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

I. Introduction
The JUpiter ICy Moons Explorer (JUICE) is a mission funded and led by the
European Space Agency (ESA) in the context of its Cosmic Vision program.
The Japanese space agency (JAXA) and the United States' NASA contributed to
this mission. The spacecraft carries 11 instruments designed to observe and
monitor the Jupiter system, with a focus on the Galilean moons Callisto,
Europe and Ganymede, in the intent of furthering our present knowledge of
the Solar System and our understanding of planetary formations, two key
themes of the Cosmic Vision programi [1].
We will present the program (GeoMAJIS) we designed, developed and deployed
to help with the interpretation of JUICE's visible and infrared spectrometer
(MAJIS) observations.

II. The MAJIS spectrometer
MAJIS is a push-broom imaging spectrometer designed to observe the atmosphere
of Jupiter and characterize the surface of its moons from the visible to the
thermal infrared domains (.4 - 5.6 µm) [1]. This instrument has two channels
sharing the same field-of-view (FOV), each equipped with a Teledyne H1RG
1024x1024 detector, with one optimised for the 0.4-2.35 µm domain (VISNIR)
and the other for the 2.25-5.6 µm domain (IR). The detectors are composed
of squared pixels with an instantaneous FOV of 75 µrad.  In the baseline mode, a x2
binning is applied on each detector to increase the signal, such that the FOV
is provided by a 1x400 binned pixels segment, i.e. a 8.5°x10-3*3.43° window.
When orbiting Jupiter from a distance of 10^6 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 in
the along-track axis by up to 2° in either direction.
As MAJIS will perform in a complex observational context, in which
occultations are common, it is necessary to complement the calibrated
observations with ancillary data to ease their interpretation.

III. The GeoMAJIS code
This program relies on the CSPICE library [3] to
perform the necessary computations to detail the observational context,
based on the spacecraft' predicted (or recomputed) trajectory and
attitude, 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 inertial
frames, the Sun's and its own position relative to them, and the
instrument' spatial resolution at their surface.
In addition, the program also performs ray-tracing computation specific
to each observation so that for each pixel, it might be known which
surface element of which object was observed, if it was fully illuminated
and visible, or shadowed or occulted from the spacecraft's perspective.
Such computations are performed either with a simple ellipsoidal
representation of the observed bodies, or, when available, also with
a detailled 3D model of the target.
All the results of the RTX computations are then written-out as a
multiple-frame FITS file, while the other results are written down into
the geometry dictionary of the PDS4 descriptor of the said FITS file,
complementing the corresponding scientific observation.

 

IV. The August 2024 Earth flyby
In complement to theoretical simulations, the GeoMAJIS program will be
tested and perfected in the shadow of the 20th of August 2024 Earth
flyby. While presently available ephemerids allow to simulate this
flyby, 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 theoretical 
or 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. Conclusions
We designed, developped and tested a segment of MAJIS' calibration pipeline. This segment aims at providing the most complete description of the 
observational 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 preliminary 
results 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