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Unmanned Surface Vehicle (USV) Tow Point Surge Reduction for Towed Body Stabilization


OBJECTIVE: This topic seeks innovation that mitigates tow point surge fluctuations on a Fleet Class Unmanned Surface Vehicle to provide a more stable tow for a sonar towed body. DESCRIPTION: The Navy (PMS 406, Unmanned Maritime Systems) intends to add a minehunting capability to the USV in the Unmanned Influence Sweep System (UISS) (Ref 1). This addition will provide USV Increment 2 with minehunting, in addition to mine sweeping, capability. The USV Increment 2 towed minehunting system is currently expected to be the AN/AQS-20A sonar (Ref 2). As described below, USV tow point motions can negatively affect towed sonar performance in a seaway, and a system that can mitigate the tow point disturbances is needed to meet minehunting system performance requirements. Minehunting sonars housed in towed bodies require stability in order to achieve the required sonar performance. Towed body motions can result in degraded sonar imagery, leading to potentially missed detections, and difficulty in accurate mine classification. Towed body motions can also reduce area coverage rates because sonar swath overlap must be increased between tracks to maintain an adequate probability of detection. Due to their size, Fleet Class USVs (Ref 3) operating in the mission-required environment through Sea State 3 can have motions that will negatively impact towed body motions, and therefore, the performance of towed sonars. The primary input to towed body motions occurs at the tow point on the USV. Heave (vertical) and sway (lateral) motions are generally damped out by the cable catenary, but surge motions typically are nearly in line with the cable at the tow point. These surge motions can translate with high efficiency directly down the cable to the towed body as fluctuations in the cable tension. Depending on the geometry of the towed body and the depth at which it is operating, the cable tension fluctuations can translate to a disturbance of the towed body pitch. In addition, due to hydrodynamics of the towed body, pitch disturbances can also translate into yaw and roll disturbances. The active control system of the towed body will work to minimize the disturbances, but some motion will always occur, and, if the disturbances are large enough, the controller can become saturated, leading to larger motions. Theoretical predictions of boat motions are described (Ref 4) and the translation of these motions to the towed body (Ref 5). Towed body testing to date with a surrogate UISS USV has demonstrated possible challenges in maintaining towed body sonar steadiness in increased sea states due to surge motions being translated from the USV to the towed body. During these periods of unsteadiness, the sonar images may become degraded, impacting the Minehunting Mission. Heave (vertical) and sway (lateral) motions are generally damped out by the cable catenary, but surge motions are not. The active controller in the tow tries to compensate motions induced by input from the tow cable, but the compensation may not be fully successful in all minehunting modes. Current state of the art cable tension and motion compensation systems are larger systems found on larger ships and are unsuitable for application to USVs with their limited footprint and payload capacities. Advertised commercially available USV towed sonar bodies do not provide the full capabilities of the AN/AQS-20A. The Navy is seeking an innovation that will mitigate the tow point surge motions of the USV, allowing full performance of the towed sonar. The system must be of sufficiently small size and weight to fit in the limited aft deck area of a Fleet Class USV. The system may include adaptive dynamic control of the cable scope, adaptive control of USV speed, a dedicated mechanism, or a combination of those and other innovative solutions. Reference 6 provides additional information on technical requirements of the system. Proposed modifications to, or new designs for, craft hull-forms or towed bodies will NOT be considered. The system is expected to reduce operational cost by improving minehunting performance. Specifically, a more stable tow body will have a wider effective path-width, thus improving hunting efficiency by increasing the time on station and area covered in a sortie for a given fuel load. Increased time on station will also have the benefit of reducing sortie rates, thus reducing the USV"s host platform manning requirements by reducing the number of sortie turnarounds required. PHASE I: The company will develop concepts for a tow point surge reduction system that meet the objectives described above. The company will demonstrate the feasibility of the concepts in meeting the needs described above and will establish that the concept can be feasibly developed into a useful product for the Navy. Feasibility will be demonstrated by analysis, modeling, and/or simulation and documented in a final report. The company will provide a Phase II development plan with performance goals and key technical milestones that address surge reduction performance, space, and weight, and technical risk reduction. PHASE II: Based on the results of Phase I and the Phase II development plan, the company will develop a surge reduction prototype for evaluation. The prototype will be evaluated to determine its capability in meeting the performance goals defined in the Phase II development plan and the Navy requirements for the tow point surge reduction system. System performance will be demonstrated through prototype evaluation and modeling or analytical methods over the required range of parameters including various seaway conditions. Evaluation results will be used to refine the prototype into an initial design that will meet Navy requirements. The company will prepare a Phase III development plan to transition the technology to Navy use. PHASE III: If Phase II is successful, the company will be expected to support the Navy in transitioning the technology for Navy use. The company will develop a tow point surge reduction system for evaluation to determine its effectiveness in an operationally relevant environment. The company will support the Navy for test and validation to certify and qualify the system for Navy use. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The technology developed under this topic will have applications in various areas of undersea exploration where USV towing sonars are or can be used and high fidelity sonar images are desired. Such fields would include environmental remediation and underwater archeology. REFERENCES: 1."Unmanned Influence Sweep System (UISS) Fact Sheet."Department of the Navy Research, Development, and Acquisition Website. Accessed 02 August 2012.2."AN/AQS-20A Minehunting Sonar System."Department of the Navy Research, Development, and Acquisition Website. Accessed 4 October 2012.3."The Navy Unmanned Surface Vehicle (USV) Master Plan."23 July 2007. U.S. Navy. Accessed 29 February 2012.4. Martin, Milton."Theoretical Prediction of Motions of High Speed Planing Boats in Waves."April 1976. David W. Taylor Naval Ship Research and Development Center. Accessed 24 September. 2012.5. Patton, Kirk T. and Schram, Jeffrey W."Equations of Motion for a Towed Body Moving in a Vertical Plane."Defense Technical Information Center (DTIC), 15 June 1996. Accessed 02 August 2012. 6."Additional Guidance for USV Tow Point Surge Reduction for Towed Body Stabilization."(Document to be provided on SITIS after the Solicitation pre-release.)
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