ACOUSTIC TOWED ARRAY MOTION DETERMINATION USING HYDRODYNAMICS AND AMBIENT NOISEFIELDS

THE NAVY-S ASW DETECTION AND CLASSIFICATION SYSTEMS ARE DEPENDING INCREASINGLY ON THE USE OF TOWED RECEIVING ARRAYS BECAUSE THEY PROVIDETHE OPPORTUNITY TO OBTAIN A LARGER MOBILE RECEIVE APERTURE WITH LESSFLOW NOISE THAN OCCURS WITH HULL MOUNTED ARRAYS. UNFORTUNATELY, ALONG WITH THE ADVANTAGES OF A TOWED ARRAY COME INHERENT PROBLEMS. THE MAIN PROBLEM IS THAT THE ARRAY CONFIGURATION CHANGES WITH TIME, DUE TO VARIOUS FACTORS SUCH AS TURBULENCE AND THE TOW-SHIP-S INABILITY TO STEER IN A PERFECTLY STRAIGHT LINE, MAKING ACCURATE BEAMFORMING DIFFICULT. ALSO, SINCE THE TOWED ARRAY ACQUIRES OTHER THAN A STRAIGHT LINE MOTION, BEAM POINTING DIRECTIONS ARE UNSTABLE GIVING THE APPEARANCETHAT TARGETS ARE WANDERING. PRECISE BEAMFORMING AND BEARING ESTIMATION ON AN ARRAY DEPEND ON KNOWING THE RELATIVE LOCATION OF THE SENSORS TO A SMALL FRACTION OF A WAVELENGTH OF THE HIGHEST ACOUSTIC FREQUENCY PROCESSED ON THE ARRAY. THE ACOUSTIC SENSOR LOCATION ERRORS MUST BE PARTICULARLY SMALL IF THE BENEFITS OF THE LARGER APERTURE AVAILABLE ON A TOWED ARRAY ARE TO BE REALIZED. IF SUCH EFFORTS ARE TO SUCCEED IT WILL BE NECESSARY TO KNOW THE ACCURATE SHAPE OF THE ARRAY. THISIN TURN REQUIRES THAT THE TOWED ARRAY-S MOTION BE CHARACTERIZED IN ANY TERMS AVAILABLE INCLUDING, MOST IMPORTANTLY, IN TERMS OF THE HYDRODYNAMIC MOTION OF TOWED BODIES. WE PROPOSE AN INNOVATIVE SOLUTION BASED ON THE USE OF THE HEADING AND DEPTH SENSORS. THE SOLUTION USES A HYDRODYNAMIC EQUATION COMPLEX ENOUGH TO INCLUDE ALL IMPORTANT DYNAMICVARIABLES BUT WITHOUT BEING SO COMPLEX AS TO HAVE TO SPECIFY A LARGENUMBER OF PARAMETERS. THE PROPOSED SOLUTION INCLUDES ADDITIONAL ARRAY MOTION ESTIMATION BASED ON ESTIMATING NOISE FIELD CHANGES CAUSED BYARRAY MOTION, PARTICULARLY SENSOR TO SENSOR PHASE CHANGES IN A HIGH SIGNALTONOISE RATIO (SNR) NARROWBAND SIGNAL.