Method for determining relative motion using accelerometer data

A method for determining motion between first and second combat system elements on a ship, comprising:

• providing first and second accelerometers associated with first and second combat system elements, respectively for generating first and second acceleration signals;
• providing first and second inertial navigation systems (INS) sensors positioned at different locations on-board the ship for generating first and second ship attitude signals;
• conditioning using a computer processor said first and second acceleration signals;
• generating first and second signals representative of rotational and translational displacements of said combat system elements based on said conditioned first and second acceleration signals, and first and second ship attitude signals;
• determining the translational and rotational displacements of the first and second combat system elements due to hull modal vibration at times T1 and T2;
• determining the translational and rotational displacements of the first and second combat system elements due to forced vibration at times T1 and T2 by differencing the first and second INS attitude signals to remove rigid body motion, and comparing the differenced attitude signals to forced vibration deflection shapes determined previously from at-sea vibration data collection, wherein the rotational displacements of the first and second combat system elements due to hull modal vibration at times T1 and T2 are determined by analyzing the filtered accelerometer signals in the frequency domain to identify the first three ship's hull vibration mode frequencies, calculating a contribution factor for each of the three modes based on the fast-Fourier transform (FFT) magnitude of the signal, and calculating the rotational displacements using mode shapes derived from a hull finite element model and the calculated mode shape contribution factors;
• and
• determining relative motion between the first and second combat system elements by differencing the translational and rotational displacements at each of the first and second combat system elements at times T1 and T2.