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This article presents an innovative geometric calibration mode for a linear array camera used in the Jilin-1 satellite. Changing the camera from observing the ground to observing space and treating stars as control points allow for the accurate determination of the geometric parameters of the camera. In this article, several important calibration procedures and strategies are presented to gradually eliminate the influence of astronomical and attitude errors. Self-verification of the calibration image results in a star-based calibration accuracy is better than 0.3 pixels. Further verification of the star image that leads to an interior orientation accuracy is better than 0.6 pixels. The accuracy of geometric positioning without control is verified by ground images of different regions to be 30 m. The interior orientation accuracy of ground images processed by star-based calibration parameters is better than 1.5 m. The ground image corrected using the star-based calibration parameters is well overlaid on the reference image. The accuracies obtained with star-based and ground-based calibrations are comparable. However, ground calibration fields require high maintenance costs and may suffer from harsh imaging conditions. Contrarily, star-based calibration is not affected by the weather and climate conditions on the ground, which can be photographed from any location along the satellite orbit. Therefore, star-based calibration can meet the requirements for high-frequency geometric calibration of subsequent satellite constellations construction.

In recent years, there has been significant interest in the deployment of Low Earth Orbit (LEO) satellite constellations as a potential solution to provide broadband internet access to remote regions [1]. One such constellation, Starlink, run by SpaceX, promises fast internet connectivity via its constellation of satellites in low Earth orbit. In this study, a comprehensive analysis of Starlink’s performance in Nigeria was given, assessing five performance metrics: latency, packet loss, throughput, routing strategy, and environmental influences. The tests were conducted for the various metrics using command-line tools like Ping (Packet Internet or Inter-Network Groper), tracert (traceroute), iperf3 (Internet Performance Working Group), and Python scripts which were used to both automating the test procedures and analysing the collected data. Our findings reveal that Starlink’s latency was higher than advertised but still low compared to most Mobile Network Operators (MNOs) in Nigeria, making it suitable for the majority of internet users. On average, the study found that packet loss remained relatively low, jitter was within acceptable limits, and throughput exhibited variations but generally maintained satisfactory levels. Also, heavy rainfall was found to affect Starlink’s performance, highlighting the impact of environmental factors.

In recent years, there has been significant interest in the deployment of Low Earth Orbit (LEO) satellite constellations as a potential solution to provide broadband internet access to remote regions [1]. One such constellation, Starlink, run by SpaceX, promises fast internet connectivity via its constellation of satellites in low Earth orbit. In this study, a comprehensive analysis of Starlink’s performance in Nigeria was given, assessing five performance metrics: latency, packet loss, throughput, routing strategy, and environmental influences. The tests were conducted for the various metrics using command-line tools like Ping (Packet Internet or Inter-Network Groper), tracert (traceroute), iperf3 (Internet Performance Working Group), and Python scripts which were used to both automating the test procedures and analysing the collected data. Our findings reveal that Starlink’s latency was higher than advertised but still low compared to most Mobile Network Operators (MNOs) in Nigeria, making it suitable for the majority of internet users. On average, the study found that packet loss remained relatively low, jitter was within acceptable limits, and throughput exhibited variations but generally maintained satisfactory levels. Also, heavy rainfall was found to affect Starlink’s performance, highlighting the impact of environmental factors. [ABSTRACT FROM AUTHOR]

Satellite constellations consist of similar satellites distributed in a specific orbital pattern to accomplish a joint mission. They are commonly used for global telecommunications, positioning, and remote sensing missions. This article utilizes a scientometric approach to analyze the structure and scientific mapping of "satellite constellations" worldwide, focusing on two leading countries: the United States and China. Additionally, a comparison is made between the United States, China, and global research outputs.The research encompasses all scientific documents in the "satellite constellations" field indexed in the Scopus database from 1969 to 2023. Knowledge maps (thematic subfields) and international cooperation maps are generated using Bibexcel and VOSviewer software. The number of documents published in this period includes 14,346 globally, with 3,738 from the United States and 3,369 from China.The findings reveal that the United States, China, Germany, Italy, and the United Kingdom are leaders in this field. The trend of scientific production in this area is increasing globally, indicating its importance and attractiveness. Conference papers constitute the most significant number of publications worldwide and in the United States, whereas, in China, most documents are journal articles. The top scientific production institutions globally include the National Aeronautics and Space Administration (NASA), the Chinese Academy of Sciences, the German Aerospace Center, Wuhan University, and the European Space Research and Technology Centre.

Recent advancements in space technology have sparked significant interest in small satellite constellations, primarily due to their reduced costs, quicker development cycles, and improved capabilities. Although the costs associated with launching these satellites remain high, innovative strategies are emerging that offer alternatives to conventional deployment methods for positioning constellations in their designated orbits. These deployment strategies can be broadly categorized into two types: direct and indirect. This paper explores an indirect deployment method that utilizes Earth's oblateness perturbation alongside the satellite's propulsion subsystem to position satellites across multiple orbital planes. The method has been rigorously simulated and analyzed in the context of two operational satellite constellations, each serving distinct purposes—remote sensing and global internet provision. Furthermore, a comprehensive launch and deployment strategy has been developed and applied to a regional satellite navigation system, consisting of 130 satellites designed to serve Iran. This approach emphasizes the influence of Earth's shape on satellite orbits to optimize deployment efficiency. The analysis focuses on key parameters such as the time required for deployment and the necessary change in velocity (ΔV). The study demonstrates how leveraging the natural nodal precession caused by Earth's oblateness can significantly enhance the deployment process, reducing fuel consumption and operational costs. This approach presents a promising alternative for future satellite constellation deployments, offering both economic and technical advantages. [ABSTRACT FROM AUTHOR]

