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The Ganymede laser altimeter (GALA): key objectives, instrument design, and performance

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Abstract

The Ganymede Laser Altimeter (GALA) is one of the ten scientific instruments selected for the Jupiter Icy Moons Explorer (JUICE) mission currently implemented under responsibility of the European Space Agency (ESA). JUICE is scheduled for launch in mid 2022; arrival at Jupiter will be by end of 2029 with the nominal science mission—including close flybys at Ganymede, Europa, and Callisto and a Ganymede orbit phase—ending by mid 2033. GALA’s main objective is to obtain topographic data of the icy satellites of Jupiter: Europa, Ganymede, and Callisto. By measuring the diurnal tidal deformation of Ganymede, which crucially depends on the decoupling of the surface ice layer from the deep interior by a liquid water ocean, GALA will obtain evidence for (or against) a subsurface ocean in a 500 km orbit around the satellite and will provide constraints on Ganymede’s ice shell thickness. In combination with other instruments, it will characterize the morphology of surface units on Ganymede, Europa, and Callisto providing not only topography but also surface roughness and albedo (at 1064 nm) measurements. GALA is a single-beam laser altimeter operating with up to 50 Hz (nominal 30 Hz) shot frequency at a wavelength of 1064 nm and pulse lengths of \(5.5\pm 2.5\) ns using a Nd:YAG laser. The return pulse is detected by an Avalanche Photo Diode (APD) with 100 MHz bandwidth and is digitized at a sampling rate of 200 MHz providing range measurements with a subsample resolution of 0.1 m and surface roughness measurements from pulse-shape analysis on the scale of the footprint size of about 50 m at 500 km altitude. The instrument is developed in collaboration of institutes and industry from Germany, Japan, Switzerland, and Spain.

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Abbreviations

ADC:

Analog-to-digital converter

AEM:

Analog electronics module

APD:

Avalanche photo diode

BELA:

BepiColombo laser altimeter

BEO:

Back-end optics

CDR:

Critical design review

DC:

Direct current

DPM:

Digital processing module

ELU:

Electronics unit

EM:

Engineering model

EMC:

Electro-magnetic compatibility

EMI:

Electro-magnetic interference

EQM:

Engineering qualification model

ESA:

European space agency

FPA:

Focal-plane assembly

FPGA:

Field programmable gate array

GALA:

Ganymede laser altimeter

JUICE:

Jupiter icy moons explorer

LEU:

Laser electronics unit

LHM:

Laser head module

MLI:

Multilayer insulation

M1:

Main mirror

M2:

Secondary mirror

MOS:

Metal-oxide semiconductor

Nd:YAG:

neodymium-doped yttrium aluminum garnet, \(\hbox {Nd:Y}_3\hbox {Al}_5\hbox {O}_{12}\)

PCM:

Power converter module

PFM:

Proto flight model

RFM:

Range finder module

RX:

Receiver

RXM:

Receiver module

STM:

Structural and thermal model

TEL:

Receiver telescope

TID:

Total ionizing dose

TRU:

Transceiver unit

TX:

Transmitter

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Acknowledgements

The authors would like to thank two anonymous reviewers for their helpful comments on a first version of the manuscript. Financial support was provided under grant 50 QJ 1401 on behalf of the DLR Space Administration by the German Bundesministerium für Wirtschaft und Energie. This research has been supported by the Spanish Ministerio de Economía y Competitividad under Contract ESP 2016–76076–R.

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Hussmann, H., Lingenauber, K., Kallenbach, R. et al. The Ganymede laser altimeter (GALA): key objectives, instrument design, and performance. CEAS Space J 11, 381–390 (2019). https://doi.org/10.1007/s12567-019-00282-8

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  • DOI: https://doi.org/10.1007/s12567-019-00282-8

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