Physical Approach to the ASM-VFM residual investigation National Space Institute, DTU 3. December 2014

New Data Set of Swarm A Extended to third noon/midnight and using pre-flight calibration Scalar residual for the whole period The pre-flight scale values are scaled down by a common factor to match ASM Noon-Midnight The residual indicate scale value drift of the VFM (this was foreseen)

Scale Value Adjustment Scale value adjustment is proposed: S New = S old · [ ] · ( ppm/yr) Re-calibration of the scale values is performed using the whole data set and allowing scale value drift The scale values of the VFM sensor are determined by the size of the spherical CSC shell. The composite material of the shell will change when exposed to vacuum. The baseline of the Swarm mission was to use the ASM as a calibration instrument for the VFM

Offset Value adjustment The DTU Perturbation Model is based on lowering the residual in the first dawn-dusk by adjusting the offsets. The Perturbation Model suggests the following offset adjustment: O new = O Old + [ -1.21, 1.82, -0.08] The ASM-VFM residual seem correlated with blinding of the star cameras No cameras was blinded in the dawn/dusk period No star camera blinding (next slide)

Offset Value adjustment (cont) Re-calibration the offsets in spring dawn/dusk seem reasonable if the residual is generated by sun in the star camera. The residuals are mainly shifted to the noon/ midnight Unfortunately the re- calibration does not hold into the next dawn/dusk Furthermore another effect seem to appear in the autumn dawn/dusk Special effort has been made pre-flight to prevent interference between the VFM and the star camera Spring dawn/duskAutumn dawn/dusk Spring dawn/dusk Eclipse residual

Thermal coverVFM Boom Bracket Legs Thermally induced perturbation Heat from sunlit baffle A thermal gradient on the VFM bracket may come from a varying temperature of the thermal cover through the thermal cover legs The black inner part of the baffle will absorb heat energy when lit by the sun The three baffles are mounted on the thermal cover. Heat transfer from the baffles to the cover is still under discussion The existence of thermoelectric currents in the VFM bracket is still under discussion but the currents are speculated to induce a field perturbation in the x-axis of the VFM sensor

Original ASM-VFM correction Sun vector in baffle Heat filter model Baffle Heat model

Original correction on New Data Set Spring Dawn/Dusk Autumn Dawn/Dusk The single baffle model does not work in the autumn dawn/dusk period

Thermal Analysis of the Optical Bench Hot Cold GFRP standoff Outer Baffle Thermal Cover Conclusion is that heat from the baffles cannot penetrate through the thermal cover and down to the Ti-plate and hereby produce sufficient thermoelectric current as suggested. This is mainly due to GFRP standoffs off the outer baffle and the area of the faces in the T 4 -radiation. The Analysis predicts a temperature gradient over the Ti-bracket in the order of 3 deg (max) A thorough study has been performed by Airbus Industry to test and verify the model above. This is considered to low to generate the thermo- electric perturbation observed The time delay (sunlight-perturbation) would also be to high

Thermal coverVFM Boom Bracket Legs Heat from sunlit baffle The ESA Way L1b ImplementationEmpirical Modelling Choose ModellingDisturbance Vector Physical Analysis NEW APPROACH

Residual in Sun-frame sun Azimuth Frame definition Side C Sun on Swarm side C The remaining scalar residual after re-calibration is correlated with the sun impingement on the satellite (or optical bench). This correlation can be seen when plotting the residuals in the sun-frame Sun above (as seen from satellite) The main perturbation seems to originate from the sun just above the satellite (zenith) and a secondary effect seems to originate from the sun radiation

Zenith Perturbation Perturbation Field (in VFM-frame) Perturbation Vector (in VFM-frame) Perturbation Distribution ( in Sun-frame) sun Sun direction of maximum disturbance Side C Side B After correction Side Perturbation Applying the zenith perturbation model reveals a new signature here referred to as the Side Perturbation

Side Perturbation sun Sun direction of maximum disturbance Perturbation Field (in VFM-frame) Perturbation Vector (in VFM-frame) Perturbation Distribution ( in Sun-frame) Side C Side B After correction After applying the Side Perturbation model the main perturbation seem to be corrected.

Model Comparison Orbit maneuvers Similar features Comparable size X-axis identical but different detailing Y-axis identical features but different size Z-axis affected by zenith perturbation in one model but not in the other Level of detailing to be discussed Uniqueness of the solution is questionable DTU ModelGFZ (Lesur) Model Disturbance Vector

Physical relation with the Disturbance Eclipse Boundary Sun in Side Baffle (B) Sun in Zenith Baffle (A) Trace on the next slide

Model Comparison (cont) Into the sunOut of the baffleInto eclipse Scalar residual (after recalibration) Depend on the recalibration performed Sharp peak when the satellite goes into eclipse Feature when satellite goes into the sun Feature when sun exits the baffle Magnitude of Disturbance The two models try to compensate different features DTU Model GFZ (Lesur) Model

What to look out for on Swarm Thermal gradients and conductive material near the VFM sensor Metallic subject exposed to sunlight close to the VFM sensor Thermocouple circuit in the proximity of the VFM sensor Cu Al T1T1 T2T2 I Presented April 2014

Suggestion of where to look Current loop connecting the inner baffles Grounding wires intercon- necting the outer baffles Current loops formed by various harness/grounding wires Inadequate insulation of CFRP thermal cover / Ti-legs / Ti-plate