Satellite constellations consist of similar satellites distributed in a specific orbital pattern to accomplish a joint mission. They are commonly used for global telecommunications, positioning, and remote sensing missions. This article utilizes a scientometric approach to analyze the structure and scientific mapping of "satellite constellations" worldwide, focusing on two leading countries: the United States and China. Additionally, a comparison is made between the United States, China, and global research outputs. The research encompasses all scientific documents in the "satellite constellations" field indexed in the Scopus database from 1969 to 2023. Knowledge maps (thematic subfields) and international cooperation maps are generated using Bibexcel and VOS viewer software. The number of documents published in this period includes 14,346 globally, with 3,738 from the United States and 3,369 from China. The findings reveal that the United States, China, Germany, Italy, and the United Kingdom are leaders in this field. The trend of scientific production in this area is increasing globally, indicating its importance and attractiveness. Conference papers constitute the most significant number of publications worldwide and in the United States, whereas, in China, most documents are journal articles. The top scientific production institutions globally include the National Aeronautics and Space Administration (NASA), the Chinese Academy of Sciences, the German Aerospace Center, Wuhan University, and the European Space Research and Technology Centre. [ABSTRACT FROM AUTHOR]

Orbiting solar reflectors (OSRs) are flat, thin and lightweight reflective structures that are proposed to enhance terrestrial solar energy generation by illuminating large terrestrial solar power plants locally around dawn/dusk and during night hours. The incorporation of OSRs into terrestrial energy systems may offset the daylight-only limitation of terrestrial solar energy. However, the quantity of solar energy delivered to the Earth's surface remains low due to short duration of orbital passes and the low density of the reflected solar power due to large slant ranges. To compensate for these, this paper proposes a constellation of multiple reflectors in low-Earth orbit for scalable enhancement of the quantity of energy delivered. Circular near-polar orbits of 1000 km altitude in the terminator region are considered in a Walker-type constellation for a preliminary analysis. Starting from a simplified approach, the equations of Walker constellations describing the distribution of the reflectors are first modified by introducing a phasing parameter to ensure repeating pass-geometry over solar power farms. This approach allows for a single groundtrack optimisation to define the constellation, which was carried out by a genetic algorithm for single and two reflectors per orbit with an objective function defined as the total quantity of energy delivered per day, to existing and hypothetical solar power projects around the Earth. When full-scale constellations are considered with a number of reflectors, the quantity of solar energy delivered is substantial in the broader context of global terrestrial solar energy generation. • Terrestrial solar energy enhancement by constellation of orbiting reflectors is proposed. • Scalable, predictable and continuous solar energy from space is aimed. • Walker constellation equations are modified for repeating reflector passes. • Groundtrack optimised by genetic algorithm to maximise daily energy delivery. • Quantity of daily solar energy delivered is considerable in the global context. [ABSTRACT FROM AUTHOR]

The next generation of spacecraft technology is anticipated to enable novel applications, including onboard processing, machine learning, and decentralized operational scenarios. Although several of these applications have been previously investigated, the real-world operational limitations associated with actual mission scenarios have been only superficially addressed. Here, we present an open-source Python module called PASEOS, capable of modeling operational scenarios involving one or multiple spacecraft. It considers several physical phenomena, including thermal, power, bandwidth, and communications constraints, and the impact of radiation on spacecraft. PASEOS can be run as a high-performance-oriented numerical simulation and/or in a real-time mode on edge hardware. We demonstrate these capabilities in three scenarios: one in real-time simulation on a Unibap iX-10 100 satellite processor, another in a simulation modeling an entire constellation performing tasks over several hours, and one training a machine learning model in a decentralized setting. While we demonstrate tasks in Earth orbit, PASEOS also allows deep space scenarios. Our results show that PASEOS can model the described scenarios efficiently and thus provide insight into operational considerations. We show this by measuring runtime and overhead as well as by investigating the constellation's modeled temperature, battery status, and communication windows. By running PASEOS on an actual satellite processor, we showcase how PASEOS can be directly included in hardware demonstrators for future missions. Overall, we provide the first solution to holistically model the physical constraints spacecraft encounter in space. The PASEOS module is available online with extensive documentation, enabling researchers to incorporate it into their studies quickly.

Recent advancements in space technology have sparked significant interest in small satellite constellations, primarily due to their reduced costs, quicker development cycles, and improved capabilities. Although the costs associated with launching these satellites remain high, innovative strategies are emerging that offer alternatives to conventional deployment methods for positioning constellations in their designated orbits. These deployment strategies can be broadly categorized into two types: direct and indirect. This paper explores an indirect deployment method that utilizes Earth's oblateness perturbation alongside the satellite's propulsion subsystem to position satellites across multiple orbital planes. The method has been rigorously simulated and analyzed in the context of two operational satellite constellations, each serving distinct purposes—remote sensing and global internet provision. Furthermore, a comprehensive launch and deployment strategy has been developed and applied to a regional satellite navigation system, consisting of 130 satellites designed to serve Iran. This approach emphasizes the influence of Earth's shape on satellite orbits to optimize deployment efficiency. The analysis focuses on key parameters such as the time required for deployment and the necessary change in velocity (ΔV). The study demonstrates how leveraging the natural nodal precession caused by Earth's oblateness can significantly enhance the deployment process, reducing fuel consumption and operational costs. This approach presents a promising alternative for future satellite constellation deployments, offering both economic and technical advantages.

