In-flight receiver calibration of the Ganymede Laser Altimeter (GALA) by passive Earth observations

Abstract. Post-launch characterization of the receiver telescope of the laser altimeter is essential for achieving precise georeferenced planetary measurements and for radiometric calibration to passively measure surface reflectance. For the Ganymede Laser Altimeter (GALA) aboard the Jupiter Icy Moons Explorer (Juice), such in-flight calibration was originally planned during a lunar flyby but could not be performed due to an unexpected reboot of the instrument. Instead, this study reports a passive noise measurement acquired during the Earth-farewell campaign on 9 September 2024 as an alternative opportunity for in-flight calibration. Using temporal variations in the GALA noise level as a proxy for photon flux incident on the GALA detector, we combine a theoretical noise model with Earth images obtained by the Jovis Amorum ac Natorum Undique Scrutator (JANUS) imager to constrain the GALA receiver boresight direction. By comparing the timing, magnitude, and temporal pattern of noise variations between observations and simulations, we find that the pre-launch boresight vector is inconsistent with the Earth-farewell observations. Our results further suggest that the GALA receiver boresight may have experienced a post-launch offset over 700 μrad, although a definitive conclusion can be drawn only with additional cruise-phase data. The methodology developed in this study offers a framework for in-flight alignment calibration of GALA during future flybys, which is also broadly applicable to other planetary laser altimeters. The radiometric calibration performed in study is also prerequisite for the interpretation of passive albedo measurements of Jupiter and the Galilean moons.