022, intended to maintain SURTASS capabilities with twin TL-29A passive arrays for improved detection.[31] [32] These new vessels, with displacements around 8,500 tons and lengths of 356 feet, will replace aging platforms to ensure continued undersea surveillance effectiveness.[33]