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.

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Publication Details

Journal
Annales Geophysicae
Published
2026-10-14
DOI
https://doi.org/10.5194/angeo-44-1075-2026
Primary Topic
Calibration and Measurement Techniques
Type
article
Field-Weighted Citation Impact
0.00

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article

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

Livio Agostini, C. Tubiana, Ganna Portyankina, Keigo Enya et al.
Annales Geophysicae
Calibration and Measurement Techniques
article

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

Livio Agostini, C. Tubiana, Ganna Portyankina, Keigo Enya, Kay Lingenauber, Jan Binger, Hauke Hußmann, Jun Kimura, Konrad Willner, P. Palumbo, Wladimir Otto Neumann, Christian Hüttig, Luca Penasa, Gaku Nishiyama, Klaus‐Dieter Matz, Kai Wickhusen, Janot Phillip George, Giulio Macrì, Alexander Stark, Masanori Kobayashi, Christian Althaus
article en

Abstract

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.

Annales GeophysicaeVol. 44(2)
University of Padua (IT), The Graduate University for Advanced Studies, SOKENDAI (JP), Hokkaido University (JP), Japan Aerospace Exploration Agency (JP), Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR) (DE), Institute for Space Astrophysics and Planetology (IT), Institute of Space and Astronautical Science (JP), Osservatorio Astronomico di Padova (IT), Technische Universität Berlin (DE), The University of Tokyo (JP), Chiba Institute of Technology (JP), The University of Osaka (JP)
Japan Society for the Promotion of Science
Openalex Percentile: Top 94%
Calibration and Measurement Techniques
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