Abstract Air pollution and climate change are two significant concerns threatening sustainable human development. Tracking and mapping atmospheric pollutants and greenhouse gases are essential to managing these issues. Due to its extensive spatio-temporal coverage, satellite remote sensing is indispensable in Earth observation systems to provide measurements of atmospheric chemical species. Since 2008, China has been gathering global atmospheric chemical composition data from space on a daily basis, and the Fengyun satellite series provide the nation’s longest record of ozone and aerosol monitoring. The Medium Resolution Spectral Imager (MERSI) onboard the Fengyun 3 (FY-3) series and the advanced Geostationary Radiation Imager (AGRI) aboard the Fengyun 4 (FY-4) series satellites monitor aerosols in sun-synchronous and geosynchronous orbit, respectively. The Total Ozone Unit (TOU) and the Solar Backscatter Ultraviolet Sounder (SBUS) are China’s first instruments to measure global total ozone columns and vertical profiles. The Ozone Monitoring Suite-Nadir (OMS-N) is the successor to the TOU, monitoring ozone and trace gases like nitrogen dioxide (NO2) and sulfur dioxide (SO 2 ) in the UV-Visible spectrum. The OMS-Limb (OMS-L) provides the capability to obtain stratospheric profiles of ozone and other species. In the infrared wavelength range, the Hyperspectral Infrared Atmospheric Sounder (HIRAS) retrieves vertical information for ozone as well as other trace gases, including carbon monoxide (CO) and ammonia (NH 3 ). For NH 3 and CO, HIRAS in low Earth orbit maps their global distribution, and the Geostationary Interferometric Infrared Sounder (GIIRS) in geostationary orbit tracks their temporal variation over East Asia. The Greenhouse-gases Absorption Spectrometer (GAS) onboard FY-3D and FY-3H is designed to detect the greenhouse gases carbon Dioxide (CO 2 ) and methane (CH 4 ).

Abstract The Polar Radiant Energy in the Far-Infrared Experiment (PREFIRE) is a low-cost CubeSat mission comprising two 6U CubeSats in separate sun-synchronous orbits that continuously measure spectral emissions up to 54 µ m and provide ongoing, time-lapsed observations of the polar processes that modulate them. PREFIRE fulfilled its prime mission between 1 August 2024 and 30 April 2025, during which time each CubeSat, PREFIRE-SAT1 and PREFIRE-SAT2 , regularly resampled itself (“self-intersections”) and the other satellite (“SAT1–SAT2 intersections”). Since PREFIRE intersections will reveal the spectral signatures of the processes that modulate thermal emission from the Arctic and Antarctic, this paper introduces methods to identify PREFIRE resampling and establishes the spatial and temporal records of subdaily PREFIRE intersections from 1 August 2024 to 30 April 2025. Our results indicate that about 76% of self-intersections and over 80% of SAT1–SAT2 intersections occur poleward of 60° latitude. Self-intersections form discrete, time-invariant latitude–temporal bands with time scales that become progressively shorter toward higher latitudes. Conversely, SAT1–SAT2 intersections are dynamic, varying in step with the increasing offset in orbital altitude between satellites, and they exhibit broader time differences between crossovers than self-intersections. Our results further suggest that the second PREFIRE CubeSat nearly quadruples the number of possible daily intersections and SAT1–SAT2 intersections yield twice as many latitude–temporal bands as self-intersections, underscoring the utility of a configuration featuring multiple CubeSats.

