Recalibration of the CHAMP satellite magnetic field measurements

The 8 years of continuous CHAMP operation allow us to investigate the long-term behaviours of the vector magnetometer on board the satellite.For the processing of the FGM vector magnetometer data primarily 9 instrument parameters are needed (3 offsets, 3 scale factors, 3 misalignment angles).These are determined in pre-flight calibration and routinely in the in-flight scalar calibration.A review of the temporal evolution shows that the scale factors exhibit a clear change with time.This variation of the sensitivity can well be described by a logarithmic function.The offsets and the misalignment angles show no long-term trend.The Z component offset performs medium-term variations within the range of 1nT.These have been corrected on a daily base.When processing the FGM data over the whole period with the derived parameters, the difference between the FGM and OVM readings stays below 0.5nT (corresponding to about 0.2nT standard deviation).For a further improvement the error model has to be extended beyond the scalar calibration.Dynamic spacecraft magnetic fields varying over an orbit are falsely interpreted by the scalar calibration and cause apparent variations of the instrument parameters.In the case of CHAMP magnetic field generated by the power system are responsible for most of the remaining errors.We have applied an extended error model that can account for the time-varying disturbance fields.When considering them properly the standard deviation of scalar errors less than 0.1nT can be achieved.