This letter proposes a novel synthetic aperture radar (SAR) time series analysis method based on the scattering power decomposition algorithm with a reasonable ensemble average in both temporal and spatial domains. We reveal that the ensemble average is effective not only in the spatial domain but also in the temporal–spatial domains in the scattering power decomposition. That is, if we extend the ensemble average window in the temporal domain, the proposed method can accurately achieve volume scattering power with a higher spatial resolution than conventional approaches. The precise volume scattering power serves accurate forest monitoring. As an application, we performed forest disturbance detection in the Amazon rainforest using Sentinel-1 time series data. The proposed method detected the disturbances earlier, in less than 2 months, compared to other methods that take about 3 months. [ABSTRACT FROM AUTHOR]

This research is focused on the problem of agile attitude maneuvering, aimed at the precise pointing of a satellite forming a typical constellation in low Earth orbit. We consider two different operational scenarios: (a) pointing toward a specific ground station, located on the Earth surface (for downlink data routing), and (b) pointing toward a companion satellite (for establishing an intersatellite connection). The two preceding operational requirements can both be formulated as attitude tracking problems. In this study, we use an inertia-free nonlinear attitude control algorithm based on rotation matrices and possessing remarkable stability properties, in conjunction with a pyramidal array of single-gimbal control momentum gyroscopes. Numerical simulations, in both nominal and nonnominal flight conditions, demonstrate that the attitude control architecture proposed in this work is effective for the purpose of performing agile attitude maneuvering, aimed at precise pointing during downlink and intersat data routing. [ABSTRACT FROM AUTHOR]

Satellite gravity missions so far are medium size satellites consisting of one or a pair of satellites flying in near-polar or sun-synchronous orbital planes. Due to the limited observation geometry and the related space–time sampling, high-frequency non-tidal mass variation signals from atmosphere and ocean cannot be observed and cause temporal aliasing. For current single-pair satellite gravimetry missions as GRACE and GRACE Follow-On (GRACE-FO) temporal aliasing is the limiting factor and represents the major error source in the gravity field time-series. Adding a second inclined satellite pair to a GRACE-like polar pair (Bender constellation) currently is the most promising solution to increase the spatio-temporal resolution and to significantly reduce the temporal aliasing error. This shall be implemented with the MAGIC mission in future. With the ongoing developments in miniaturization of satellites and gravity-relevant instruments (accelerometers and intersatellite ranging), in future constellations of multiple small satellite pairs may solve this problem even beyond the capabilities of a Bender constellation. Therefore, in this study the capabilities of such constellations flying in specific formations are investigated in order to enable a retrieval of the temporal gravity field on short time scales. We assess the performance of up to 18 satellite pairs. The satellite configurations cover satellite pairs in polar and inclined orbits flying in pair-wise or pearl-string formation with varying mean anomalies and right ascensions of the ascending node (RAAN). As future potential miniaturized instruments optomechanical accelerometers with similar performance as those flying on GRACE-FO are a candidate, while for the intersatellite ranging instrument still some technological development is required. Therefore, in this study a microwave ranging system equivalent to the GRACE and GRACE-FO instruments performance is taken as baseline assuming that such instruments can be miniaturized in future as well. In numerical closed-loop simulations, up to nine different satellite configurations with up to 18 satellite pairs are evaluated based on the retrieval of the non-tidal temporal gravity field on a monthly basis. From the simulation results it is concluded that the best-performing satellite constellation of 18 polar satellite pairs already is outperformed by a typical Bender-like constellation of 1 polar and 1 inclined pair. In general, we identify that increasing the number of satellite pairs leads to an improved gravity field retrieval, either at low spherical harmonic degree and order (d/o) by the shift in RAAN or at high d/o by the shift in mean anomaly. By a two-step simulation approach, co-estimating also (sub-)daily gravity fields for selected configurations with a large number of satellite pairs distributed equally over the globe, it is possible to retrieve stand-alone gravity fields at 24, 12 and 6 hr temporal resolution. Ultimately it is concluded that a network of miniaturized satellites with instrument performances similar to GRACE-FO and flying in a well-defined constellation has the potential to observe (sub-)daily mass variations and therefore could drastically reduce the problem of temporal aliasing due to high frequency mass variations in the Earth system. [ABSTRACT FROM AUTHOR]

We introduce a methodology for estimating the risk posed to the space environment by a spacecraft over an arbitrary period of time following a risk mitigation strategy, in terms of aggregate collision probability. Our methodology enables estimation of residual risk and maneuver frequency, where residual risk is defined conceptually as the risk to a spacecraft which remains even after adherence to a risk mitigation strategy. The key parameters considered which affect residual risk for a general risk mitigation strategy are the risk mitigation maneuver (RMM) threshold, the risk reduction factor, and the maneuver execution time. We present an analytic result regarding the necessary residual risk (per-satellite) to ensure the total aggregate collision probability of a satellite constellation of arbitrary size be below a target value. This approach offers a more complete model of spacecraft safety and potential risk to the space environment by studying more than just the RMM threshold, which has historically been used as a common benchmark in space situational awareness and regulatory compliance literature. Our analysis shows that the RMM threshold is but one of a few factors which have significant effects on residual risk. We provide evidence that the RMM threshold alone is an incomplete indicator of the actual risk posed to the space environment by an operational spacecraft. We demonstrate the effectiveness of this methodology by presenting numerical results which model realistic satellites and satellite constellations and draw key insights from this analysis which will aid in managing the safety and sustainability of the space environment and inform risk mitigation strategies for large satellite constellations. [ABSTRACT FROM AUTHOR]