Everything else being equal, operating at a very low Earth orbit (VLEO) gives better ground resolution compared to operating at higher altitudes. However, operating at a VLEO faces marked atmospheric drag and various other uncertain interference factors. In this paper, we describe a pioneering satellite named ELITE (Extremely Low-earth Imaging and Technology Explorer) for high-resolution imaging and solar activity observation in VLEO. ELITE is planned to launch to an altitude of 550 km (Sun-synchronous orbit), then maneuver to a VLEO within the range of 200 to 350 km, and maintain that altitude for a year in VLEO through electrical propulsion. To address the challenges of VLEOs, a multiphase altitude planning framework is proposed, covering 4 critical scenarios that account for atmospheric drag, solar activity forecasts, and the strict requirements of time delay integration imaging. This enables robust and energy-efficient altitude adjustment strategies for stable long-term operation. An atmospheric density model is also developed, integrating solar activity predictions, drag coefficient estimation, public datasets, and onboard measurements, and validated using real VLEO mission data. A risk-aware collision avoidance strategy based on a beta-distributed differential evolution algorithm is introduced to balance safety and fuel efficiency, ensuring fast convergence during different evolutionary stages. These innovations ensure that ELITE can successfully accomplish its VLEO mission objectives: a groundbreaking 0.5-m ground sampling distance in color and innovative measurements of ionospheric intensity and atomic oxygen in VLEO. Our mission planning confirms ELITE’s capability to accomplish its missions, marking an important milestone in the field of VLEO satellites.

Abstract As a member of the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) constellation, GECAM-C is an all-sky gamma-ray monitor with in-flight trigger capability aboard the SATech-01 experimental satellite. Operating in a Sun-Synchronous Orbit, GECAM-C generates many in-flight triggers, some of which correspond to important astrophysical bursts, such as gamma-ray bursts and soft gamma-ray repeaters, which may require follow-up observations. However, there are also a substantial number of non-astrophysical triggers, such as particle events. Therefore, a prompt and accurate classification of in-flight triggers is essential for scientific research. In this work, we propose an automatic trigger classification algorithm for GECAM-C in-flight triggers with Bayesian inference. Applying this method to GECAM-C triggers from 2022 December to 2023 December, we demonstrate that it can successfully categorize all trigger types with an accuracy of about 95%, thereby providing effective support for rapid follow-up observations.

Classical mechanics teaches us that the orbital plane of a point mass orbiting a spherical planet remains fixed relative to distant stars. For Earth, the rotation-induced equatorial bulge adds a quadrupole term to the gravitational potential and exerts a torque on satellites that makes their orbital angular momentum precess. We show how choosing a satellite’s orbit makes this precession match Earth’s annual motion about the Sun, yielding Sun-synchronous orbits and, as a special case, dawn--dusk orbits with nearly constant illumination. We also explain why perfect synchronicity with the Sun is impossible: a satellite on a circular orbit keeps mean solar time, whereas sundials follow apparent solar time. The equation of time involves the angle between Earth’s perihelion and the vernal equinox. We estimate its secular drift from Earth’s spin-axis precession and from planetary perturbations of the perihelion, using a simple ring-averaged approach.

Live coral cover is a key indicator of coral reef composition, health, and functioning. Airborne imaging spectroscopy provides verifiably accurate estimates of live coral cover to seawater depths of 25 m, yet satellite-based approaches have not achieved the same level of performance. The new Tanager-1 satellite carries a high-fidelity imaging spectrometer in sun-synchronous Earth orbit, providing an opportunity to transition from airborne to spaceborne imaging of live corals and other benthic constituents. We coordinated overpasses of Tanager-1 and Global Airborne Observatory (GAO) imaging spectrometer measurements of coral reef to a depth of 25 m in Hawaiʻi. Tanager-1 has a spatial resolution of 30 m, while the GAO data were collected at 2 m resolution, requiring detailed modeling to simulate 30 m data for subsequent comparison to the satellite data. At 30 m resolution, the two sensors generated similar geographic patterns of live coral, macroalgal, and sand cover. Field validation indicated similar precision and accuracy of live coral cover estimates, and the ratio of live coral to macroalgal cover proved similar between sensors. Overall results indicate that live coral cover can be mapped with high-fidelity imaging spectroscopy from Earth orbit. With the advent of more spaceborne imaging spectrometers, a new era of live coral monitoring will be possible, filling a critical gap for repeated assessments of reef compositional change at a global level.

