Acoustic Tomography Using Tactical Sensors

The US Navy has the national responsibility of maintaining freedom of navigation of the seas and securing the world from nuclear attack. Anti-submarine warfare (ASW) is an important part of these responsibilities. The Multi-Static Active Coherent (MAC) system developed by the Naval Air Warfare Center (NAWC) and deployed from the Boeing Maritime Patrol Aircraft (MPA) provides a wide area ASW search capability. These systems enable the Navy to project ASW capabilities rapidly at any location on the globe, detecting and tracking threat submarines over tactically relevant timescales.  NAWC’s MAC system consists of air-dropped SSQ-125 sources and SSQ-101 (Air Deployed Active Receiver [ADAR]) receiver buoys spread over a wide area to be searched for adversary submarines. The patterns in which the sources and receivers are deployed, particularly the spacing between buoys, are adapted to the presumed ocean conditions. The Navy’s oceanographic assets maintain global ocean circulation models, such as NCOM or HYCOM. These models are based on partial differential equations that capture the known physics of the ocean and are continually adjusted for observations reported from around the globe. Forward deployed US Navy forces and systems, such as the MAC system, receive ocean forecasts from NCOM/HYCOM for the time and location where systems will be deployed. These forecasts provide 3D ocean sound speed and current fields to be used as inputs to acoustic wave propagation models that calculate transmission loss (TL) and travel times. These are key inputs to tactical decision aids and target localization and tracking. The ocean is complex and temporally evolving, and sound speed forecasts degrade with time, thereby impacting processes  that depend upon them.  Sound speed directly impacts localization of submarines because it is needed to convert target echo travel times to ranges and target locations. Any degradation to the accuracy of the sound speed along the path of travel of the target echo will translate into degradation in target location and tracking. Ocean acoustic tomography (OAT), a technique of imaging the ocean sound speed field, similar to a Computerized Axial Tomography (CAT Scan), provides the opportunity to improve NAWCs real-time understanding of the sound speed field. The ability to transmit coherent signals and measure travel times on all receivers enables tomography to be used to track the 4D ocean sound speed field, inverting the measured travel times of the rays making up the direct blast signals received on each buoy.  Standing up a tomography system using the sources and receivers organic to the MAC system by integrating state-of-the-art data assimilation methods with the Navy’s ocean forecasts will enable the MAC system to produce better target location and tracking estimates as well as optimize detection performance.