Ship-Suspended Acoustical Transmitter Position Estimation and Motion Compensation

An acoustical transmitter was suspended at multiple depths to 800 m from the research vessel R/V Melville at several stations in the North Pacific in 2004. The 3-D position of the transmitter varied with time due to ship motion and subsurface currents. The transmitter 3-D position and velocity were subsequently estimated using a cable dynamics model forced by ship position, as measured by high-precision global positioning system (GPS), and subsurface currents, as measured by the onboard acoustical Doppler current profiler. These estimated positions and velocities varied in the horizontal up to 10 m from the station “center” position, and 0.5 m/s from zero, respectively. Auxiliary measurements indicate that these estimates were accurate along either horizontal coordinate to better than 2 m and 0.05 m/s, respectively. Transmitter motion dilates the apparent time base of the radiated signal, producing time-varying Doppler effects. Simulation and analysis are used to determine when the induced Doppler effect is significant, and a technique is presented that “de-dopplerizes” a received signal for arbitrary interplatform motion. One example, involving the transmitter motion solutions determined here, shows that the transmitter motion induces a root mean square (RMS) variability of roughly for a 75-Hz ranging signal on time scales of several minutes: a 41-point de-dopplerizing filter reduced this to .