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https://doi.org/10.5194/egusphere-2026-2064
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2026-2064
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
22 Apr 2026
| 22 Apr 2026
Juice/SWI during the Lunar-Earth-Gravity-Assist (LEGA). IV. Antenna pointing and beam characterisation
Abstract. The Submillimetre Wave Instrument (SWI) onboard the Jupiter Icy Moons Explorer (Juice) spacecraft consists of a 29 cm off-axis paraboloid antenna and two double-sideband heterodyne receivers operating in the frequency ranges of 530–638 and 1066–1286 GHz. This instrument is designed to study Jupiter’s atmosphere and the exospheres of the Galilean moons.
We used the Lunar-Earth-Gravity-Assist (LEGA) data from August 2024 to characterise this antenna using several SWI observations pointing towards the Earth. The data analysis of the LEGA observations dedicated to the antenna beam allows us to characterise the antenna beam Full Width at Half Maximum (FWHM), sidelobes, absolute pointing and receiver coalignement. Moreover, an estimate of the calibration efficiency factor has been obtained.
How to cite. Moreno, R., Rezac, L., Formánek, T., Cavalié, T., Jarchow, C., Lellouch, E., Hartogh, P., Murk, A., Kotiranta, M., Carrasco Gallardo, A., Goodyear, S., Schulz-Ravanbakhsh, A., Dabrowski, B., Herpin, F., Kasai, Y., Murtagh, D., Olberg, M., Rengel, M., Sagawa, H., Szutowicz, S., and Wirström, E.: Juice/SWI during the Lunar-Earth-Gravity-Assist (LEGA). IV. Antenna pointing and beam characterisation, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-2064, 2026.
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LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
Ladislav Rezac
Max-Planck-Institute for Solar System Research, Göttingen, Germany
Tomáš Formánek
LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
Thibault Cavalié
Univ. Bordeaux, CNRS, LAB, UMR 5804, F-33600 Pessac, France
Christopher Jarchow
Max-Planck-Institute for Solar System Research, Göttingen, Germany
Emmanuel Lellouch
LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
Paul Hartogh
Max-Planck-Institute for Solar System Research, Göttingen, Germany
Axel Murk
Institute of Applied Physics, University of Bern, Switzerland
Mikko Kotiranta
Institute of Applied Physics, University of Bern, Switzerland
Alberto Carrasco Gallardo
Max-Planck-Institute for Solar System Research, Göttingen, Germany
Samuel Goodyear
Max-Planck-Institute for Solar System Research, Göttingen, Germany
Ali Schulz-Ravanbakhsh
Max-Planck-Institute for Solar System Research, Göttingen, Germany
Borys Dabrowski
Max-Planck-Institute for Solar System Research, Göttingen, Germany
Fabrice Herpin
Univ. Bordeaux, CNRS, LAB, UMR 5804, F-33600 Pessac, France
Yasuko Kasai
National Institute of Information and Communications Technology, Tokyo, Japan
Institute of Science Tokyo, Tokyo, Japan
Donal Murtagh
Department of Physics and Astronomy, Chalmers University of Technology, Sweden
Michael Olberg
Department of Physics and Astronomy, Chalmers University of Technology, Sweden
Miriam Rengel
Max-Planck-Institute for Solar System Research, Göttingen, Germany
Hideo Sagawa
Faculty of Science, Kyoto Sangyo University, Kyoto 603-8555, Japan
Slawomira Szutowicz
Centrum Badań Kosmicznych Polskiej Akademii Nauk, Bartycka 18A, Warsaw, Poland
Eva Wirström
Department of Physics and Astronomy, Chalmers University of Technology, Sweden
Short summary
The Submillimetre Wave Instrument (SWI) onboard the Jupiter Icy Moons Explorer (Juice) spacecraft consists of a 29 cm antenna and two radio receivers operating at submillimeter wavelength. This instrument is designed to study Jupiter's atmosphere and the thin atmospheres of its moons.
We used Earth observations obtained during the Lunar-Earth-Gravity-Assist (LEGA) . This data analysis allows us to characterise the antenna beam and pointing corrections.
The Submillimetre Wave Instrument (SWI) onboard the Jupiter Icy Moons Explorer (Juice)...
General Comments:
The paper describes the observed properties of the beams of the two
frequency bands of SWI onboard Juice. For this a point source and an
extended source (earth) were repeatedly observed to construct 1D cuts
and 2D maps. These allowed to derive half-power beamwidths,
ellipticity, alignment, pointing and errors. Beam properties were
compared with simulations of the far-field beam pattern. The paper
is well written, to the point, concise, and represents an important
contribution to the success of this mission. I still have a couple of
largely minor points:
Table 1 gives a mean of CTS1 FWHM_AT of 8.32+-0.07. How is this
error of 0.07 derived ? What is it ? The standard deviation of the
measured FWHMs is much larger, 0.78 ! This comment concerns all
errors or 1sigma values given in the paper.
Specific Comments:
+ Abstract: say that the calibration efficiency factor was obtained
from simulations only.
+ Sec.2: CTS has 1GHz or 4.4GHz bandwidths ? The text is confusing.
+ Sec.3.1: When discussing observations of a point source,
which source was actually observed and what was its angular diameter ?
+ "...scans that cross the limb of the target" for targets
that are much larger than the main beam.
+ Sec.4: Observations are done in DSB mode, i.e. both sidebands are observed
simultaneously. When comparing the observations with the GRASP
simulations, how were the two sidebands taken into account ?
+ Sec.3.1.1: "samling >=2arcmin" Better give range of sampling,
i.e. 2' to 6'.
+ "two pairs of observations" Scanning Back and forth ?
+ Figure 1: The CT scans show the temperatures to vary over the
limb of earth. For 1107.45GHz, there even is a discontinuity.
I suggest to add a sentence, describing in how are such effects
were taken into account and how they may effect the overall
accuracy of the beam properties.
+ Figure 6: The extended main beam seems to be not exactly
symmetrical, showing a broadening or "shoulder" at negative
offsets, which is especially prominent at CT 1119GHz.
This seems worth mentioning. In the future, 2D maps may help to
study this feature in more detail.
+ "mb=3xFWHM" seems arbitry. Maybe say that this corresponds
roughly to the main beam down to the 1st null of the simulated
diffraction pattern.
Technical Corrections:
+ slated -> scheduled/foreseen to arrive...
+ Sec.2: Say that each band is observed in single polarisation
+ Sec.3.1.2 "arrange the measured data by" -> plot as function of...
+ "fit them with a model" refer to section in which the model is presented.
+ Figure 4: Explain the white circle in the centers.