1. Optical Bench Anomaly Investigation and Modelling
Presented by:
Thomas Usbeck
Prepared by John Leif Jørgensen
Peter S. Jørgensen

2. Outline Summary of the observed Inter CHU alignment effect
Detailed study of Swarm A:
Constructing a combined frame
Description of the individual CHUs in the combined frame
Modelling the individual CHU's in the fixed frame

3. Summary of the observed Inter CHU alignment effect
Variation of the Inter Boresight Angles (IBA) has been observed on all three Swarm S/Cs
The effect varies in magnitude and distriburtion between axes
The major component of the IBA variation is correlated with temperature
(The plots to the right show orbital averages of the IBA between CHU-A and CHU-B on all three Swarm satellites)
Sw-A
Sw-B
Sw-C

4. Swarm A: IBA A-B and temperature
The observed IBA variation follow the large scale temperature variation
The individual CHU has an internal thermistor
There are also several thermistors on the optical bench and boom
Which temperature should be used?
THT00032: Thermistor on the optical bench (OB)

5. Spacecraft thermistor temperatures
Telemetered Temperatures from the optical bench
T00032
T00029
T00115
Courtesy of ESA

6. Spacecraft thermistor temperatures SW-A
Some features of the IBA variation are better represented by SC thermistors than by internal CHU temperature.

7. What proxy temperature to use?
Best fit using linear model of internal CHU temperatures

8. Temperature proxy solution
Combination of: T1=THT00032 and T2=THT00029
Tgradient=2T T2

9. Spacecraft thermistor temperatures SW-B

10. Spacecraft thermistor temperatures SW-C

11. Inflight Inter CHU Orientation –Decomposition
The relative orientation between CHUs can be investigated by taking the angle between CHU measurement frame axis (XX, YY, ZZ)
The signature is seen both for pointing (Z) and roll about boresight (X + Y)
The best resolution is for ZZ (same as IBA)
Inter axis angles between CHU-A and CHU-B on SW-A and CHU temperatures

12. Inflight Inter CHU Orientation –Decomposition vs temperature (Swarm A: CHU A-B)

13. Inflight Inter CHU Orientation –Decomposition XX-YY-ZZ (Swarm A: CHU A-C)

14. Inflight Inter CHU Orientation –Decomposition XX-YY-ZZ (Swarm A: CHU B-C)

15. Summary of the effect
The relative orientation between CHU’s on the optical bench have been observed to vary
The variation is most clearly seen in the Inter Boresight Angle (IBA), but is also present about the other axes
The variation is clearly correlated to temperature and gradient across the optical bench. It can thus be modelled
However, to arrive at a stable solution relative to the VFM, more details are needed on which CHU moves. This is investigated in the following

16. Swarm A: Detailed Study
IBA for A-C and B-C pairs vary significantly (½) less than A-B
This suggests that CHU C boresight (Z axis) is stable on the bench
Further, CHU-A boresight appears to be stable in the axis perpendicular to the A-C plane
This suggests using CHU-C and CHU-A boresights (Z-axis) to construct a common stable frame

17. Swarm A: Combined frame
A combined frame using the two Z-axis of CHU-C and CHU-A is constructed
The following assumptions are made:
Boresight of CHU-C stable
Boresight of CHU-A does not vary perpendiculary to the ZC - ZA plane
Thus
Zfixed= ZA× ZC
Yfixed= ZC
Xfixed= ZC× ZFixed

18. Swarm A: Combined frame
The combined frame is constructed for each update where CHU-A and C are valid (and without Big Bright Object, BBO flag set)
Only information about the two boresights are used.
The individual CHU can now be evaluated in the combined frame by calculating the angle between axes
Angles are averaged over one orbit

19. Swarm A: Boresight stability vs combined frame
For CHU-A the boresight varies ~5”
For CHU-B the boresight varies ~10”
For CHU-C the boresight is fixed by definition to the combined frame
Zfixed= ZA× ZC
Yfixed= ZC
Xfixed= ZC× ZFixed
The variation of the boresight is seen to follow the absolute temperature of the OB

20. Swarm A: Rotation about boresight
The rotation of the individual CHU relative to the combined frame is evaluated by taking the angle between the Y/X axis of the CHU and the relevant axis in the combined frame
To avoid cross-coupling to boresight motion the CHU frame is offset by a fixed rotation to acheive angles of ~90 deg
CHU-A rotation about boresight relative to combined frame (~20”)
The rotation of CHU-A about the boresight follows the OB absolute temperature

21. Swarm A: Rotation about boresight
CHU-B rotation about boresight relative to combined frame (~25”)
CHU-C rotation about boresight relative to combined frame (~30”)
The rotation of CHU-B and CHU-C occur at ~double frequency of the temperature

22. Swarm A: Rotation about boresight
The double frequency suggests gradients between temperature
Gradients between CHUs
Not obvious

23. Swarm A: Rotation about boresight
The double frequency suggests gradients between temperature
Gradients between CHUs and bench (CFRP tube)
Correlation between active and quiet periods?
The search for a good model continues

24. Modelling – next step
The next step is to construct a model for the orientation of the individual CHU relative to the combined frame
This will then be applied to always get the best corrected attitude of the three individual CHU’s

25. Conclusion
The relative orientation between CHU’s on the optical bench have been observed to vary
The variation is most clearly seen in the Inter Boresight Angle (IBA), but is also present about the other axes
The variation is clearly correlated to temperature and gradient across the optical bench
For Swarm A, a combined stable frame can be constructed using the boresights of CHU-A and CHU-C
The individual CHU can be caracterized relative to this combined frame
Next step is to establish a model for the individual CHU based on the combined frame and to apply this model