2.4.2 Magnetometer Triad

The magnetometer triad measures the magnetic field vector at its location within the external sensor unit. Magnetic sources include the earth, electric currents, structures surrounding the vehicle, electric motors, and the vehicle structure. The purpose of the measurement of the magnetic field is to provide a reference vector pointing North in order to complete the determination of the vehicle's attitude.
The earth's magnetic field at the latitude of the SSL is about 18° from vertical. The state estimator uses this vector, along with a vector that points "down" to determine the attitude of the vehicle. The algorithms for attitude determination are discussed in detail in Section 4.2.1.
The analog magnetometer data is collected by the ESU 6811 microprocessor through a New Micros 12 bit ADC board. The sensor information is collected, and returned to the main processor via a RS422 serial communications line at a rate of about 200 Hz.