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Flight Calibration of the Van Allen Probe Magnetometers

The twin Van Allen Probes each carry a triaxial fluxgate magnetometer. Their orbits transverse a large of range of magnetic intensity. A procedure is described that calibrates the magnetometer in the spinning-spacecraft frame and in the presence of a persistent gradient of the magnetic field. This is done by extending a model based on a constant background magnetic field to a model with a changing magnetic field during intervals used for calibration. Model parameters derived from short intervals are obtained by direct calculation, and uncertainties are propagated throughout the analysis. A daily ensemble analysis of interval-derived parameters and uncertainties is then made to correct for sensor bias and misalignment. The whole analysis is iterated to convergence in a few steps based on the estimated uncertainties associated with the model parameters.