Heavy rain is associated with strong winds and extreme waves in a tropical cyclone (TC). In this letter, a practical algorithm for rain rate retrieval in TCs is proposed through 24 dual-polarized [vertical-vertical (VV) and vertical-horizontal (VH)] Sentinel-1 (S-1) synthetic aperture radar (SAR) images acquired in interferometric-wide (IW) swath mode, in which 13 images are collocated with the observations from stepped-frequency microwave radiometers (SFMRs). TC winds are directly obtained from VH-polarized images utilizing the geophysical model function (GMF) S-1 IW mode wind speed retrieval model after noise removal (S1IW.NR). The normalized radar cross section (NRCS) at VV-polarization channel is simulated using GMF CMOD5N and VH-polarized SAR wind. It is found that the difference between the simulated NRCSs and measurements from SAR is linearly related to the rain rate and oscillates with the incidence angle. Following this finding, an empirical algorithm for SAR rain rate retrieval is developed, denoted as CRAIN2_S1, which considers the influence of the radius of the maximum wind speed. The proposed algorithm is applied to 11 images in the dataset, and the validation of the rain rate (up to 35 mm/hr) against the products from global precipitation measurements (GPMs) has a root mean square error (RMSE) of 1.74 mm/hr, a correlation coefficient of 0.92 and a scatter index (SI) of 0.29. Collectively, it is concluded that the algorithm CRAIN2_S1 can be practically applied for dual-polarized SAR rain rate retrieval without any external information. [ABSTRACT FROM AUTHOR]

This paper presents a method of planning spacecraft maneuvers for mobile target tracking. Agile, maneuverable spacecraft have been proposed as a means of modifying satellite orbits to observe discrete targets on the Earth on demand. This work adapts this concept to propose a method of using maneuverable spacecraft to observe a mobile target as it moves across the Earth. Previous work has shown the potential of such an approach to increase persistence of coverage of a moving target. This work applies a suitable optimizer to select possible maneuvers for a spacecraft, or a constellation of spacecraft, to repeatedly observe a moving target. A biased random key genetic algorithm is used, which adjusts the ΔV for each maneuver to minimize the total ΔV used and maximize target coverage over the full sequence of maneuvers. The developed method is applied to two case studies concerning monitoring of tropical storms. The results indicate that, using relatively small maneuvers, spacecraft orbits can be adjusted to improve the quantity and quality of views of a moving target. In the Typhoon Megi case study, a single spacecraft using less than 2.5  m/s ΔV is shown to double the access time and provide two additional observations of the storm eye compared with a nonmaneuvering spacecraft in an identical initial orbit. Opportunities for observations increase as the number of maneuverable spacecraft are increased, with a three-spacecraft constellation able to provide five complete observations of the storm eye for a total change in velocity of 12 m/s. [ABSTRACT FROM AUTHOR]

This research is focused on the problem of agile attitude maneuvering, aimed at the precise pointing of a satellite forming a typical constellation in low Earth orbit. We consider two different operational scenarios: (a) pointing toward a specific ground station, located on the Earth surface (for downlink data routing), and (b) pointing toward a companion satellite (for establishing an intersatellite connection). The two preceding operational requirements can both be formulated as attitude tracking problems. In this study, we use an inertia-free nonlinear attitude control algorithm based on rotation matrices and possessing remarkable stability properties, in conjunction with a pyramidal array of single-gimbal control momentum gyroscopes. Numerical simulations, in both nominal and nonnominal flight conditions, demonstrate that the attitude control architecture proposed in this work is effective for the purpose of performing agile attitude maneuvering, aimed at precise pointing during downlink and intersat data routing.

The United States has plans to develop two new missile defense programs in the space domain: a space-based sensor architecture and a space-based missile intercept layer. Both proposed systems rely on a network of satellites in low Earth orbit to offer full or partial coverage of the Earth's surface, precisely tracking a missile during its flight in one case, or shooting it down entirely in the other. A space-based sensor system could expand current capabilities for monitoring missile launches and warrants further study. The deployment of a space-based missile intercept layer, however, would require launching hundreds or thousands of weapons into space -- an expensive, inefficient, and provocative idea. The technical discussion surrounding space-based interceptors should be decoupled from that of space-based sensors -- a much more plausible proposal. Despite decades of support from influential policymakers, the resources required to deploy space-based interceptors would be better spent on other layers of US missile defense. [ABSTRACT FROM AUTHOR]

Several commercial organizations have recently launched or plan to launch constellations containing thousands of satellites. Such large constellations potentially adversely affect astronomical observations. This study formulates a set of indicators that assess the impact of light pollution from different constellations on ground-based visible band astronomy. These include the statistically expected number of visible and sunlit satellites above ground-based observers, as well as the number that are also expected to be brighter than the currently recommended limit for constellation satellites. The latter indicator provides a consolidated means to evaluate the potential for a constellation to affect ground-based astronomy too severely, by simultaneously accounting for the effects of constellation population, orbital distribution as well as brightness magnitude and variability. For existing constellations, the evaluation process incorporates actual satellite photometric brightness measurements, which are becoming increasingly available in web-accessible databases and repositories. For proposed constellations, a semi-empirical method allows rough approximations of pre-launch light pollution levels, based on observed brightness distributions observed of currently orbiting analog satellites. [ABSTRACT FROM AUTHOR]

