The Swarm Advanced stellar Compass Professor John Leif Jørgensen

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

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

ASC Architecture

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

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.

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.

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

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

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)

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

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

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”)

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

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??

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

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

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.

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

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

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