Real-time Operational Tomography from Multistatic Active Sonar

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).