TWOSTEP: A Fast Robust Algorithm for Attitude-Independent Magnetometer-Bias Determination
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TWOSTEP: A Fast Robust Algorithm for Attitude-Independent Magnetometer-Bias Determination Roberto Alonso' and Malcolm D. Shuster Abstract A fast robust algorithm is developed for the inflight estimation of magnetometer biases when the attitude is not known. This algorithm combines the convergence in a single step of an heuristic algorithm currently in use with the correct treatment of the statistics of the measurement, and does this without discarding data. The new algorithm works well even when the magnetometer bias is comparable in magnitude to the ambient magnetic field. The algorithm performance is examined using simulated data for both spinning and inertially stabilized spacecraft.
Introduction At orbit injection, often the only attitude sensor operating is the vector magnetometer. Frequently, the spacecraft is spinning rapidly, and, if the spacecraft is not in an equatorial orbit and not at too high an altitude, it is possible on the basis of this sensor alone (and, of course, a knowledge of the spacecraft position) to determine the spin rate and the spin-axis attitude of the spacecraft. At the same time, the accuracy of the magnetometer data may be compromised by large systematic magnetic disturbances on the spacecraft, often the result of space charging during launch or from electrical currents within the spacecraft. Thus, some means is usually needed to determine this bias quickly. Since the three-axis attitude of the spacecraft usually cannot be determined at this stage, the desired algorithm must not require a knowledge of the attitude as input.
'Jefe, Grupo de Control de Actitud, Comisi6n Nacional de Actividades Espaciales (CONAE), Avenida Paseo Colon 751, (1063) Buenos Aires, Argentina. 2Director of Research, Acme Spacecraft Company, 13017 Wisteria Drive, Box 328, Germantown, Maryland, 20874. email: [email protected].
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The above situation occurs for nearly every spacecraft. For spacecraft equipped with only a vector magnetometer and a Sun sensor, three-axis attitude will rely on the magnetometer data. In this case, the spacecraft attitude cannot be used directly to determine the magnetometer bias vector by transforming the reference magnetic field to magnetometer coordinates using the computed attitude and then comparing this transformed reference field with the magnetometer measurement. For such a mission, which occurs quite often, algorithms of the type discussed in this paper are required. 3 Our study focuses on near-Earth spacecraft, for which a reasonably accurate magnetic field model exists (certainly for magnetic latitudes of less than 70 deg). Obviously, for scientific studies in which one wished to refine the geomagnetic field model, one would require complete attitude knowledge, at least as a practical matter. The algorithm studied in this work is adequate for calibrating parts of the attitude control system, such as for determining the ambient magnetic field for momentum dumping. For a spacecraft with moderate attitude accuracy requirements (say, approximate
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