the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
- Preprint
(3274 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 22 Sep 2026)
- RC1: 'Comment on egusphere-2026-3463', Anonymous Referee #1, 11 Aug 2026 reply
-
RC2: 'Comment on egusphere-2026-3463', Anonymous Referee #2, 20 Aug 2026
reply
The authors present a Brief Communication discussing some potential benefits of inclined geosynchronous orbits (IGOs) for Earth observation. They motivate the discussion in particular in terms of atmospheric composition and on the need for observational coverage at high latitudes that cannot be attained from geostationary (GEO) orbit.
The perspective is a useful one to share with the field; many in the atmospheric sciences are likely unfamiliar with IGOs and the type of sampling they can enable. The topic is suitable for this journal and the narrative is mostly clear and logically presented. I recommend publication after some minor revisions. I have one substantive comment on a topic I feel should be addressed in the article. My other comments point out some minor ways the paper can be improved and identify some technical corrections.
General comment.
The paper invokes high-latitude coverage as a primary motivator for IGO observations, as those areas are poorly captured by GEO. They further state “two identical satellites in an IGO inclined at 40° at a similar equatorial longitude to TEMPO and offset by 12 hours could give observations with good VZAs covering almost all of North, Central and South America. This would be more efficient that using two GEO satellites for the low latitudes and multiple HEO satellites spanning the circumpolar high latitudes. The same increased VZA coverage range would occur for a pair of IGOs observing Europe and Africa from a similar equatorial longitude to Sentinel-4 or a pair of satellites observing East Asia and Oceania from a similar equatorial longitude to GEMS as shown in Figure 2. In general, two IGO satellites can give essentially the same revisit rate as two GEO satellites (to cover the same sized area) but can provide better mid- and high latitude viewing angles over any north-south continental pairing (North-South America, Europe-Africa, East Asia-Oceania).” A perspective that I think is missing is whether LEO could accomplish the same objectives as well or better. The scenario outlined above involves a total of 6 instruments in IGO across the three regions discussed. Am I correct in presuming that this would provide more or less hourly coverage for all of the domains shown in Figure 2? One could imagine an alternative strategy involving multiple LEO sensors. For example, a constellation of 12 TROPOMI-like sensors in LEO would provide hourly (daytime) data everywhere on Earth. Less than 12 would be needed to measure the high latitudes at this frequency since the LEO orbits converge towards the poles. If one also considers the fact that putting an instrument into LEO (~800km) is a lot less expensive than into GEO (~36,000 km), and the measurement advantages that come with being about 5x closer to the target, one wonders whether the same science goals could be achieved more readily with a comprehensive LEO strategy. I am open to persuasion either way, and possibly there are critical factors that I am not considering (for example, perhaps there are hosting limitations or the debris issues the authors allude to). But in any event I feel this topic should be addressed explicitly in the paper.
Minor suggestions.
Introduction: there are a lot of acronyms being introduced that force the reader to keep track of them all. To improve readability I suggest limiting this to terms that are used frequently throughout the paper. For example, EO and CEOS both appear just one more time in the paper (both at the very end), and AC-VC never reappears. IGSO is used twice but not defined.
Line 39-41, “The maximum tolerable view zenith angle (VZA) depends on the specific observable, with a typical VZA limit of ~60° for retrievals where large airmasses are more susceptible to biases, such that retrievals are unacceptable poleward of ~58° latitude even at the satellite longitude.”. Please provide a citation or otherwise clarify how you are making this specific determination. Figure 2, again, since you are using 60 degrees as a VZA threshold one wants to know how you selected this specific value. And what about SZA?
Line 41, “Horizontal resolution also decreases for large VZAs”. It would help make your case if you can give us a back-of-the-envelope quantification of this effect … such as “(e.g., by a factor of ~X at a VZA of Y)”
Line 96-97, “Preliminary analysis…” Consider fleshing this out with a few words to provide some sense of context. For example, fuel requirement approximately X times that of a standard GEO configuration?
Lines 43-54, I suspect many readers will be unfamiliar with HEO. I understand there are many possible HEO configurations, but is it possible to expand Figure 1 to show a relevant HEO example? Or perhaps a separate panel that shows both LEO and HEO?
Line 20, Recommend stating a range here rather than “a few hundred kilometers”. In fact many LEO orbits are in the 700-800 range which is more than most people will associate with “a few hundred”.
Line 73-74, “An additional simulation is also conducted for i=25° and an equatorial longitude of 25°E.” State here the purpose of this simulation?
Technical issues.
Abstract, “for expanding the coverage *of* the atmospheric composition virtual constellation”?
Abstract, “presenting” --> “and thus presents”?
Line 30, “The *current* constellation”?
Line 78, delete comma after “is”
Line 89, capitalization “A”
Line 94, “It is not possible…” Awkward sentence, hard to parse
Line 97, “and *is* thus”
Figure 2, please add row and column titles to the figure to make it more readily understandable.
Figure 2 caption, “are equal to that for the corresponding IGO equator crossing”
Line 106, “since pair of identical satellites formation”, grammar
Line 111, “could give *daytime* observations”?
Line 112, “that” --> “than”
Citation: https://doi.org/10.5194/egusphere-2026-3463-RC2
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 61 | 16 | 9 | 86 | 9 | 7 |
- HTML: 61
- PDF: 16
- XML: 9
- Total: 86
- BibTeX: 9
- EndNote: 7
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
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.