the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Mar 2026
Juice/SWI during the Lunar-Earth-Gravity-Assist. III. Observations of the Earth as Calibration Target
Abstract. On August 19th and 20th 2024 the Jupiter Icy Moons Explorer (Juice) executed during its cruise phase towards Jupiter a combined Lunar Earth Gravity Assist (LEGA) maneuver. These close flybys of the Moon and the Earth provided so far the best opportunity to test the behavior, performance, and calibration of the Submillimetre Wave Instrument (SWI) onboard Juice. This paper shows typical data taken during the Earth Gravity Assist and the following few days. Data quality and problems resulting from unexpected behavior of the hardware are discussed.
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Status: open (until 24 Apr 2026)
- RC1: 'Comment on egusphere-2026-1096', Michael Küppers, 29 Mar 2026 reply
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RC2: 'Comment on egusphere-2026-1096', Vincent Kofman, 15 Apr 2026
reply
Dear authors,
The near-Earth observations clearly have provided great test grounds for the submillimetre wave instrument (SWI). The paper is clearly written and provides interesting examples of the observations taken using the Juice spacecraft. The results are clearly presented and the radiative transfer model reproduces the observations well. Juice/SWI will no doubt be a great asset for studying the Jovian system in the future. This manuscript describes the calibration issues and strategies with the (unexpected) limited life time of the calibration flip mirror, and demonstrates the performance of the methods. I would recommend a few corrections to be made to the manuscript listed below, but recommend this work to be published.
Minor comments:
6 remove word 'essentially'20 Perhaps elaborate on the 'telescope efficiency numbers'.
40 consider adding a statement like: at the scales and level of details considered (mention beam size on disk?), the databases used (MERRA2) are expected to accurately reproduce the Earth.
50 although for this part of the spectrum GHz is the units typically used, perhaps add the wavenumbers and/or microns too. This helps readers less familiar with this part of the spectrum to orient.
49 & 54. In 49 it is mentioned that the receivers can be operated simultaneously, in 53-54 you mention only one type can be operated at one time. I assume what is meant here is that both receivers can be operated without changing the instrument configuration, but not at the same time. Please clarify.
105 were these in-flight observations?
140. Can you elaborate on the choice of the latitudinal averages? How does the beam size compare to the the 10 deg. averages? This seems to be because you probe above 6 KM, where spatial and temporal variations are much smaller.
Eq 8. Watch type setting in this equation. Currently it looks like there are vertical offsets between different elements of the summation.
150. Are there any more applications of this radiative transfer model? Perhaps it could be of interest to refer to these documents.
165. Since 2008, several updates of HITRAN have been released. I recognize that code development and implementation of new databases are tedious, but this could be relevant if more of the observations will be analyzed in more details.
Fig 4. Does the S represent the South Pole? I see that this is the case as described in the caption of figure 5. Please ensure that the caption of Figure 4 reflects this as well.
Fig. 9 Clarify what the different traces are in the figures.
Fig. 10 Please indicate the units on the vertical axis, even though are are normalized
334. The statement here indicates the methods used here are advantageous compared to another approach, presumably plotting simulated spectra separated by wavelength versus the double sideband together. Without an actual example however, it is not clear how 'this spectral scan demonstrates clearly'. Perhaps rephrase to something along the lines of: it is apparent from the figures that the simulated spectra, with their double sideband signals combined, reproduce the signals from the observations well.
340. It is tough to follow the different transitions over the frequency range between the different panels. The specific ozone transition could be highlighted in the figures to guide the readers eye.
Citation: https://doi.org/10.5194/egusphere-2026-1096-RC2
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- 1
The manuscript is part of a series of papers describing the observations of the SWI instrument onboard JUICE during the Moon-Earth gravity assist. The presentation is clear and accessible for a non-specialist.
It is important to document the in-flight calibration activities and results, in particular for a mission with a long cruise phase like JUICE. Therefore the manuscript deservers publishing.
Overall the paper is well structured and complete. I just have some minor suggestions for improvements the authors may want to consider:
Introduction: There seem to be 4 papers describing different aspects of the SWI observations around the swingby. It maybe useful to shortly describe which aspects are covered in which paper.
Instrument description:
“There are a few things about SWI the reader needs to know to better understand the data presented in this paper”: The phrase sounds a bit strange for a scientific publication. Maybe say something like: “In this section we shortly summarize the instrument and its measurement principle.”
Section 3.1: Why are the atmospheric data resampled to lower spatial resolution given that, according to the introduction, one of the sources for differences between data and model are small-scale variations in the atmosphere?
Section 4.1: “Another important thing simplifying the calculation of the expected Earth spectrum is given by the fact that”. Maybe simpler ”Further simplification of the calculations is possible because…”.
Section 4.1.1: “which could be used in this way for the intended analysis” -> “which could be analyzed this way”
Section 4.1.2: “To obtain the wanted nadir viewing spectra”. Suggest to remove “the wanted”
Section 4.1.4: In addition to the values above 1170 GHz in the high frequency receiver, there seems also be a deviation much larger than 4% below 540 GHz in the low frequency receiver. Can you comment on that?
Section 4.3.2: Any idea of the reason for the discrepancy at 563.5 GHz? Also, several of the spectra in the low frequency band show discrepancies towards low IF frequencies. In addition, some tunings in the high frequency band show discrepancies (e.g.1218.9 and 1221). Those other discrepancies should be mentioned/discussed as well.
Conclusions: I would remove “Obviously”from the start of conclusion 3.
Maybe the conclusions should also summarize where observations differ from expected data, the possible reasons, and the foreseen way forward.