1. In flight calibration of the experimental ASM vector mode on board the Swarm satellites
Thomas Jager, François Bertrand, Viviane Cattin & Jean-Michel Léger, CEA-LETI Isabelle Fratter, CNES Laura Brocco, Pierre Vigneron, Xavier Lalanne et Gauthier Hulot, IPGP

2. Context of ASM vector mode development
SWARM satellite
VFM sensor
ASM DPUs
ASM sensors
Source: EADS-ASTRIUM
Role of ASM is to provide absolute scalar data to calibrate the Vector Field Magnetometers
Proposal by Leti to take advantage of improved ASM characteristics wrt the Overhauser magnetometers flown on Oersted & Champ (increased resolution and bandwidth) in order to deliver simultaneously scalar and vector data (technological demonstration)
Option acknowledged by CNES & ESA provided it neither results in scalar performances degradation nor implies any constraint on the scalar instrument design  tradeoff between achievable performance and aforementioned requirements
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

3. ASM Vector measurement basics
Vector modulations are superimposed onto ambient field B0 thanks to 3 vector coils
Vector field is rebuilt in ASM orthogonal reference frame thanks to a specific calibration process
Vector modulation amplitudes of the order of 50 nT (HW set)
Vector modulation frequencies adjustable in ASM SW (presently # Hz)
Real–time measurement of the 3 projection amplitudes gives the orientation of the magnetic field => both scalar and vector measurements are provided at 1 Hz (same location and time)
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

4. Environment noise assessment (ASM burst mode)
Vector mode operation requires a very low background noise
No data available from previous missions with the required resolution (< 1 pT/Hz)
 need to sense the in-flight environment noise to eventually tune the modulation
frequencies
Burst mode Fs = 250 Hz, thus allowing noise spectrum analysis up to 100 Hz
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

5. Residual instrument noise (see poster # 18 )
Burst mode also revealed some unexpected noise sources:
Heaters 58 kHz , but with numerous harmonics…  perturbations for B0 = 2070 nT + i x 4140 nT +/- [100 nT]
Motor activations also affect the scalar measurements (# 10 pT for about 2 s, but with an impact on vector measurement due to the intrinsic amplification factor)
Additional outliers not yet linked to any identified source (work in progress) detected by looking at field variation rates anomalies
Total of pts:
6600 noise outliers (4%)
11500 motor activations (7 %)
7600 HF heater perturbations (4.4 %)
FILTERED DATA
(84.6 %)
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

6. Vector calibration parameters (first step)
Basic vector field reconstruction parameters:
Vector coils transfer functions  3 parameters
Vector coils deviation from orthogonality  3 angles
No offsets 
Determination of the vector calibration parameters:
least-squares minimisation algorithm: minimisation of the ASM scalar residual r= 𝐵 0 𝐴𝑆𝑀 − 𝐵 𝑉𝑒𝑐𝑡 𝐴𝑆𝑀 between ASM scalar field measurement and the modulus obtained through vector field data
Validated during ground calibration, both on thin shell runs and orbit simulations
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

7. Phenomena affecting the vector reconstruction
Vector coils thermal expansion coefficients
measured on-ground and confirmed in-flight, Peek CTE # 47 ppm/K
Modulation of the laser pumping light intensity due to the rotor angular position (ASM sensor isotropy is obtained by controlling the rotor orientation wrt the magnetic field direction)
The optical coupling is periodically modulated
The He4 atoms response to vector coils excitations is also periodically modulated
Refined model taking into account these phenomena accounted for by 1 +(3*3) additional correction parameters
Model dependent
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

8. Analysis of the scalar residuals at instrument level
Scalar residuals analysis provides a real time quality assessment of the vector data, unaffected by stray fields: example of SWARM A, 02-03/06/14
 Possible remaining orbital low frequencies signatures to be investigated (latitude, …)?
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

9. Vector data analysis (short term, based on daily calibration)
Statistical analysis of the ASM scalar residuals:
Scalar residual distribution and separated perturbations analysis
Deviation from gaussian profile due to the dependence of vector resolution on field modulus (ASM-V  constant attitude resolution)
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