The development of large constellations of satellites (i.e., so-called megaconstellations or satcons) is poised to increase the number of LEO satellites by more than an order of magnitude in the coming decades. Such a rapid growth of satellite numbers makes the consequences of major fragmentation events ever more problematic. In this study, we investigate the collisional risk to on-orbit infrastructure from kinetic anti-satellite (ASAT) weapon tests, using the 2019 Indian test as a model. We find that the probability of one or more collisions occurring over the lifetime of ASAT fragments increases significantly in a satcon environment compared with the orbital environment in 2019. For the case of 65,000 satellites in LEO, we find that the chance of one or more satellites being struck by ASAT fragments of size 1 cm or larger is more than 25% for a single test. Including sizes down to 3 mm in our models suggests that impacts will occur for any such event. Finally, we apply our methods to examine the November 2021 Russian ASAT test, also finding a significant collision probability over the lifetime of the fragments. The heavy commercialization of LEO demands a commitment to avoiding debris-generating ASAT tests. [ABSTRACT FROM AUTHOR]

For data communications over long distances, optical wireless satellite networks (OWSNs) can offer lower latency than optical fiber terrestrial networks (OFTNs). However, when is it beneficial to switch or crossover from an OFTN to an OWSN for lower latency data communications? In this article, we introduce a crossover function that enables to find the crossover distance, i.e., a distance between two points on the surface of the Earth beyond which switching or crossing over from an OFTN to an OWSN for data communications between these points is useful in terms of latency. Numerical results reveal that a higher refractive index of optical fiber (or $i$) in an OFTN and a lower altitude of satellites (or $h$) in an OWSN result in a shorter crossover distance. To account for the variation in the end-to-end propagation distance that occurs over the OWSN, we examine the crossover function in four different scenarios. Numerical results indicate that the crossover distance varies with the end-to-end propagation distance over an OWSN and is different for different scenarios. We calculate the average crossover distance over all scenarios for different $h$ and $i$ and use it to evaluate the simulation results. Furthermore, for a comparative analysis of OFTNs and OWSNs in terms of latency, we study three different OFTNs having different refractive indexes and three different OWSNs having different satellite altitudes in three different scenarios for long-distance intercontinental data communications, including connections between New York and Dublin, Sao Paulo and London, and Toronto and Sydney. All three OWSNs offer better latency than OFTN2 (with $i_{2}$ = 1.3) and OFTN3 (with $i_{3}$ = 1.4675) in all scenarios. For example, for Toronto–Sydney connection, OWSN1 (with $h_{1}$ = 300 km), OWSN2 (with $h_{2}$ = 550 km), and OWSN3 (with $h_{3}$ = 1100 km) perform better than OFTN2 by 18.11%, 16.08%, and 10.30%, respectively, while they provide an improvement in latency of 27.46%, 25.67%, and 20.54%, respectively, compared to OFTN3. OWSN1 performs better than OFTN1 (with $i_{1}$ = 1.1) for Sao Paulo–London and Toronto–Sydney connections by 2.23% and 3.22%, respectively, while OWSN2 outperforms OFTN1 for Toronto–Sydney connection by 0.82%. For New York–Dublin connection, all OWSNs while for Sao Paulo–London connection, OWSN2 and OWSN3 exhibit higher latency than OFTN1 as the corresponding average crossover distances are greater than the shortest terrestrial distances between cities in these scenarios. Multiple satellites (or laser intersatellite links) on its shortest paths drive up the propagation distance to the extent that OWSN3 ends up with a higher latency than OFTN1 for the Toronto–Sydney intercontinental connection scenario although the related average crossover distance is less than the shortest terrestrial distance between Toronto and Sydney. The challenges related to OWSNs and OFTNs that may arise from this work in future are also highlighted. [ABSTRACT FROM AUTHOR]

Despite the rapid and continuous development of communication and networking technologies, the goal of universal connectivity (i.e., the ability to communicate with any user at any time and in any place) is still unrealized. In 2019, a study found that 40% of Earth’s regions lack network coverage, which means that there are still 4 billion people on the planet without Internet access. Moreover, even as the COVID-19 pandemic increased the need for online working, learning, and accessing services—and, thus, increased the number of Internet users—2.9 billion people remain offline in developing countries. Satellite communications (SatCom) have been emerging as a potential and indispensable solution to extending broadband coverage to underserved areas. [ABSTRACT FROM AUTHOR]

A low Earth orbit (LEO) satellite constellationnetwork (SCN) has become a promising solution for nonterrestrial networks. In LEO-SCNs, the shortest distance path (SDP) and the minimum hop path (MHP) are two types of important paths. This letter focuses on the proposition that all the SDPs belong to the MHP set and studies the conditions when the proposition holds or not. Based on the topological regularity and link distance variation patterns, this letter proves several simplified equivalent propositions and derives a discriminant function to judge if the proposition holds in an arbitrary constellation. Simulations verify the judging method and find that all the SDPs belong to the MHP set in constellations with small inclinations (less than 68$^\circ$) or large phasing offsets. The propositions can help to simplify the calculation of the SDP. [ABSTRACT FROM AUTHOR]