Abstract The Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission is a NASA Small Explorer to determine the cross-scale processes that unify the solar corona and heliosphere. PUNCH has two science objectives: (1) understand how coronal structures become the ambient solar wind, and (2) understand the dynamic evolution of transient structures, such as coronal mass ejections, in the young solar wind. To address these objectives, PUNCH uses a constellation of four small spacecraft in Sun-synchronous low Earth orbit, to collect linearly polarized images of the K corona and young solar wind. The four spacecraft each carry one visible-light imager in a 1 + 3 configuration: a single Narrow Field Imager solar coronagraph captures images of the outer corona at all position angles, and at solar elongations from 1.5° (6 R ⊙ ) to 8° (32 R ⊙ ); and three separate Wide Field Imager heliospheric imagers together capture views of the entire inner solar system, at solar elongations from 3° (12 R ⊙ ) to 45° (180 R ⊙ ) from the Sun. PUNCH images include linear-polarization data, to enable inferring the three-dimensional structure of visible features without stereoscopy. The instruments are matched in wavelength passband, support overlapping instantaneous fields of view, and are operated synchronously, to act as a single “virtual instrument” with a 90 ∘ wide field of view, centered on the Sun. PUNCH launched in March of 2025 and began science operations in June of 2025. PUNCH has an open data policy with no proprietary period, and PUNCH Science Team Meetings are open to all.

Each substance in the Universe has its own spectrum signature in which we can identify. hyperspectral scanners allow us to identify any of these substance. Hyperspectral remote sensing imagers mounted on spacecrafts give the opportunity to collect the reflected spectrum waves in digital data. The analysis of these collected data will lead to very important information. These information assist in evolving effective and sustainable development plans. Most of the African and Arab countries are suffering lack of natural resources information. The proposed Regional Resource Monitoring Micro-satellite (R2M2) is an advanced Earth observation satellite. The proposed R2M2 shall carry two imager. The first shall be a multispectral imager with wavelength ranged (0.4 - 0.9) μm. The second shall be hyperspectral imager in the middle infrared and short wave thermal infrared with wavelength range (1.0 – 2.5) μm. The R2M2 will regionally monitor vegetation cover on the Earth surface and give data on the soil types, minerals and may predict underground water resources. This study presented the technical feasibility of the R2M2 project. Four orbits two polar sun-synchronous orbits and two inclined circular orbits were studied. Two configuration designs concepts presented and discussed as well as structure strength analysis based on number of launch vehicles. Thermal loads affecting the satellite and the disturbing moment’s evaluation mathematical models had presented. Estimated power required for satellite operation is presented and discussed as well as the space environment parameters affecting the satellite in orbit are modeled mathematically. The final conclusion shows possibility of R2M2 realization.

Quantitative assessment of satellite proliferation impacts on Antarctic astronomy reveals that 14.5% (14.14–14.86%) of images from Kunlun station are contaminated, exceeding Hubble's design tolerance over four times and ACS/WFC baselines (10.0%) by 45% and threatening humanity’s ultimate pristine window to the universe, immediate space traffic control is needed to improve the space environment. Using Kunlun Station images (05/2008, n=11477) and Space-Track.org satellite catalogues, we annotated satellite trails using LabelImg and modeled orbital mechanics to quantify contamination risks. We estimate the errors and uncertainties via Poisson Noise. Consistent with the contamination metrics, orbital density model visually demonstrates that polar-orbiting satellites over Antarctic (70°-90°S) have an areal density four times higher than equatorial orbits due to sun-synchronous orbits. This degradation poses a unique challenge because Antarctica offers unparalleled conditions for cosmic discovery. Its stable atmosphere, minimal light pollution and the world’s lowest atmospheric interference make Antarctic is the only capable of high-precision studies of the cosmic microwave background (CMB) and dark matter exploration. Without immediate action, these critical observations may be impossible in the future. Urgent mitigation strategies, such as satellites orbit regulations, must be implemented to preserve this vital window into the universe.

