A Proposal to Cross-Validate ESA's Swarm Level 1b Data with Absolute Scalar Magnetometer Experimental Vector Mode Data
Hulot, Gauthier1; Leger, Jean-Michel2; Crespo-Grau, Raul1; Bertrand, François2; Chulliat, Arnaud1; Jager, Thomas2; Lalanne, Xavier1; Vigneron, Pierre1

Each of the three ESA Swarm satellites, to be launched in the summer of 2013, carries a vector field magnetometer (VFM) and an absolute scalar magnetometer (ASM), positioned on a boom away from the body of the satellite to minimize undesired magnetic perturbations, and distant enough from each other to avoid crosstalk between instruments. The VFM further shares an optical bench with a star imager (STR), to which it is thus rigidly attached. The rational behind this set-up is that the ASM will provide very accurate absolute field intensity measurements, while the combination of the VFM with the STR will provide vector field measurements oriented in the terrestrial frame of reference. The ASM will also provide the values needed to calibrate and improve the accuracy of the three components of the field measured by the VFM in the satellite frame (next converted into field components in the Earth frame of reference thanks to the STR). It is the output of this procedure (using the implemented baseline Swarm level 1b algorithms, also correcting for known satellite perturbations) that will provide the very accurate level 1b data the Swarm mission is aiming at. The above procedure is entirely based on the baseline 1 Hz scalar ASM output. Following an agreement between ESA and CNES, ASM instruments will however also provide two additional, non-nominal types of data, which we intend to use for some useful diagnostic and validations of the nominal level 1b data. The first type of data we intend to use is that of a 250 Hz "burst mode" to be operated during the commissioning phase. It will be used to explore the spectral content of the field encountered on orbit, and identify the frequency bands within which three modulations could be used for the need of the additional, and experimental, absolute vector field ASM data. This second type of data consists in absolute vector field measurements produced as a by-product by the ASM and synchronized with the nominal 1 Hz scalar measurements, the usefulness of which will however depend on the outcome of the series of analysis made with the "burst mode". We intend to make use of these data to carry on some comparisons with the nominal level 1B vector data. The ASM 1 Hz vector field measurements will measure the field at the location and in the frame of reference of the ASM. After checking this data for internal consistency, they will be used for carrying comparisons with level 1b 1 Hz VFM data in the VFM frame of reference to reconstruct estimates of the Euler angles between the two instrument frames of reference. This procedure may not require much more than an hour of data to recover useful parameters. This information could thus be used to assess the stability of both instrument's frames of reference and of the boom mechanically linking them. If conclusive, these tests would then lead us to envision a direct estimate of the Euler angles linking the ASM frame of reference to the STR and Earth frames of reference using a procedure similar to that used for the reconstruction of the nominal Level 1b data. This would then allow the production of 1 Hz vector data (almost) independent from the nominal 1 Hz level 1 b data for further comparison.

Details of the intended studies, and results of various simple simulations, will be presented. If available, preliminary results will be reported.