| 11 Aug 2026
Brief Communication: Inclined Geosynchronous Orbits for Earth Observation Satellites
Abstract. The use of geostationary orbits (GEO) for Earth observation has led to advances in weather prediction and more recently, new spectroscopic observations of air quality through a virtual constellation of GEO satellites predominantly observing over the northern hemisphere. In this brief communication, we introduce the concept of Inclined Geosynchronous Orbits (IGOs) for Earth observation and contrast them to GEO and other orbits for expanding the coverage from the atmospheric composition virtual constellation. Many key advantages of IGOs presented here for observing atmospheric composition extend to other types of Earth observation (e.g. surface imaging) in which the area of interest reaches to the high latitudes of both hemispheres presenting challenges for GEO.
The manuscript is well written; the topic of the brief communication fits well into the scope of the EGU sphere. I recommend publication after addressing the issues identified below.
General
1. Title and the abstract are very general; they do not specify the targeted type of Earth observation or target application area. Later in the manuscript the focus is on “rapid-revisit UV-vis spectroscopic observations for air quality”. Please consider making the title more specific and introducing in the abstract the focus … or clarifying that the IGO advantage is illustrated exemplarily for “rapid-revisit UV-vis spectroscopic observations for air quality”.
2. Atmospheric composition observation using Earthshine is limited not only by the viewing zenith angle but also by the solar zenith angle at the target. The benefit of IGO is discussed with a strong focus on the former. I recommend discussing both in a balanced way. Please consider adding a plot showing the latitudinal dependence of the fraction of the year in which targets are illuminated with a solar zenith angle exceeding a threshold (e.g. 60 degrees).
Minor Issues
The term “continuous coverage” is inaccurate. Atmospheric composition observations are typically acquired by scanning the field of regard, which leads to non-continuous coverage. Please consider a reformulation.
22: please add: For example, the Copernicus Sentinel-5 Precursor satellite “with the” Tropospheric Monitoring Instrument (TROPOMI) “on board” …
25: either “revisits are less frequent” or “revisit rates are lower” (rates can be high or low but not frequent).
25: I propose reordering and splitting the sentence: “Orbits with an altitude of 35,786 km are synchronized with Earth’s rotation. Geostationary orbit (GEO) satellites remain essentially stationary over a specific longitude on the equator have a continuous view on a limited region of the Earth.”
36: “High revisit rates” or “Frequent revisit …”
36: Rephrase: “…but observing from Earth’s equatorial plane implies that targets at high latitudes are seen with very large viewing angles,…”
40: The term “airmasses” is not introduced, its relevance is not explained; The statement that “airmasses are … susceptible to biases” is inaccurate. Please consider removing reference to airmasses altogether or explain.
40: Proposed change: “retrievals are unacceptable” -> “retrievals are typically considered as unacceptable”
41: Resolution “decreases” -> “degrades”
44: “varying” -> “various”
50: “although …” needs rewriting.
53: “(AOD) .. to derive fine particulate matter (PM2.5)” -> “(AOD) … to infer information on fine particulate matter (PM2.5)” (the link between AOD and PM is complex.)
54: I suggest adding a rationale why NO2 is important: it is a key air pollutant by itself, it controls the production of other air pollutants including ozone and secondary aerosol.
106: “a” role, “a” pair of
108: “edge of the acceptable observing range” -> “edge of the area with acceptable viewing angles”
112: for clarity: This would be more efficient that using “a combination of” two GEO satellites for the low latitudes and multiple HEO satellites
Figure 1: Purely cosmetic comment: The orbit inclination in the left panel seems less than 40 degrees. I suggest making it 40 degrees to match the situation depicted on the right and supporting the statement made in the caption (“highest latitude equal to the orbital inclination”).
Figure 2: The perspective chosen for the globes in the upper and lower rows seem suboptimal. Areas that benefit from IGO appear at the edge of the visible disc some are even behind the horizon; A view from the northernmost (upper row) and southernmost point of the analemma would provide a better view of the areas that benefit from IGO.