SBIR-STTR Award

A multi-static active system, such as the MAC system, uses measured travel times of target echoes to infer range to target, including combining multiple observations to form target tracks. However, all such inferences rely on forward propagation models that are based on the presumed sound speed field in the ocean. Operational forces use ocean state forecasts that grow stale as the true ocean evolves in time, with changing currents and soundspeeds. At the same time, the MAC system constantly interrogates the ocean with its transmitted active waveforms looking for submarines. Each active transmission is received on multiple receivers and via multiple acoustic paths, providing an opportunity to measure travel times. Each organic transmission of the MAC system samples the ocean soundspeed along all of its paths and to all receivers, and can be used as the basis for a tomography system. AOS will use forecasted ocean soundspeed and current fields produced by NCOM/HYCOM to establish a likely initial state of the evolving ocean, including likely deviations. AOS will assimilate travel time measurements produced by the MAC system to refine presumed buoy locations and ocean state (soundspeed and currents).

Benefit:
There is ample commercial pull for improving our understanding and ability to predict the state of the ocean, to support many aspects of the emerging blue economy. Although there are many instruments that are surveying the ocean, their measurements are often less useful than they could be if their locations were better known, particularly instruments that infrequently breach the ocean surface to get GPS reads. Such instruments often rely on dead-reckoning with uncertainty in their locations growing with time since their last GPS reading. In principle, outfitting such platforms with modest acoustic sensing and processing, if they do not already have it, would enable them to be used as integral parts of a worldwide tomography and underwater GPS system that both improved navigation for such systems, and also provided additional receivers for probes of the ocean state, to be used to invert for both currents and soundspeed. The vastly increased number of platforms available for sampling the ocean would greatly improve our understanding of the ocean and lead to more-informed planning and environmental impact evaluation as efforts to combat climate change take on increased urgency. This tomography effort will produce advanced signal processing and understanding of the long-range acoustic channel that will inform efforts to design acoustic communications systems. Currently, long-range acoustic communications is severely limited due to the limited bandwidth and difficult channel presented by the ocean. The proposed tomography system will advance the technology for understanding the channel, knowledge that may lead to better strategies for acoustic communications, particularly at ranges of hundreds of kilometers.

Keywords:
dictionary learning, dictionary learning, ocean acoustic tomography, Data Assimilation, Ray Tracing, Reduced Order Modeling, acoustic wave propagation modeling, optimal basis sets, time of arrival estimation

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. NAWCs 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 Navys 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 Navys ocean forecasts will enable the MAC system to produce better target location and tracking estimates as well as optimize detection performance.

Benefit:
This SBIR seeks to expand the functionality of the MAC-E system by re-processing its data using tomography to estimate the ocean sound speed field, a key input to estimating buoy and target tracks from measured travel times of direct blast arrivals (source to receiver) and target echoes (source to target and target to receiver). Sound speed is a key input at several stages of the processing. First, we need the sound speed to track buoy locations as they drift under the influence of ocean currents. Second, we need sound speed to estimate target locations, in converting measured travel times to ranges, so that target locations can be isolated onto intersecting ellipses. The current MAC system uses a single sound speed to convert travel times to ranges (average of sound speeds at the source and receiver depths). This may be adequate when the signals are confined to surface ducts, but when the acoustic paths include deep-diving refracted paths, this grossly overestimates the sound speed. Sound speed alone is not enough. Sound travels along many paths and it is not clear which travel time corresponds to which path, or eigenray. It is critical to assign travel times to the right eigenrays, before we can invert travel times for ranges and locations. Incorrect ray identification can result in inconsistent ranges that lead to nonsensical results. Ray identification is also a key step in setting up tomographic inversions. Ray identification is eased by having the correct sound speed (for modeling eigenrays) or a tighter range interval over which to match eigenrays to travel times. This greatly improves buoy and target tracking, and is the key to standing up a stand-alone, automated tomography system. The tomography system enabled by ray identification provides improved sound speeds for improved buoy and target tracking. Commercialization Every US Navy active system relies upon sound speed as a key input to own platform and target localization and tracking. All active systems ensonify the ocean, thereby providing measurements for refining our knowledge of ocean state, which can then be used to improve target detection and tracking. Further, different systems can receive each others direct blast signals, and incorporate them in a basin scale tomography process that captures data to be incorporated into the Navys ocean forecasting systems, such as NCOM and HYCOM. More broadly, the ocean is becoming instrumented to progressively greater degrees by drifting sensors and robotic vehicles. However, the data produced by these sensors is made much more valuable if locations are provided for all measurements. For systems that have no surface expression, long-range acoustic ranging (as in DARPAs POSYDEN project) can be used as a substitute for GPS. The technology used to stand up tomography for the MAC-E system will be directly applicable to such future navigation systems.

Keywords:
Ocean acoustic tomography (OAT), Target Localization and Tracking, Ocean Sound Speed field characterization, anti-submarine warfare (ASW), In-stride ocean-acoustic data analysis and assimilation, Maritime Patrol Aircraft (MPA), Ocean Acoustic travel times, Multi-Static Active Coherent (MAC) System