China's next-generation atmospheric carbon monitoring satellite, TanSat-2, will utilize nitrogen dioxide (NO2) as a tracer to identify anthropogenic CO2 emissions. TanSat-2 will employ an innovative medium Earth orbit frozen, Sun-synchronous elliptical orbit with a wide-swath instrument to enhance monitoring capabilities over the Northern Hemisphere. An end-to-end simulation framework was established based on the UNL-VRTM radiative transfer model and the differential optical absorption spectroscopy algorithm. The impacts of various instrumental and observational parameters on NO2 vertical column density retrieval were systematically assessed. Results indicate that high solar zenith angles encountered during winter significantly challenge retrieval accuracy. In addition, the variable integration time required to maintain a constant spatial resolution across the elliptical orbit results in a dynamic signal-to-noise ratio (SNR) environment. Our analysis demonstrates that a spectral resolution better than 0.6nm and an SNR exceeding 800 are necessary to resolve the fine absorption features of NO2 effectively. The impact of instrumental artifacts inherent to the wide-swath design was also quantified, revealing that uncorrected "smile" and "keystone" effects can introduce retrieval biases exceeding 1×1015molecules/cm2. Finally, simulations of pollution plumes underscore the importance of high spatial resolution for characterizing emission structures and dispersion patterns. This study confirms the feasibility of the TanSat-2 mission concept and provides a quantitative basis for finalizing its payload specifications.

Volcanic sulfur dioxide monitored from a constellation of FengYun hyperspectral infrared sounders in dawn-dusk, mid-morning, and afternoon sun-synchronous orbits

Ocean surface wind vector (OWV) is a key variable for ocean remote sensing and tropical cyclone (TC) monitoring. This study presents the first comprehensive intercomparison of Ku-band OWV products from FY-3E/WindRAD and HY-2B/SCA scatterometers using full-year data from 2022 (583,805 spatiotemporal collocations), with both sensors sampling the morning–evening local-time sector in sun-synchronous orbits. Results indicate strong agreement in wind speed (R = 0.95; mean bias −0.47 m/s; RMSE 1.30 m/s) and wind direction (mean bias 0.22°; std 28.13°) for wind speeds ≥ 3.4 m/s (Beaufort scale B3 and above), with the highest consistency across Beaufort scale 3–8 (B3–B8); however, at wind speeds greater than 20.8 m/s (B9) the bias increases. A fusion leveraging FY-3E’s fine resolution and HY-2B’s wide coverage is implemented and applied to Super Typhoon Hinnamnor (2022), enhancing the spatial coverage and structural detail of TC winds. Quadrant 34 kt wind radii (R34) are estimated from the fused wind fields and evaluated against the best-track data from the Joint Typhoon Warning Center (JTWC), showing close agreement during compact, symmetric TC stages but larger differences during structural reorganization. Overall, the findings confirm inter-satellite consistency for the two Chinese scatterometers and demonstrate the practical value of a multi-source fusion approach that benefits TC monitoring, wind radii estimation, and marine weather services.

