Earth observations as opportunities for in-flight calibration of the Ganymede Laser Altimeter (GALA) aboard the Jupiter Icy Moons Explorer (Juice) mission

Precise in-flight calibration of laser altimeter receivers is a fundamental requirement for achieving georeferenced planetary topography measurements and for enabling passive radiometric observations of surface reflectance. In the same manner as other laser altimeters, such a calibration is essential for the Ganymede Laser Altimeter (GALA) aboard the Jupiter Icy Moons Explorer (Juice) mission. GALA is designed to obtain surface topography data of the Galilean moons to investigate geologic processes and to measure tidal deformation of Ganymede induced by Jupiter, providing clues to its interior structure. As GALA is also capable of passive observations to map surface reflectance at 1064 nm, accurate in-flight characterization of the receiver telescope is essential both for precise estimation of footprint positions on the surface of a celestial body and for radiometric calibration of future passive albedo measurements. In-flight calibration of GALA was originally planned during a lunar flyby as part of the Lunar Earth Gravity Assist maneuver in 2024 but could not be performed due to an unexpected reboot of the instrument. 

As an alternative opportunity for inflight calibrations, we exploit Earth observations during the Earth-farewell campaign on 9 September 2024, in which GALA was operated in a passive mode and its noise level was modulated by reflected sunlight from the Earth entering the detector. Using temporal variations in the GALA noise level as a proxy for incident photon flux, we combine a theoretical noise model with Earth images obtained by the Jovis Amorum ac Natorum Undique Scrutator (JANUS) imager and compare the timing, magnitude, and temporal pattern of noise variations between observations and simulations to constrain the GALA receiver boresight direction. This establishes a framework for in-flight alignment and radiometric calibration of GALA using Earth observations, which is also broadly applicable to other planetary laser altimeters.

Application of this framework reveals that the pre-launch boresight vector is not consistent with the Earth-farewell observations, suggesting a post-launch offset potentially exceeding 700 µrad, though a definitive conclusion requires additional cruise-phase data. The radiometric response of GALA is simultaneously characterized with respect to radiance measured by JANUS's filter-12, which covers the same wavelength as GALA, providing a prerequisite for future passive reflectance measurements of Jupiter and the Galilean moons. These results underscore the value of Earth observations as calibration opportunities for instruments during the cruise phase. In this presentation, we also discuss our predictions on upcoming calibration opportunities, including observation during the Earth Gravity Assist maneuver of Juice in September-October 2026.