Technique for the measurement of hull azimuth angles in an NDBO directional wave measurement system

The azimuth angle of a floating platform can be accurately calculated from the platform´s pitch and roll angles and the earth´s magnetic field vector components measured in the platform frame of reference. The relationship among the earth´s magnetic vector, the total magnetic vector, and the magnetic vector arising from the platform itself is analyzed and described in this paper. For the case of negligible induced fields, equations are developed for the calculation of the needed earth´s field components as functions of the total field components and the components of the magnetic field fixed in the platform. Practical methods for the calculation of these fixed fields from measured data in buoy hulls are described and applied to measured data taken aboard a discus hull. The application of these hull-fixed fields and hull pitch and roll measurements to the routine measurement of directional wave spectra is noted.