Abstract. MATS (Mesospheric Airglow/Aerosol Tomography and Spectroscopy) is a Swedish satellite mission designed to investigate atmospheric gravity waves. In order to observe wave patterns, MATS observes structures in the O2 atmospheric band airglow (light emitted by oxygen molecules in the mesosphere and lower thermosphere), as well as structures in noctilucent clouds (NLCs) which form around the mesopause. The main instrument is a telescope that continuously captures high-resolution images of the atmospheric limb. Using tomographic analysis of the acquired images, the MATS mission can reconstruct waves in three dimensions and provide a comprehensive global map of the properties of gravity waves. The data provided by the MATS satellite will thus be three-dimensional fields of airglow and NLC properties in 200 km-wide (across track) strips along the orbit at altitudes of 70 to 110 km. By adding spectroscopic analysis, by separating light into six distinct wavelength channels, it also becomes possible to derive temperature and microphysical NLC properties. Based on those data fields, further analysis will yield gravity wave parameters, such as the wavelengths, amplitudes, phase, and direction of the waves, on a global scale. The MATS satellite, funded by the Swedish National Space Agency, was launched in November 2022 into a 580 km sun-synchronous orbit with a 17.25 local time of the ascending node (LTAN). This paper accompanies the public release of the Level 1b (v. 1.0) dataset from the MATS limb imager. The purpose of the paper is to provide background information in order to assist users to correctly and efficiently handle the data. As such, it details the image processing and how instrumental artefacts are handled. It also describes the calibration efforts that have been carried out on the basis of laboratory and in-flight observations, and it discusses uncertainties that affect the dataset.

Remotely sensed nighttime lights (NTLs) have become essential in urban and environmental research but are typically captured at fixed local times by sun-synchronous satellites, limiting their ability to capture changes throughout the night. In contrast, in situ measurements of night sky brightness (NSB) can provide continuous records over time, but direct comparisons with NTLs have remained rare. This study first examines the relationship between in situ NSB and remotely sensed NTLs using multi-temporal imagery from Luojia-1 and the International Space Station (ISS), focusing on 10 sites in Hong Kong and Macau. We find moderate to strong correlations between NSB and Luojia-1 (R = 0.73) and between NSB and ISS imagery (R = 0.8–1.0), though notable spatial and temporal variations persist. Even images captured within seconds differ in brightness across locations (R = 0.88–0.96), driven by factors such as changing viewing angles in dense urban areas, variations in light transmission paths, and atmospheric conditions, all influenced by satellite position. Our further analysis reveals distinct temporal patterns across land use categories: port facilities and airports are brightest late at night, whereas commercial districts peak earlier and gradually dim throughout the night. Within individual ISS images, transportation-related lighting tends to be red, and commercial areas appear blue compared to other urban areas, which may be due to lamp type differences (high pressure sodium, LED). This study highlights the need to cross-examine in situ and remotely sensed data in NTL research, emphasizing that factors such as local pass time, viewing geometry, color sensitivity, and atmospheric conditions can influence observations and ultimately affect the conclusions.

Using electrodynamic tethers to perform orbit maneuvers in sun-synchronous satellites

Observing carbon monoxide and volatile organic compounds from Canadian wildfires in 2023 from FengYun-3E/HIRAS-II in a dawn-dusk sun-synchronous orbit

Tidal estimates from sun-synchronous satellite altimeters in the Bohai Sea via an improved harmonic analysis model

The paper presents the results of thermal vacuum tests of the PolyITAN-HP-30 university nanosatellite of the 2U CubeSat format un- der the conditions of ground-based physical modeling of the main operating factors of outer space, which satisfy the sun-synchronous orbit with an altitude of up to 550 km. The tests were conducted following the requirements of the European Community for Space Standardization (ECSS) and are standardized and regulated by documents. The performed thermal vacuum tests are a mandatory and important stage for the development of reliable space equipment, which extends its operational lifespan. The equipment for thermal vacuum tests available at National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute» (Igor Sikorsky Kyiv Polytechnic Institute) provides modeling of “cold” and “black” space, fosters the implementation of scientific and technical approaches and solutions for the development and implementation of new space technologies and methods for conducting thermal vacuum research/tests, and helps to develop measurement tools and systems to obtain reliable technical characteristics of working components and assemblies of the spacecraft. The article describes the structure and construction of the university nanosatellite (NS). The main characteristics of the electronic components, the payload of the NS in the form of a thermal stabilization system from mini-heat pipes, are presented. The authors emphasize that the electronic platforms include mass-produced elements that are usually not designed to operate in the harsh conditions of outer space and, therefore, require careful verification of their functioning in these conditions. Besides, before launching into space, they must always be subjected to thermal vacuum tests. The article specifies the main requirements for conducting ground-based research and testing of a university NS. References are given to the main characteristics of the bench equipment used in the work, as well as the quantitative characteristics of the NS. The test results are presented in the form of graphical dependences of the temperature changes of the NS’s elements on time. The analysis of the data obtained from thermal balance tests in the cycling mode in the temperature range from –20 °С to +50 °С was carried out. The flight sample of the NS was degassed at ambient temperatures in the experimental chamber from 50 °С to 55 °С and a vacuum of 1.0105 Torr. Losses after degassing the NS amounted to no more than 0.1 % of its mass. The duration of degassing after thermal stabilization in the chamber lasted 4.5 hours. The tests performed ensured verification of thermal regimes of various components of the spacecraft and onboard equipment in conditions close to their operation in Earth orbit. The results obtained showed the correctness of modeling the main factors of space in thermal cycling and degassing regimes and confirmed the ability of the spacecraft to perform operational functions with stability of parameter values within the limits specified by the regulatory and technical documentation.

