Exploring the icy moons of Jupiter: The Ganymede Laser Altimeter (GALA) for ESA&#8217;s JUICE Mission
Authors: Luedicke Fabian , Hussmann Hauke , Lingenauber Kay , Kallenbach Reinald , Enya Keigo , Kobayashi Masanori , Thomas Nicolas , Touhara Kazuyuki 
Published: October 2020

Exploring the icy moons of Jupiter: The Ganymede Laser Altimeter (GALA) for ESA&#8217;s JUICE Mission
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Fabian L&#252;dicke (1), Hauke Hussmann (1), Kay Lingenauber (1), Reinald Kallenbach (1), Keigo Enya (2), Masanori Kobayashi (3), Nicolas Thomas (4), Luisa Lara (5), Kazuyuki Touhara (2) and the GALA team.
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(1) DLR Institute of Planetary Research, Berlin, Germany (fabian.luedicke@dlr.de), (2) ISAS/JAXA, (3) Chiba Institute of Technology, Japan, (4) Physikalisches Institut, University of Bern (UBE), (5) CSIC, Instituto de Astrof&#237;sica de Andaluc&#237;a (IAA), Granada, Spain
&#160;AbstractThe Ganymede Laser Altimeter (GALA) is one of ten instruments selected for ESA&#8217;s Jupiter Icy Moons Explorer (JUICE) mission. We will present an overview on the scientific goals as well as on the status of the instrument development and performance analysis.ESA&#8217;s JUICE mission will explore Jupiter, its magnetosphere and satellites first in orbit around Jupiter before going finally into polar orbit around Ganymede [1]. GALA is one of ten payloads on-board the spacecraft and is developed under responsibility of the DLR Institute of Planetary Research in collaboration with industry and institutes from Germany, Japan, Switzerland and Spain. Its major objectives are to measure the surface topography and the tidal deformation of the satellite.JUICE (Jupiter Icy Moons Explorer) will be the first orbiter around a moon (other than Earth&#8217;s moon) in solar system exploration. Its launch is planned for May/June 2022 followed by an interplanetary cruise of 7.6 years. Jupiter orbit insertion will take place by the end of 2029. An orbit maneuver will bring the spacecraft into a 500-km circular orbit in which it will be staying for at least 132 days until end of nominal mission. The latter phase will be the main period for GALA taking data. In addition data will be taken at Europa, Ganymede, and Callisto at closest approaches of flybys in the Jupiter orbiting phase.GALA has two main objectives: (1) by range measurements it shall obtain Ganymede&#8217;s topography on global, regional and local scales. This will reveal how surface features have formed and how they are connected with the shallow interior ice shell. Global shape measurements will tell us whether the satellite is in hydrostatic state with respect to rotational and tidal forces. (2) Obtaining range measurements distributed in time along the orbital cycle, tidal variations of surface elevations will be measured. The tidal amplitudes are indicative for the presence of a subsurface ocean and would (together with complementary measurements) constrain the ice-I shell thickness [2].GALA is a single-beam altimeter: a laser pulse (at 1064 nm wavelength) is emitted by using an actively Q-switched Nd:YAG laser firing at 30 Hz in nominal operation. A small fraction of the pulse is guided through fiber optics onto the detector characterizing the outgoing pulse and time of emission. After about 3 msec the Lambertian reflection of the pulse from the surface is received by a aperture telescope and transferred to the detector, an Avalanche Photo Diode. The signal is digitized at a sampling rate of 200 MHz and transferred to the range finder module, which determines (a) the time of flight between the emission and receiving of the pulse (b) the pulse shape, in particular the pulse-width, and (c)&#160; the energy of the received pulse. From the time of flight of the wave-package and the spacecraft position and attitude, the distance for each shot can be converted into height above a reference surface in post-processing of the data.&#160;&#160;(1)&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; (2)(3)&#160;Figure 1: (1) GALA Tranceiver, (2) Electronics Unit (ELU), (3) Laser Electronics Unit (LEU)&#160;In 2019 the GALA EM (Engineering Model) was delivered to Airbus Defence and Space Toulouse and was successfully integrated into the JUICE EM and tested. The EM is used to test the functionality of GALA; it can be operated only under ambient conditions. Later it will be used as a ground reference model for checks for the Flight Model.For the GALA &#160;delivery in 2021 the different GALA flight model subsystems are currently &#160;tested and integrated at &#160;DLR, and at industries in Japan (Meisei) and Germany (Hensoldt Optronics). Flight hardware of the Power converter Module (PCM) and the Range Finder Module have been received from our partner institutes in Spain (IAA, CSIC) and&#160; Switzerland (UBE). The main units of GALA are the TRU, the LEU and the ELU, see Figure 1. The TRU consists of the laser/detector system, including an Avalanche Photo Diode. ELU includes the main CPU, Data Processing Module, Range Finder Module for the analysis of the laser pulses and the Power Converter Module which provides the power. A detailed description of GALA can be found in [3]. All parts were tested in terms of functionality and performance, here in particular the detector. &#160;Besides electrical testing a big part is the testing of the on-board software, the Application Software. Mainly it manages the commanding of the above mentioned parts, the memory management, FDIR (Fault Detection Isolation Recovery) and the interface (I/F) to the spacecraft (S/C). For I/F testing to the S/C a SIS (Spacecraft Interface Simulator) is used to check the GALA commanding and the data output.After testing at DLR the above mentioned GALA parts will be assembled to the GALA PFM at Hensoldt Optronics Oberkochen. Here additional tests will take place like thermal vacuum tests, EMC and further functional testing, especially for the laser system, but also for the complete system before it will be delivered for final integration into the JUICE spacecraft in 2021.&#160;[1] Grasset, O. and 17 colleagues (2013), JUpiter ICy moons Explorer (JUICE): an ESA mission to orbit Ganymede and to characterise the Jupiter system, Planet. Space Sci., 78, 1-21.[2] Steinbr&#252;gge, G., A. Stark, H. Hussmann, F. Sohl, and J. Oberst (2015), Measuring tidal deformations by laser altimetry. A performance model for the Ganymede Laser Altimeter, Planet. Space Sci., 117, 184-191.[3] Hussmann, H. and 38 colleagues 2019. The Ganymede Laser Altimeter (GALA): key objectives, instrument design, and performance. CEAS Space Journal (Special Issue on Space Lidar and Space Optics) 11, 381 &#8211; 390.