Potential impact of long-life environmental sonobuoys on littoral ASW

The focus of military activity has recently shifted from large area engagements to regional conflicts. Consequently, supportive Naval maritime operations have continued to evolve toward littoral warfare in complicated shallow-water, near-shore environments. This evolution requires new sensors, advanced Concept of Operations, and improved data-analysis capabilities, among others. Planning operations in these harsh-environment areas is difficult because accurate predictions of tactical sensor performance depend on detailed knowledge of the local environmental conditions. Tactical mission planning is thus seldom optimal or efficient, often resulting in coverage gaps, increased risk, and reduced mission success. According to a Navy Mission Need Statement, "Air ASW tactical execution, especially in littoral seas, requires in-situ environmental updates for preflight mission planning. In the conduct of ASW operations, an urgent need for explicit knowledge of environmental variables is required to optimize the effectiveness of operational acoustic sensors, as well as acoustic sensors in development " The Naval Air Systems Command has considered extended-life environmental sonobuoy concepts to better characterize the littoral environment. Most designs contain a thermistor string to measure ocean temperatures and also hydrophones to measure ambient noise. This type of complex sonobuoy would be more expensive than a traditional single-measurement AXBT but it could provide a more thorough littoral environment assessment. This paper examines the trade-off between increased sensor complexity and improved ASW performance, in terms of cumulative detection probability. Some advantages of an extended-life combined thermistor string/hydrophone approach, compared to AXBTs and tactical hydrophones, are: 1) higher accuracy of the raw data; 2) temporal averaging to smooth out fluctuations; 3) extended area coverage during drift; 4) less chance for surface temperature anomalies (e.g., mixed-layer-depth errors) caused by various electronic and mechanical variability upon impact; 5) opportunities to discover thermal and acoustic feature boundaries during drift; and 6) less need to re-seed thus allowing longer tactical mission times. These advantages are evaluated relative to the following disadvantages: 1) increased cost; 2) potential drift outside the mission area; and 3) need for increased battery life for longer durations. The analysis is tempered by considering how a potential new system might be used. One assumption is that an environmental sampling decision aid is available to determine the minimum number and best initial locations of drifting sensors to meet performance objectives. The November 2007 Requirements Document from the Naval Oceanographic Office states “Sampling guidance: Development of guidance on the best way to deploy, spatially and temporally, observation systems in order to meet various forecasting, model assimilation, and model evaluation objectives is needed.” Work in this area is reported in this Oceans '09 Conference in a paper entitled “Uncertainty-based Adaptive AXBT Sampling with SPOTS”, which addresses optimal sampling requirements. For this trade-off analysis, temperature data from watersampling flights in the Sea of Japan off the east coast of Korea were used to simulate expected capability of a long-term drifting thermistor string. Then optimal initial positions for three notional buoys were determined followed by a simulation of drifting positions and data collected over 12 days. Ocean nowcasts were constructed and used to determine acoustic performance of a notional tactical sonobuoy field. The analysis shows that a drifting extended-life thermistor string can provide significant improvement in environmental characterization, tactical planning, and ASW detection performance.