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.
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RC1: 'Comment on egusphere-2026-3463', Anonymous Referee #1, 11 Aug 2026
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AC1: 'Reply on RC1', Ray Nassar, 23 Oct 2026
Note: Author responses tor reviewer comments are the "Bold" bullets below.
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
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”.
- Our intention was to introduce the concept of IGO for Earth Observation and use UV-vis spectroscopic observations of air quality as a relevant example, but the IGO concept is applicable to other areas of EO. Our preference is to maintain a general title and focus on one example of current relevance in the text.
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).
- We fully agree with the reviewer that UV-vis atmospheric composition observations require reflected sunlight or “Earthshine”. Our intention was not to imply otherwise, but for IGO, the available sunlit fraction of the Earth is the same as for GEO, as it is based on the Sun and Earth positions, rather than the satellite orbit. Since this should be covered elsewhere by GEO papers, our preference is not to add another figure. A brief communication in this journal has a limit of 3 figures, so adding a new figure would require removing or combining with an existing figure to avoid disqualifying our submission from this manuscript format.
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.
- Continuous coverage is possible from GEO if the field-of-view (FOV) matches the field-of-regard (FOR). If the FOV is smaller than the FOR, then scanning the FOV across the FOR is necessary and viewing is non-continuous, as the reviewer correctly identifies is the case for all existing air quality missions. We were careful not to actually state that air quality observations are continuous, but only to say that continuous viewing from GEO is possible or that from HEO, it is possible with two satellites. Lines 25 and 36 refer to this possibility from GEO and lines 43 and 45 refer to this possibility from HEO. From LEO, continuous viewing is only possible with a rather large number satellites (likely hundreds), due to geometrical limitations regardless of the size of the FOV. Line 25 is revised based on another comment from this reviewer below.
22: please add: For example, the Copernicus Sentinel-5 Precursor satellite “with the” Tropospheric Monitoring Instrument (TROPOMI) “on board” …
- This change has been made.
25: either “revisits are less frequent” or “revisit rates are lower” (rates can be high or low but not frequent).
- Revised to “revisits are less 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.”
- Revised text to very close to the reviewer’s recommendation “Geostationary orbit (GEO) satellites orbit at an altitude of 35,786 km and are synchronized with Earth’s rotation. As they are essentially stationary over a specific longitude on the equator, they have a continuous view of a limited region of the Earth.”
36: “High revisit rates” or “Frequent revisit …”
- Revised to “Frequent revisits”
36: Rephrase: “…but observing from Earth’s equatorial plane implies that targets at high latitudes are seen with very large viewing angles,…”
- Revised as recommended.
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.
- We thank the reviewer for alerting us of this omission. In the GHG retrieval community the term “airmass” and the link between biases and high airmasses are well-known. One airmass is the quantity of air encountered by light on the shortest path through Earth’s atmosphere (a perfectly vertical column).
- In general, the number of airmasses = 1/cos(VZA) + 1/cos(SZA) (e.g. Nassar et al. 2023, https://doi.org/10.3389/frsen.2023.1233803). To avoid complicating the text in the 2nd paragraph of the introduction with equations and definitions, we rephrase by replacing “large airmasses” with “a longer solar light path through the atmosphere” in the introduction (and Figure 2 caption). The effect of a longer light path on bias is mentioned in Moore et al. (2018), which was cited elsewhere in the manuscript and is now added as a reference here too.
40: Proposed change: “retrievals are unacceptable” -> “retrievals are typically considered as unacceptable”
- Revised as recommended.
41: Resolution “decreases” -> “degrades”
- Revised as recommended.
44: “varying” -> “various”
- Revised as recommended.
50: “although …” needs rewriting.
- We have removed “although”, removed mention of space weather and separated to two sentences for clarify.
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.)
- Revised as recommended.
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.
- We thank the reviewer for this suggestion. We have now added: “NO2 is a key anthropogenic air pollutant that controls the production of other air pollutants including ozone and secondary aerosol.”
106: “a” role, “a” pair of
- Revised as recommended.
108: “edge of the acceptable observing range” -> “edge of the area with acceptable viewing angles”
- Revised as recommended.
112: for clarity: This would be more efficient that using “a combination of” two GEO satellites for the low latitudes and multiple HEO satellites
- Revised as recommended.
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”).
- Since this is just a schematic, the inclination angle was only approximated. The figure has now been modified so that it is closer to 40°.
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.
- We deliberately maintained the same vantage point in all panels (i.e. an orthographic projection centered on the specified equatorial longitude), although the area within the VZA restriction is skewed toward the limb of the Earth, because we felt that this best illustrated the difference in VZA-related coverage available from IGO compared with GEO.
Citation: https://doi.org/10.5194/egusphere-2026-3463-AC1
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AC1: 'Reply on RC1', Ray Nassar, 23 Oct 2026
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RC2: 'Comment on egusphere-2026-3463', Anonymous Referee #2, 20 Aug 2026
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 -
AC2: 'Reply on RC2', Ray Nassar, 23 Oct 2026
Note: Author responses to reviewer comments are the Bold bullets below.
