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Using AUV-acquired survey data to derive a magnetic model for a surface vessel

To develop an inverse model of the magnetic signature of a surface vessel, autonomous underwater vehicles (AUVs) equipped with tri-axial, flux-gate magnetometers were used to acquire magnetic field measurements underneath the moving surface vessel. Measurements were taken by AUVs operating in a formation at 3-7 m depth below the water surface. Multiple magnetic dipoles were used to model and estimate the magnetic signature of the surface vessel from the measurements. AUV orientation was estimated from onboard inertial measurement unit (IMU) and magnetic compass measurements in an attempt to transform the measured magnetic field vector components into the Earth's inertial frame. An uncertainty propagation study predicted ~18 nT root-mean-squared (RMS) noise in the magnetic field vector components resulting from uncertainty in AUV orientation estimation and uncertainty in the magnetometer measurements. The impact of network latency in the AUV distributed control system on the magnetic field vector components was discussed. A measured deviation in the transformed, inertial frame magnetic field vertical component of ~220 nT was compared with predicted values.