Geometric in-flight calibration of MAJIS (JUICE) during early cruise phase and LEGA flyby

Abstract. In August 2024, MAJIS, the visible and infrared mapping spectrometer onboard the ESA-Juice spacecraft, collected its first observations of resolved targets during Lunar and Earth Gravity Assist maneuver (LEGA). One month before, during payload checkout 2 (PC2), MAJIS performed two starfield observations to evaluate its post-launch geometric performances that can be compared with the LEGA data. This work presents an overview of the spatial distribution of the MAJIS signal collected at different scan angles and their position in MAJIS field of view. In PC2 starfields, 14 stars were identified, allowing us to derive a new in-flight alignment with an accuracy of 0.7 ± 0.4 MAJIS instantaneous field of view (IFOV). The offset between the nominal boresight of Juice remote sensing platform (+Z) and MAJIS boresight was evaluated to be 27 samples (along-slit) and 31 lines (cross-slit). With this new alignment, MAJIS observations on the Moon are co-aligned with JANUS camera with an accuracy of 2.2 ± 0.9 IFOV which could be improved to 1.1 ± 0.6 IFOV with bundle adjustment. A comparison of MAJIS Earth observations with simultaneous weather satellites observations confirmed that the MAJIS alignment is accurate for the full close Earth flyby. Residual misalignments with JANUS and NavCam remain visible in the Earth dataset but they can be easily corrected using small timing and scan angle adjustments (<10 IFOV). The instrument and frame kernels for MAJIS will be updated on the basis of these results. The improved co-alignment between MAJIS, JANUS and NavCam ensures strong synergy between the remote sensing instruments during the nominal scientific phase when they operate at the same time.