Optical Intersatellite Links (OISLs) are being considered for future Global Navigation Satellite System (GNSS) constellations. Thanks to OISLs, the constellation incorporates improved clock synchronization and precise ranging among the satellites, which are essential features to achieve accurate time and orbit determination. High data rate communications within the space segment also reduce ground segment dependency, by means of decentralized access to information. However, the dual optimization of data and navigation performance metrics requires a careful assignment of OISLs to the available laser communication terminals on-board. To this end, we present a contact plan design scheme based on a degree constrained minimum spanning tree heuristic applied to such OISL-enabled GNSS (O-GNSS) constellations. Results on the Kepler system, a novel GNSS proposal, show that a fair distribution of connectivity among the constellation can be ensured while optimizing its range-based position estimation capabilities (PDOP). A PDOP improvement of 85 % is reached on average by the optimized contact plan with respect to a generic scheduler that disregards the geometrical distribution of the chosen links. [ABSTRACT FROM AUTHOR]

Passive microwave satellite observations provide critical information for global forecast models, particularly in cloudy and/or precipitating conditions. The limited temporal sampling provided by current operational polar orbiters cannot capture rapidly changing conditions such as the development of convective storms. This is a significant issue for open-ocean weather systems such as tropical cyclones and hurricanes that can only be effectively monitored from satellites. The recent development and demonstration of miniaturized microwave radiometers on-board low-cost CubeSat satellites has the potential to dramatically improve the temporal and spatial sampling of all-sky microwave observations by deploying a substantial constellation of satellites in low Earth orbit. Two constellations of 60 CubeSats in 550 km orbits are compared to the current operational microwave sensors. One approach employs all polar orbiters, while the other approach uses multiple inclination orbits for increased sampling over convective storm regions. Both approaches reduce average revisit times to approximately 20–30 min globally, and the multi-inclination approach also provides irregular 5–10 min sampling over selected latitudes. Improved global temporal sampling would provide all-sky observations to global forecast models over rapidly changing environments, while millimeter-wave observations over convective storm regions would be valuable for both forecasting and studying the development of convective storms. This article demonstrated that a constellation of low-cost CubeSats with microwave radiometers has the potential to provide equivalent temporal resolution to that observed from sensors on geostationary orbit .

This article studies the problem of analyzing the coverage capacity of satellite constellations with respect to ground regions. The coverage characteristics and formulations are first investigated, and a corresponding symbol system is established. On this basis, the coverage state function and the corresponding extension and intermediate modes for basic coverage analysis problems are defined. By introducing the parameter wildcard characters, the properties of three basic spatial and temporal factors are described. An integrated coverage theory system is constructed accordingly to formalize the general coverage analysis problems in a uniform way, which is versatile and works on representing any type of coverage analysis problems. Additionally, in accordance with the properties of the coverage theory, a boundary function is proposed. The corresponding algorithm framework is also constructed for the calculation of complicated practical coverage analysis problems. Experiments simulating different types of coverage analysis problems are carried out, and results reveal the correctness and the effectiveness of the proposed coverage theory system and algorithm framework. [ABSTRACT FROM AUTHOR]

Digital connectivity has become the foundation of prosperity and an essential need for functioning societies. Despite this dependence, limitation on Internet access remains a prevalent issue, largely hinging on socioeconomic and geographic factors. A promising solution to attain global access equality is based on integrated terrestrial-satellite networks that rely on low Earth orbit (LEO) mega constellations. While the benefits of LEO constellations complement the shortcomings of terrestrial networks, their incorporation impacts the network design, adding complexity and challenges. This article presents a systematic analysis of next generation LEO mega satellite constellations, outlining opportunities by virtue of the many benefits these constellations can provide and highlighting the major challenges. Furthermore, it provides a synopsis of analytic models and underscores modern simulation approaches for next generation mega satellite constellations. This article provides network designers with the necessary insights into satellite network performance. [ABSTRACT FROM AUTHOR]

Several billion people currently lack reliable access to the Internet and thus to a tremendous source of knowledge. In this article, we describe a field-tested communication approach combining CubeSat platforms and delay-tolerant networking (DTN) solutions to provide asynchronous connectivity to populations and regions that are underserved by the Internet. The resulting class of networks is known as ring road networks (RRNs), a networking approach that is built on technology developed for the construction of a solar system Internet. The necessary self-sufficiency of DTN nodes enables network access to be deployed incrementally at low cost, supporting communities that cannot be profitably served by Internet satellite constellations. We present the RRN architecture and evaluate the expected performance by means of simulations. Based on the latter, we discuss µD3TN: a lightweight and open source DTN protocol stack for RRNs and other DTN classes. µD3TN has been flight-tested in ESA's OPS-SAT in low Earth orbit during December 2020 and May 2021. This work discusses the experiment results as we validated the RRN approach in concrete application use cases. The reported outcomes motivate a new application domain. [ABSTRACT FROM AUTHOR]

Mega-LEO satellite constellations are becoming a concrete reality. Companies such as SpaceX, Virgin Orbit, and OneWeb have already started launching hundreds of LEO satellites and are turning their services on. Even if the aim of such LEO satellite constellations is just, for now, to offer worldwide Internet access equality, their deployment proves their feasibility and suggests usefulness for further purposes. In this article, we shed some light on the possible integration of the in-network computing paradigm in mega-LEO satellite constellations. Terrestrial and/or non-terrestrial nodes can benefit from offloading the computing to an orbital edge (OE) platform reachable through the satellite constellation, exploiting its fast and distributed computational capability. In this context, a preliminary analysis highlights that task offloading strategies can lead to performance improvements that open up novel challenges in the design and setup of OE platforms. [ABSTRACT FROM AUTHOR]

