Vector Field Magnetometer (VFM)
Swarm Optical Bench
The Vector Field Magnetometer (VFM) measures the magnetic field vector at the tip of the optical bench on the boom. The sensor is a 3-axis Compact Spherical Coil (CSC) with a 3-axis Compact Detector Coil (CDC) inside. The instrument operates as a closed-loop system adjusting the compensating CSC currents to maintain a null field at the detector coils within the sphere. The currents in the CSC coils are measured and digitized (by an ADC) and this constitutes the raw measurements of the instrument. See [AD-8] for a detailed description of the instrument, and for the Level 1b algorithm described by the instrument supplier.
The VFM is an analogue instrument and as such subject to temporal changes due to radiation and aging effects of the electronics. The effects are only significant in the bias and linear scale parameters of the characterization; hence these parameters are estimated daily through a comparison of the VFM output with the ASM scalar magnetometer output. Additionally, the non-orthogonality angles of the VFM sensor may also be estimated in this process. The allowed change from day to day in the parameters is controlled group-wise by weight parameters specified in the CCDB. The parameters estimated daily are stored in a Temporal Calibration File (TCF) as an auxiliary data product.
VFM Algorithm Overview
The overall VFM processing is sketched in the Figure below. Only data with a common DPU_id is to be processed, minority data (DPU_id wise) is rejected. Next, the raw output (Level 1a.VFM.Sci.EU, rate: 50 Hz, timestamps t0,VFM) of the VFM is corrected for timestamp, processing, and filter delays. Then, it is corrected and converted to physical units (nT) using the CCDB.VFM.Cal parameters and the Level 1a.VFM.Sci.TX temperatures. This is the Level 1bInst VFM vector product. The rate is 50 Hz and the time-instants tVFM.
Then, the preliminary vector field measurements, B'VFM, are computed using the TCF.VFMinit parameters. TCF.VFMinit is the most recent (w.r.t. data being processed) TCF.VFM record among the TCF.VFMinput (from the previous Level 1b Product) and the CCDB.L1BP. VFM.TCF_Aux array - with DPU_id corresponding to Level 1a.VFM.Sci.DPU_id. Formally, let t0 = Level 1a.VFM.Sci.t0, then:
Next, outliers are detected and accordingly rejected or flagged as suspicious samples. Now, the magnetic stray fields from the rest of the S/C at the VFM sensor position - at the time instants of the VFM measurements, tout,VFM (= tVFM) - are computed. The phase linearity and fast response of the VFM Bessel filter makes it unnecessary to apply a filter to the stray fields as was the case for the ASM. However, the characterization of the AOCS torquer coil disturbances possibly needs to take the VFM filter into account.
Next, the internal temporal changes of the VFM electronics and possibly any change in the non-orthogonalities of the CSC are modeled by the TCF.VFM parameters which are estimated by comparison of VFM data with the fully corrected ASM scalar data (FASM). The new estimates of the TCF.VFM parameters, TCF.VFMoutput, are applied to all the VFM data of the actual day. Together with the correction for stray magnetic fields the Level 1b.Mag-H.BVFM data are obtained. I.e. fully converted and corrected magnetic vector data in the orthogonal VFM sensor frame. The rate is 50 Hz and the time instants tout,VFM.
Then Level 1b.Mag-L.BVFM, the 1 Hz magnetic vector product in the VFM frame at UTC seconds, is extracted. Finally, the magnetic field vectors BVFM at 50 Hz and 1 Hz are transformed via the Common Reference Frame (CRF) into the NEC frame. This completes the generation of the Level 1b.Mag-H.BNEC and Level 1b.Mag-L.BNEC products.
Note: the rotation from the VFM to the CRF frame is estimated pre-flight and refined inflight. Hence periodic updates of CCDB.Structure.STR_q_VFM are foreseen and consequently reprocessing of the final step above is required regularly.
VFM processing overview