GeoMAJIS, providing the observational context for the JUICE/MAJIS spectrometer

I. IntroductionThe JUpiter ICy Moons Explorer (JUICE) is a mission funded and led by theEuropean Space Agency (ESA) in the context of its Cosmic Vision program. The Japanese space agency (JAXA) and the United States' NASA contributed tothis mission. The spacecraft carries 11 instruments designed to observe andmonitor the Jupiter system, with a focus on the Galilean moons Callisto, Europe and Ganymede, in the intent of furthering our present knowledge ofthe Solar System and our understanding of planetary formations, two keythemes of the Cosmic Vision programi 1. We will present the program (GeoMAJIS) we designed, developed and deployedto help with the interpretation of JUICE's visible and infrared spectrometer (MAJIS) observations. II. The MAJIS spectrometerMAJIS is a push-broom imaging spectrometer designed to observe the atmosphereof Jupiter and characterize the surface of its moons from the visible to thethermal infrared domains (. 4 - 5. 6 m) 1. This instrument has two channelssharing the same field-of-view (FOV), each equipped with a Teledyne H1RG1024x1024 detector, with one optimised for the 0. 4-2. 35 m domain (VISNIR) and the other for the 2. 25-5. 6 m domain (IR). The detectors are composedof squared pixels with an instantaneous FOV of 75 rad. In the baseline mode, a x2binning is applied on each detector to increase the signal, such that the FOVis provided by a 1x400 binned pixels segment, i. e. a 8. 5x10-3*3. 43 window. When orbiting Jupiter from a distance of 10⁶ km (or Ganymede at 500 km), MAJIS' spatial resolution will be of 150 km/pxl (respectively 75 m/pxl). The instrument also possesses a mirror which allows to shift its FOV inthe along-track axis by up to 2 in either direction. As MAJIS will perform in a complex observational context, in whichoccultations are common, it is necessary to complement the calibratedobservations with ancillary data to ease their interpretation. III. The GeoMAJIS codeThis program relies on the CSPICE library 3 toperform the necessary computations to detail the observational context, based on the spacecraft' predicted (or recomputed) trajectory andattitude, as well as celestial bodies' ephemerides. GeoMAJIS details for instance which objects are present in MAJIS' FOV, their state (i. e position and velocity) with respect to different inertialframes, the Sun's and its own position relative to them, and theinstrument' spatial resolution at their surface. In addition, the program also performs ray-tracing computation specificto each observation so that for each pixel, it might be known whichsurface element of which object was observed, if it was fully illuminatedand visible, or shadowed or occulted from the spacecraft's perspective. Such computations are performed either with a simple ellipsoidalrepresentation of the observed bodies, or, when available, also witha detailled 3D model of the target. All the results of the RTX computations are then written-out as amultiple-frame FITS file, while the other results are written down intothe geometry dictionary of the PDS4 descriptor of the said FITS file, complementing the corresponding scientific observation. IV. The August 2024 Earth flybyIn complement to theoretical simulations, the GeoMAJIS program will betested and perfected in the shadow of the 20th of August 2024 Earthflyby. While presently available ephemerids allow to simulate thisflyby, as illustrated by Fig. 1. Fig. 1: 2D plot of the Local Time at the surface of the Earth, according the predicted trajectory of the JUICE spacecraft at the present time. The plot depicts the simulation results of 196 acquisitions with the VISNIR channel (one every 15 minutes, vertical axis) taken with 400 pixels (horizontal axis). As one can understand from the results of Fig. 1, during this flyby, the spacecraft ingresses the Earth system in the shadow of the Earth and makes its egress while imaging most of the day-side and part of the evening-side terminator. The local time was evaluated and corrected by the sun's elevation with respects to the local normals of the surface. Given the suite of instruments aboard the spacecraft, the reconstructed trajectory of the spacecraft will be computed with at best a sub-meter accuracy. Such accuracy will allow to compare and find-out any differences between simulations and observations that might be due either to a theoreticalor a factual origin. These efforts will constitute a new evaluatory review of the program and its algorithm and any correction will be included in the eventual updated version of the calibration pipeline. V. ConclusionsWe designed, developped and tested a segment of MAJIS' calibration pipeline. This segment aims at providing the most complete description of theobservational context of the varied MAJIS acquisitions. We shall present the algorithm of this program, as well as the results of present simulations of observations in the Jupiter system as well as the preliminaryresults from the review of the August 2024 Earth flyby. References: 1. Grasset et al. , PSS, http: //dx. doi. org/10. 1016/j. pss. 2012. 12. 002 2: Acton et al. , Planet. Space Sci. . Vol. 44, No. 1, pp. 65-70. 1996