The 3GM Radio Science Experiment on board the ESA JUICE Mission

The JUpiter ICy moons Explorer (JUICE) is a mission of the European Space Agency devoted to the exploration of the Jupiter system. The main mission themes are the study of Jupiter as an archetype for gas giants and exoplanetary systems, and the characterisation of the potentially habitable worlds Europa, Ganymede and Callisto. During the Jovian tour, JUICE will encounter multiple times the moons Europa and Callisto, while its final target, Ganymede, will be thoroughly investigated in the last phase of the mission, when JUICE will enter orbiting around it. The spacecraft payload consists of a suite of ten instruments, comprising a remote sensing, a geophysical, and a plasma and waves in situ package, plus a radio science instrument, named 3GM (Gravity and Geophysics of Jupiter and the Galilean Moons). An additional experiment, having no dedicated onboard hardware, will be carried out using ground-based VLBI measurements. The 3GM instrument is made up of two separate and independent units incorporated in the spacecraft TT&C subsystem: a Ka band Transponder (KaT) and an Ultra Stable Oscillator (USO). The KaT will enable two-way range and range-rate measurements at Ka band (34.5-32.2 GHz), accurate respectively to 20 cm and 3 μm/s (at 1000 s integration time). The gravity experiment relies on the Ka band radio link to help unveiling the interior structure of Ganymede, inferring the extent of the subsurface global ocean through measurements of the moon's gravity field and tidal response. For Europa and Callisto, only the low degree gravity coefficients can be measured during flybys. 3GM will refine the moment of inertia of the two moons and will determine departures from the hydrostatic equilibrium. The USO will be used for one-way downlink occultation experiments. It will generate on board a highly stable 4.79 MHz reference signal with an Allan deviation of few parts of 10-13 at 1-1000 s integration time. Then the signal will be routed to the DST after an internal x12 multiplication stage, and downlinked at X- and Ka bands to sound the Jupiter's neutral atmosphere and the moons' ionosphere. The 3GM experiment will be also supported by the spacecraft High Accuracy Accelerometer (HAA) needed to calibrate the spacecraft internal dynamical disturbances, in particular due to the propellant sloshing.