Geosynchronous (GEO) orbit is a significant gathering point for navigation, meteorological, and communications satellites that require close monitoring and protection. To achieve successful observation of GEO objects, we developed a constellation of two satellites based on the localized leak-proof observation strategy of natural rendezvous. The constellation's two satellites are spread in equal phase on the coorbital plane of the sun-synchronous orbit, and scene modeling determines the field of view of the satellite's detector. We separated the detector's field of view into two parts: along orbit (AOFOV) and across orbit (COFOV), and we established the detector's threshold field of view in the binary constellation. The results show that when the AOFOV and COFOV thresholds are both 16°, the binary constellation's coverage rate to GEO orbital targets can reach 99.12%. Compared with the designed four-star constellation, the field of view waste index is reduced by 50.51% while the coverage rate remains unchanged, the revisit period is 1 day, and all-weather continuous observation is achievable.

Abstract NUSES is a pathfinder for new satellite platforms developed by THALES and cutting-edge photo sensing technologies, such as SiPM and their associated low-power-consuming electronics. It is financed by the Italian Ministry and conducted by the Gran Sasso Science Institute (GSSI), INFN sections and the University of Geneva. NUSES hosts two payloads: Ziré is devoted to low-energy cosmic rays to investigate aspects related to space weather, and gamma-rays from gamma-ray bursts; the Terzina telescope will achieve the first observation from space of the Cherenkov light emitted by atmospheric showers induced by ultra-high-energy cosmic rays (UHECRs) within its field of view. Terzina might catch also a few Earth-skimming neutrinos above about 100 PeV. This faint light may only be detected by Terzina from a sun-synchronous orbit at 535 km of altitude while pointing to the limb and viewing the dark side of the earth and atmosphere. In such a configuration, this space-based telescope would not be constrained by the day-night cycle, unlike ground-based Cherenkov telescopes or payloads in non-polar orbit. Terzina is a Schmidt-Cassegrain telescope with dual mirror optics with a 935 mm effective focal length and a primary mirror with a diameter of 430 mm. Its SiPM-based camera is composed of 2 rows of 5 tiles of 8 × 8 SiPM 3x3 mm2 pixels. The University of Geneva collaborated with the FBK Research Foundation to define these tiles. We measured in the laboratory the equivalent effect of radiation in space on the SiPM. Understanding the light noise in situ is vital for future larger missions or constellations of such satellites in the plans in the US and Europe and also for other missions employing SiPMs. For this study we utilized a 50 MeV proton beam and a beta-radioactive source of Strontium-90. As a matter of fact, radiation damage increases the DCR, and consequently to keep the signal-to-noise ratio constant the trigger threshold has to be increased. However, we developed an annealing approach suitable for a space-based middle-size satellite to limit the effect of radiation damage while efficiently lowering the SiPM’s energy detection threshold.