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.
- We thank the Reviewer for this positive overall assessment.
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.
- The reviewer raises a fair point in that the emphasis of our comparison is with GEO not LEO and they are correct that we would expect to achieve ‘roughly’ hourly coverage over the domains shown in Figure 2 from IGO. The international atmospheric composition community (via the CEOS AC-VC, see comment below) is actively discussing and proposing an expansion of the GEO atmospheric composition constellation, hence we are simply presenting an alternative to the proposed idea of adding more GEO satellites and would welcome others to conduct the necessary analysis to present a LEO alternative. We acknowledge that there are measurement advantages from LEO being 50x closer than GEO (the reviewer stated ~5x closer above), however, it should not be assumed that LEO satellites with early morning or late afternoon overpasses would necessarily have the same swath width as missions flying near midday (e.g. Sentinel-5P TROPOMI and Sentinel 5) since these missions mainly observe with low solar zenith angles (SZAs). In the early morning and late afternoon, larger VZAs that enable a wide swath may not work well with large SZAs.
- As a compromise, we have added the statement: “A potential alternative approach to rapid delivering revisit observations with global coverage would be via a LEO satellite constellation; however, assessing the number of satellites required and technical considerations for such a LEO constellation are beyond the scope of this work” to our Introduction as well as the following statement to the final paragraph of our “Results and Discussion” section: “Achieving rapid revisit observations equivalent to those proposed here from IGO is also possible from LEO, although it is expected to require a relatively large number of satellites. The specific configuration and details of a LEO alternative are beyond the scope of this Brief Communication.”
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.
- We thank the reviewer for pointing this out. “IGSO” was erroneously included and should have been updated to IGO. Both acronyms mean Inclined Geosynchronous Orbit, but we transitioned from the slightly more common IGSO to the simpler, shorter IGO prior to submission of the manuscript. Although “CEOS” and “AC-VC” appear infrequently in the manuscript, we think they are important since these acronyms are common in the atmospheric composition satellite community as the organization takes a leadership role in coordinately space agency efforts in “virtual constellations”.
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?
- As Figure 2 illustrates, a VZA limit produces a radius cut-off rather than a single latitude cut-off, therefore one can state a maximum latitude or provide an approximate latitude, where the coverage generally reaches. Trishchenko et al. (2019) provides the equations for such calculations and displays the GEO coverage for VZAs of 55°, 62° and 70°, so we now cite Trishchenko et al. (2019).
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)”
- We have now revised to: “Horizontal resolution also degrades for large VZAs, with pixels 1.86 times larger at VZA=55° and 3.23 times larger at VZA=70° (Trishchenko et al. 2019).”
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?
- We thank the reviewer for this suggestion. Our delta v calculations have determined that for an assumed satellite mass of 1000 kg, the fuel needs for active orbital maintenance required for a stable ECT would be about 2 orders of magnitude more than typical GEO fuel requirements for orbital maintenance. We therefore deem this fuel requirement to be unrealistic. The text now states: “would be prohibitively expensive due to the fuel requirements, which are approximately two orders of magnitude more than required for GEO for the same satellite mass”.
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?
- Our intention here is to clearly introduce IGO by showing how it is similar but different from GEO as both are types of Geosynchronous orbits. Our intention is not to explain the full range of orbit options including LEO and HEO. Instead, we have provided references for HEO, while LEO publications are ubiquitous. (Note: Based on a comment by the other reviewer, we have increased the inclination of the IGO on the left schematic of Figure 1 to better approximate 40°).
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”.
- We have now revised to: “with typical orbits of ~600-800 kilometres in altitude”. We deliberately only quote altitudes to one significant digit to emphasize that it is approximate range and not a firm range or definition of LEO.
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?
- We have now revised by adding: “with the objective of improving coverage of Africa.”
Technical issues.
Abstract, “for expanding the coverage *of* the atmospheric composition virtual constellation”?
- Revised as recommended.
Abstract, “presenting” --> “and thus presents”?
- Revised as recommended.
Line 30, “The *current* constellation”?
- Revised as recommended.
Line 78, delete comma after “is”
- Done
Line 89, capitalization “A”
- Done
Line 94, “It is not possible…” Awkward sentence, hard to parse
- Sentence has now been revised for clarity of language.
Line 97, “and *is* thus”
- Done
Figure 2, please add row and column titles to the figure to make it more readily understandable.
- We thank the reviewer for this suggestion. We have now added short column titles “91°W”, “0°” and “128°E” to the figure. Our preference is not to clutter the figure with row labels but rather improved the caption explanation to emphasize what the rows present.
Figure 2 caption, “are equal to that for the corresponding IGO equator crossing”
- Revised as recommended.
Line 106, “since pair of identical satellites formation”, grammar
- Revised to “since a pair of identical …”
Line 111, “could give *daytime* observations”?
- Revised as recommended.
Line 112, “that” --> “than”
- Done
Citation: https://doi.org/10.5194/egusphere-2026-3463-AC2
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AC2: 'Reply on RC2', Ray Nassar, 23 Oct 2026
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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.