10. Consistent with a # 2 nT/√Hz gaussian noise
Vector data analysis
Statistical analysis of the ASM scalar normalized residual:
Scalar residual distribution analysis of filtered points
The global gaussian behavior is affected by outliers in the statistical analysis
Consistent with a # 2 nT/√Hz gaussian noise
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

11. Vector data analysis – Allan variance
Short term stability related to ASM vector resolution ~2 50 µT
Mid term stability : σ = 0,2 nT
Orbital period,   0,07 nT
Calibration parameters drift
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

12. Vector data analysis (short term)
Statistical analysis of the ASM scalar residual: all models
Scalar residual distribution and separated perturbations analysis 02-03/06/2014
Perturbations affecting the scalar residual characteristics can be easily removed (most efficiently on FM1A and FM2A, the FM3A having an higher intrinsic noise level)
Points selection
SWARM A – ASM FM1A
SWARM B – ASM FM2A
SWARM C – ASM FM3A
Std (nT)
Mean (nT)
All points
2.33
-0.027
2.412
-0.031
4.78
0.14
Noise outliers
7.213
0.129
5.965
-0.089
7.886
-0.01
HF heater perturbations
2.416
-0.021
2.433
-0.055
4.666
0.115
Motor activations
4.166
3.84
4.866
0.063
Filtered points
1.6
-0.029
1.895
-0.024
4.613
0.163
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

13. Vector data analysis (long term trends)
Long term analysis with a single calibration –SWARM A
Overall daily scalar residual mean value <+/- 7 nT !!!
Periodic variation linked to satellite daylight duration?
Overall daily scalar residual std value ~ 2.5 nT
A single vector calibration is not yet stable enough  daily calibration recommended
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

14. Long term vector data analysis (daily CCDBs vs single CCDB)
The scalar residual stability daily evolution is highly improved
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

15. Transfer functions long term trends
Vector coils transfer function daily evolution – SWARM A
+/-170 ppm, 5-6 month signature
Only medium correlation with other axes behaviour, but poorly excited axis
+/-170 ppm, 5-6 month signature
This behavior is suspected to be linked with a temporal evolution of the thermal configuration on the ASM sensor (thermal gradients, satellite daylight duration,…) similar results on B & C
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

16. Deviation from orthogonality evolution
Vector coils orthogonality parameters daily evolution – SWARM A
Orthogonality angles are rather stable over the first 7 months of the mission (similar results on B & C ) CCDBs with fixed orthogonality angles will be evaluated
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

17. Vector data analysis summary
Calibration parameters evolution:
Most of the evolution is related to the vector coils transfer function
Evolution with thermal gradients distribution on the ASM sensor, linked either to long term (sun incidence and daylight duration w.r.t satellite attitude) and short term (maneuvers) evolutions
Justification for the vector calibration performed on a daily basis
Vector coil orthogonality angles and other vector calibration parameters (rotation parameters) are quite stable
Scalar residual and vector performances:
Nominal 1 Hz (BW 0.4Hz) scalar residual characteristics:
Mean value in the +/- 20 pT range
Daily standard deviation of non filtered points 2,2-2,6 nT
Daily standard deviation after perturbations removals 1,5-1,8 nT
In agreement with the expected ASM level of performances:
Vector resolution ~2 B0 = 50 µT
Orbital scalar residual stability plateau # 0,2 nT
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

18. Perspectives (Swarm…and beyond)
Investigation of remaining orbital low frequency signatures in the ASM scalar residual:
Geographical dependence?
Others correlations?
Evaluation of CCDBs with:
Fixed orthogonality angles
With long term periodic correction coefficients of fixed vector coils transfer functions:
Taking into account the satellite daylight duration evolution
 possible unique instrument CCDB with additional input parameters?
Exploitation of the ASM-V data for science studies, see poster 40
Design improvement guidelines for next ASM-V generation…?
ASM in-flight vector calibration – Jean-Michel Léger- 3rd Swarm science meeting - 20/06/2014

19. Thank you for your attention
ASM Vector data calibration – Thomas Jager - 3rd Cal/Val meeting - 05/06/2014