1. The Swarm Advanced stellar Compass
Professor John Leif Jørgensen

2. Configuration of the Swarm µASC
Each satellite features one redundant µASC
The µASC is equipped with 3 camera head units (CHU)
The CHU’s are mounted on the optical bench also supporting the VFM sensor unit
The CHUs are pointed such as to always having two or more CHUs pointed to nominal night skies

3. The µASC unit
The µASC processor onboard Swarm is fully cold/hot redundant
Each side may be assigned all, some, or none of the CHUs
Each CHU is equipped with an optimized two stage straylight suppression baffle system.
The baffles for Swarm has been optimized for sunlight suppression while keeping the heat load to the optical bench at a minimum

4. ASC Architecture

5. Use of the µASC data Each CHU will output a quaternion once per second
A quaternion packet contains
The timestamp of COI
The four quaternion components
Validity flag
Stars detected
Stars used
BBO flag
Estimated noise level
Sequence and other flags and operations levels
Measured pointing of CHU A of Swarm Alpha

6. Non-nominal skies (I), Solar intrusion
When the straylight from an approaching bright object becomes significant, the BBO flag is set.
When the Sun enters the FoV, the CHU will be blinded. Here is shown the performance of CHU-A on Alpha in camera coordinates.

7. Non-nominal skies (II), Lunar intrusion
When the Moon enters the FoV, the CHU will be semi-blinded. Here is shown the performance of CHU-C on Charlie of a full Moon passage
Typical behavior of the BBO flag from a full Moon passage.

8. Non-nominal skies (III), Radiation
Despite the µASC is featuring full EDAC, radiation and especially energetic protons may result in measurable effects. When an energetic particle passes the CHU CCD, the particle will deposit energy liberating electrons which may be mistaken for signal electrons.
The morphology of the distribution of such radiation induced is easy to separate from star centroid signal, and is thus filtered out.
Signal from passing the SAA

9. Thruster activations and elevated angular rates
At nominal rates, the stars will fom elipical centroids on the CCD.
At elevated angular rates, e.g. maneuvers the centroid shape will be changed by the motion of the S/C during integration
Linear angular acceleration will result in uneven illumination of the centroids
More complex accelerations and jerks will result in unpredictable centroids.
The µASC therefore evaluates the centroid shape and reports the average shape in the high rate flag and elevated noise in the residual estimate

10. Inflight performance, common frame
The attitude measurement from two or three CHUs may be combined to achieve an improved accuracy
Since the measurement noise is about 11 times worse about the boresight compared to the pointing accuracy, this combination must be done in a way that takes this asymmetry into account.
An efficient solution is given in: ASC-DTU-TN-3066 Combination of Star Tracker Attitude Measurements
Measured deviations from a stable platform (15orbits)

11. CHU performance compared to merged solution
The performance of the individual CHU is estimated by comparing the individual CHU measurement to the merged solution
Removing daily mean gives the diurnal variation. Shown with 270s moving average (red), which is expected to be dominate by thermo-elastic distortion
After thermo-elastic bias removal the estimated performance for the individual CHU is obtianed

12. Inflight performance, stability
By calculating the attitude transfer matrix between pairs CHU of each bench, and comparing the result to the pre-flight measured values, the stability of the system can be assessed.
Preflight values
Mean
Mean deviation from preflight
[deg]
["]
Swarm A
EA1
EA2
EA3
A vs B
-90,91835
-45,52396
34,26685
-90,91324
-45,53097
34,27022
18,39
-25,25
12,13
A vs C
-90,62000
44,53864
-35,75287
-90,62486
44,53673
-35,74337
-17,49
-6,90
34,20
B vs C
90,11496
55,52102
-125,63444
90,12024
55,52417
-125,64048
19,01
11,35
-21,74
Swarm B
-88,72687
-45,20856
34,27475
-88,70678
-45,21292
34,31701
72,33
-15,66
152,15
-88,02569
44,82124
-36,37904
-88,04742
44,81704
-36,33703
-78,22
-15,10
151,22
90,06104
55,22822
-125,94160
90,05584
55,21526
-125,91839
-18,74
-46,66
83,55
Swarm C
-89,65281
-45,38820
34,54679
-89,63907
-45,38905
34,56076
49,46
-3,04
50,30
-88,99476
44,43076
-34,76885
-88,99205
44,43086
-34,76006
9,75
0,35
31,66
89,68738
54,98028
-124,04633
89,68938
54,97681
-124,05129
7,21
-12,49
-17,87

13. Inflight performance, noise per CHU
The attitude noise performance of a given CHU, may be assessed by comparison to the common optical bench frame
Naturally, such an assessment will contain any error in the estimation of the common frame
Conversely, any bias at large scale, larger than the common frame captures will be suppressed
Measured deviation from the common frame (15orbits). Here CHU-B on Swarm Alpha.
Subtracting the diurnal variation from the common frame, result in a conservative measurement RMS noise estimation of better than (3.5”, 3” and 52”) about the X, Y and Z axes respectively.
(See ”SW-DTU-RP-3080 v1_1 Swarm STR & VFM In-Orbit Commissioning Report”)

14. Inter Boresight Angle (IBA)
For a stable platform, the measured angle between any pair of CHU pointing directions (IBA) shall remain constant at the level of the measurement noise
The measured IBA has been found to vary with temperature
The variation appears to be of a parametric nature

15. Inter Boresight Angle (IBA) (II)
The question is, are the temperature dependencies solely rooted in the absolute temperature or does the gradient across the optical bench also play in??

16. Inter Boresight Angle (IBA) (III)
IBA Correlation with temperature (”/ deg C)
IBA variation with temperature falls in three categories
No or little variation (< 0.2 ”/deg C)
Medium variation (0.3 – 0.4 ”/deg C)
Large variation ( ~0.9 ”/deg C)
CHU A vs CHU B
CHU A vs CHU C
CHU B vs CHU C
Swarm A
+0.35
+0.15
-0.09
Swarm B
+0.34
+0.39
+0.44
Swarm C
+0.85
+0.17

17. Simulation of the CHU mount
Case 1 : Only one spherical washer is allowed to slide (sliding is exaggerated in the graphics):
Results in deformation translated into angle is about 21 arcsec of deformation

18. Simulation of the CHU mount (II)
Case 2: two spherical washers are allowed to slide:
Result translates into appr. 45 arcsec of deformation about the z axis.

19. Unexpected BBO triggering
Throughout the mission unexpected BBO triggering has been observed
Images acquired at times with BBO triggering show some unexpected objects near the CHU
The objects does not have a signature compliant to another S/C, and analyses show that it cannot be another Swarm S/C
Throughout the mission unexpected BBO triggering has been observed

20. Unexpected BBO triggering (II)
The objects was frequent at the beginning of the mission, but is still seen by Bravo 1 month ago
The observed objects does not affect accuracy or bias
Natural deep space objects does not trigger the BBO
The objects was frequent at the beginning of the mission, but is still seen by Bravo 1 month ago
The objects was frequent at the beginning of the mission, but is still seen by Bravo 1 month ago
The observed objects does not affect accuracy or bias

21. Conclusion All units operating as designed
Attitude availability as designed
Attitude noise as designed
Solar suppression as designed
Radiation handling as designed
A few unexpected effects has been observed
Thermo-elastic behavior of optical bench mounting
Accurate correction model can/shall be made
Nearby objects trigger BBO
No effect on performance