The rapid progress in space exploration, mining, and tourism has been fuelled by both public and private sector investments. The latter has led to the need to reduce manufacturing and launch cost of space hardware to create a competitive and sustainable space economy. A major step in making space accessible is to develop affordable power systems for "commercial space" use. Photovoltaics has in the past and will in the future be a key component. Metal halide perovskite solar cells show the greatest potential of all emerging technologies for low‐cost space photovoltaics. They have demonstrated the highest rate of power conversion efficiency improvement. Compared to the triple junction III–V compound semiconductor cells commonly used for space applications, perovskite cells have a higher power to weight ratio and are significantly cheaper to be manufactured. They have high radiation tolerance and can be fabricated onto flexible substrates for expand‐on‐demand solar panels. This paper outlines the major space markets for photovoltaics, and research and development opportunities for perovskite space solar cells in the context of their recent progress. [ABSTRACT FROM AUTHOR]

Driven by the ever increasing penetration and proliferation of data-driven applications, a new generation of wireless communication, the sixth generation (6G) mobile system enhanced by artificial intelligence, has attracted substantial research interests. Among various candidate technologies of 6G, low Earth orbit (LEO) satellites have appealing characteristics of ubiquitous wireless access. However, the costs of satellite communication (SatCom) are still high, relative to their counterparts of ground mobile networks. To support massively interconnected devices with intelligent adaptive learning and reduce expensive traffic in SatCom, we propose federated learning (FL) in LEO-based satellite communication networks. We first review the state-of-the-art LEO-based SatCom and related machine learning (ML) techniques, and then analyze four possible ways of combining ML with satellite networks. The learning performance of the proposed strategies is evaluated by simulation and results reveal that FL-based computing networks improve the performance of communication overheads and latency. Finally, we discuss future research topics along this research direction. [ABSTRACT FROM AUTHOR]

The network control plays a vital role in the mega satellite constellation (MSC) to coordinate massive network nodes to ensure the effectiveness and reliability of operations and services for future space wireless communications networks. One of the critical issues in satellite network control is how to design an optimal network control structure (ONCS) by configuring the least number of controllers to achieve efficient control interaction within a limited number of hops. Considering the wide coverage, rising capacity, and no geographical constraints of space platforms, this paper contributes to designing the ONCS by constructing an optimal space control network (SCN) to improve the temporal effectiveness of network control. Specifically, we formulate the optimal SCN construction problem from the perspective of satellite coverage factors, and apply geometric topology analysis to derive both the conditions for constructing the optimal SCN and the formulaic conclusions for SCN and MSC configurations (i.e., scale and structure). From numerical results, we investigate the tradeoff between network scale, the number of controllers, and control delays in several satellite network control scenarios, to provide guidelines for the MSC control. We also design the optimal SCN for an existing MSC system to demonstrate the effectiveness of the proposed ONCS. [ABSTRACT FROM AUTHOR]

Offering space-based Internet services with mega-constellations of low Earth orbit (LEO) satellites is a promising solution to connecting the unconnected. It can complement the coverage of terrestrial networks to help bridge the digital divide. However, there are challenges from operational obstacles to technical hurdles facing the development of LEO satellite access. This article provides an overview of the state of the art in LEO satellite access, including the evolution of LEO satellite constellations and capabilities, critical technical challenges and solutions, standardization aspects from 5G evolution to 6G, and business considerations. We also identify several areas for future exploration to realize tight integration of LEO satellite access with terrestrial networks in 6G. [ABSTRACT FROM AUTHOR]

The internet of satellites (IoS), containing multiple low earth orbit (LEO) satellite constellations, can support tremendous traffic, massive connectivity, and vast coverage. But it also puts forward higher demands for resource management due to the high dynamics of the IoS networks, especially in the malicious jamming environment, where the jammers launch jamming attacks to reduce the efficiency and reliability. Thus, this paper investigates the problem of resource management in malicious jamming environment for IoS, which is divided into three sub-problems: traffic prediction problem, anti-jamming decision problem and resource matching problem. To solve these problems, we proposed a distributed resource management framework (DRMF), which consists of three sub-algorithms. Firstly, the traffic prediction algorithm (TPA) is proposed to deeply mine and accurately predict the traffic rule. Meanwhile, the dynamic anti-jamming algorithm (DAA) is developed to make anti-jamming decision autonomously. Then, based on the outputs obtained by TPA and DAA, the distributed resource matching algorithm (DRMA) is proposed for IoS, and the satellites with insufficient resource can apply for assistance from neighboring satellites with excess resource, thereby improving the safety and efficiency of the entire IoS network. Finally, experiment results and algorithm analysis verify the proposed scheme has better performance than the existing algorithms. [ABSTRACT FROM AUTHOR]

The advent of CubeSats has empowered the vision of using a satellite constellation to provide seamless Internet coverage across the globe. This vision, known as the Internet of Space (IoS), is the key enabler of the Internet of Things (IoT). One of the major challenges in making IoS a reality is the development of an antenna system that can sustain a high data-rate link while being amenable to integration with the small CubeSat form factor. This challenge is further magnified by the requirement that the antenna dynamically reconfigures its radiation pattern to illuminate geographical areas that demand more coverage. The aim of this article is to provide an overview of antenna concepts developed by representative researchers that can overcome these challenges. If properly tailored for specific CubeSat applications, these concepts can make CubeSat-based IoS a realizable vision. We also highlight the open challenges and technology gaps that future research must address to make IoT a reality. [ABSTRACT FROM AUTHOR]

