Synthetic Aperture Sonar ( SAS ) , High Resolution and Automatic Target Recognitioni

The Autonomous Operations Future Naval Capability (AOFNC) program developed a 12.75” diameter autonomous underwater vehicle (AUV) and a Synthetic Aperture Sonar (SAS12) payload. This system falls under the lightweight designator of the Unmanned Undersea Vehicle (UUV) master plan. Bluefin Robotics Corporation and the Applied Research Laboratory of The Pennsylvania State University (ARL/PSU) developed the vehicle/payload system. In addition to the previous team members, Naval Surface Warfare Center Panama City (NSWC PC) developed the synthetic aperture image processing. The system will include motion compensation and beam formation software, real time data handlers, and automatic target recognition algorithms. NSWC PC provided test range services and test planning to the project, as well. The AUV design is an open frame that allows modular payloads to be attached. The modules are selfcontained and the surround is free-flooded. A plastic fairing covers the payload and vehicle subsystems. The payload power and communications are supplied through common interfaces. The vehicle hosts a suite of inertial, environmental, and heading sensors, as well as, a Doppler Velocity Log (DVL). Data from this sensor suite is combined to provide the information necessary for proper SAS operation. This data is used both in the SAS ping timing and ultimately in the correction of errors due to aperture misalignment. Vehicle and payload data and logs are recorded and used to evaluate system performance. The SAS payload is designed using COTS data acquisition and communication hardware. The SAS operates at 180 kHz in the side looking mode. A suite of arbitrary waveforms can be transmitted to optimize SAS performance in a given environment. The broadband receiver is designed for minimal channel-to-channel gain and phase errors necessary for acquisition of high fidelity signals. Signals are filtered and decimated then passed to the recorder and processing systems. The individual element aperture determines the ultimate resolution limit. In principle, SAS12 can be processed for 25mm resolution at all ranges out to a maximum of 150 meters. One advantage of SAS is that the data collected can be processed to whatever resolution is defined by the user, within this limit. This is useful in resolution studies because the same data set can be processed for different resolutions. Typically real apertures have a fixed resolution proportional to the physical length. Depending on the real aperture system, this resolution may be constant or vary as a function of range. The system will include real time automatic target recognition (ATR). The ATR consists of a set of algorithms developed by several different contributors. The master algorithm uses a rule based system to combine the information generated by the individual contributors. The result produces a lower false alarm rate that any single algorithm. The authors present performance for comparison to the existing data bases that relate ATR performance to image resolution. ATR performance is affected by clutter, bottom type, target aspect, and many other characteristics, as modified by the SAS resolution. Imagery is presented with ATR performance measures. Sonar performance is discussed in qualitative terms, and is based on image appearance and knowledge of what targets are present in the field. Quantitative performance measures are also presented in terms of requirements of the ATR, Probability of Detection (Pd), and Probability of False Alarm (Pfa). [Work supported by Mr. James Valentine, the Office of Naval Research.]