A cooperative blind spectrum detection with the Doolittle decomposition and support vector machine (SVM) classification is proposed to improve the performance and reduce the complexity and latency of spectrum detection in hybrid Geostationary Earth Orbit and Low Earth Orbit (GEO-LEO) satellite constellation networks. First, a principal component analysis (PCA) algorithm is adopted to process perceive signals. Second, the maximum-to-minimum eigenvalue ratio of a covariance matrix is utilized to construct statistics with self-learning ability by using the Doolittle decomposition. Third, primary user and noise signals are labeled with "+1" and "$-$1" as training labels, respectively. Finally, a high-performance PCA-SVM classification is generated by training the labels and the corresponding statistics. Innovations mainly include the Doolittle decomposition of the covariance matrix in the SVM and the PCA to preprocess the perceived signals. Simulation results show that the detection probability of the proposed scheme outperforms the energy detection and the SVM algorithm by 55 and 15% at a signal-to-noise ratio (SNR) of $-$15 dB, respectively. The average error probability of the proposed scheme is 20% lower than that of the SVM at an SNR of $-$20 dB. Therefore, in hybrid GEO-LEO satellite constellation networks, the cooperative blind spectrum detection with the Doolittle decomposition and SVM classification can detect the spectrum with low complexity and high performance effectively. [ABSTRACT FROM AUTHOR]

Recently, the ultra-dense low Earth orbit (LEO) satellite constellation over high-frequency band has served as a potential solution for high-capacity backhaul data services. In this paper, we consider an ultra-dense LEO-based terrestrial-satellite network where terrestrial users can access the network through the LEO-assisted backhaul. We aim to minimize the number of satellites in the constellation while satisfying the backhaul requirement of each user terminal (UT). We first derive the average total backhaul capacity of each UT, based on which a three-dimensional constellation optimization algorithm is proposed to minimize the number of satellites in the constellation. Simulation results verify our theoretical capacity analysis and show that for any given coverage ratio requirement, the corresponding optimized LEO satellite constellation can be obtained by the proposed three-dimensional constellation optimization algorithm. Given the same number of deployed LEO satellites, the average coverage ratio of the proposed LEO satellite constellation is at least 10 percentage points higher than that of Telesat constellation. [ABSTRACT FROM AUTHOR]

Several massive satellite constellations are currently being deployed to complement evolving terrestrial communication networks. Low Earth orbits (LEO) are selected for these constellations due to their relative low latency and reduced path-loss where each satellite dwells for a very short period of time within the user’s field-of-view. This inherent nature prompts frequent handovers between the satellites and thus causing an increased signaling overhead and possible service interruptions. This letter lays a tractable approach for characterizing satellites’ pass-duration using analytic tools, where it captures the satellites positions as a spherically wrapped homogeneous point process and accordingly derives the statistical distribution of the pass-duration. The letter further compares the presented results with well-designed constellations based on Walker-delta pattern. The obtained mean pass-duration is shown to follow a simple dependency on the altitude and provides a close approximation for practical constellation deployments. [ABSTRACT FROM AUTHOR]

This is the first of two companion papers that investigate the operations of distributed satellite systems. This first article presents a survey of conventional methods of operations of spacecraft constellations, investigates its scalability for growing number of spacecraft, and identifies operational paradigm shifts. The second article focuses on the classification of distributed satellite systems and evaluates commercial tools for automated spacecraft operations. The trend of using distributed space systems such as satellite constellation instead of monolithic systems has been growing in the last decade. Recently, a variety of large satellite constellations were announced and the production of some has started. Several of these announced constellations feature more than 1000 satellites. While the "mass production" of satellites is feasible and has already started, there are no effective solutions existing for the "mass operations" of satellites. In some instances, conventional spacecraft operations involve manual control by skilled human operators, following at least a 4-eyes principle. Even when operators batch multiple telecommands together, the scheduling process is still challenging for growing spacecraft numbers. This approach is not (linearly) scalable to large satellite constellations: new operational methods need to be established and the automation level of the constellation increased. To motivate the research activities in this framework and pave the way for automated management of large distributed satellite systems, this paper gives an overview of some conventional methods of spacecraft operations. From this description, the weakness areas in terms of scalability are deduced, identifying potential bottlenecks for the operations of such systems. Following, based on three use case studies, the operational paradigm shifts related to the operation of large distributed satellite systems are identified. [ABSTRACT FROM AUTHOR]

Recent developments have seen a trend towards larger constellations of spacecraft, with some proposals featuring constellations of more than 10.000 satellites. While similar concepts for large constellations already existed in the past, traditional satellite deployments hardly ever feature groups of more than 100 satellites. This trend towards considerably larger satellite numbers originates from non-traditional design and operations of spacecraft by non-traditional space companies. The evolution in the space sector, precipitated by new players, is often referred to as "Space 4.0" or "New Space". It necessitates a rethinking of the way satellites and satellite constellations are planned, designed, and operated. New operational paradigms are needed to enable automatic, optimal task definition, and scheduling in a holistic approach. This is the second of two companion papers that investigate the operations of distributed satellite systems. This second article investigates the classification of distributed satellite systems and evaluates commercial tools for automated spacecraft operations, whereas the first article performed a survey of conventional and "new space"operations of spacecraft constellations. Classification metrics for constellations are derived and evaluated with respect to their informative value concerning the operation, the automation, and the scalability of the constellation. The proposed classification system is applied to the Dove and RapidEye constellation and allows for a comparison between the presented automation approaches. Commercial tools for automated spacecraft operations are evaluated for several mission task elements, such as orbit control, orbit maintenance, and collision avoidance. Subsequently, the trends, benefits, and standardization needs for operational automation are identified. [ABSTRACT FROM AUTHOR]