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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.

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.

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]

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]

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]

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]

Laser intersatellite links (LISLs) are envisioned between satellites in upcoming satellite constellations, such as the one in phase I of SpaceX's Starlink. Within a constellation, satellites can establish LISLs with other satellites in the same orbital plane (OP) or in different OPs. We present a classification of LISLs based on the location of satellites within a constellation and of LISLs' duration. Then, using the satellite constellation for phase I of Starlink, we study the effect that varying a satellite's LISL range has on the number of different types of LISLs it can establish with other satellites. In addition to permanent LISLs, we observe a significant number of temporary LISLs between satellites in crossing OPs. Such LISLs can play a vital role in achieving low-latency paths within next-generation optical wireless satellite networks. [ABSTRACT FROM AUTHOR]

Within the realm of satellite remote sensing, optimal data acquisition to study natural phenomena under time, resource, and cost constraints is a well-known problem. Furthermore, since the sensors themselves are at remote locations with sparse ground connectivity, the optimal method must use a computationally light forecasting algorithm, which assimilates information from the observations at possibly irregular intervals, in near real time. In this article, we propose and demonstrate the ensemble -guided cyclone track forecasting (EGCTF) method for application in remote tropical cyclone tracking. The algorithm uses ensemble data produced by numerical weather prediction models to guide the forecasting process while assimilating measured cyclone center positions. The algorithm was tested and analyzed with the Global Ensemble Forecasting System (GEFS) data and the National Hurricane Center data for the 2018 year hurricanes within the Atlantic basin. Compared with a baseline method that uses the GEFS-issued mean ensemble track (AEMN) for forecasting and no data assimilation, the proposed algorithm exhibited positive forecast skill for more than 290 test cases over forecast periods spanning 6–48 h. The skill is seen to improve with lengthening forecast periods, with five test cases showing greater than 75% skill for a forecast period of 6 h to 247 test cases for the forecast period of 48 h. [ABSTRACT FROM AUTHOR]

Free-space optical communications in space can realize broadband and large-capacity communications using smaller and lower-power-consumption onboard equipment. Recently, many mega-constellation systems have been planned and launched from various countries. Although the frequency spectra will soon be exhausted, optical communications have potentially wider bandwidths in the optical spectrum and are a viable alternative. Because optical communications do not have regulatory restrictions or licensing issues, this technology can revolutionize the space system architecture. This article describes the recent trends and future prospects of space laser communications. [ABSTRACT FROM AUTHOR]

The satellite tracking, telemetry, and command system, or TT&C, is utilized to transmit the telemetry and telecommand data, as well as to determine the satellite orbit. It is the lifeline of a satellite, and thus its safety and reliability are of fundamental importance to the satellite. After years of development, the traditional satellite TT&C technologies are relatively mature, while there are still many open problems to be solved for long distance deep space explorers and satellite mega-constellation. In this article, we start with a brief review regarding the evolution of the TT&C system applied in space missions. Then we raise the challenges and potential solutions for the TT&C system in solar system explorations and suggest a new networked TT&C system that is suitable for large-scale satellite constellation management tasks. In the end, some potential TT&C technologies are indicated considering the future technology. In general, the goal of this article is to provide an overview to understand the motivation, challenges, methodology, and solutions of the satellite TT&C system. [ABSTRACT FROM AUTHOR]

The rapid growth of maritime transportation and its regularization have imposed an increasing demand for developing a global maritime surveillance capacity. The existing cooperative maritime traffic monitoring relies on the shipborne automatic identification system (AIS), which is now confronting severe security threats due to the vulnerabilities of AIS at both the implementation and protocol levesl. In this article, the possibility of passive AIS signal localization using low Earth orbit satellite constellations is investigated, which provides a more reliable vessel position determination. To this end, the article first presents an overview on two categories of passive localization methods. Realizations of these methods on the satellite-based AIS emitter position determination are then demonstrated and compared. To evaluate the system performance, the lower bound for the localization error is also derived. Focusing on the system realization, some general design considerations and localization accuracy optimization are further discussed. [ABSTRACT FROM AUTHOR]

Advanced receiver autonomous integrity monitoring (ARAIM) is under development to support both lateral and vertical guidance. A key component of ARAIM is the integrity support message (ISM), which contains validated input parameters including the probabilities of satellite and constellation failure. A service commitment for the former is provided in the global positioning system (GPS) performance standards, whereas further commitments from GLONASS, Galileo, and Beidou are expected to follow. However, air navigation service providers may demand some flexibility in setting the ISM. This article presents a procedure for setting ISM parameters employing a means to determine the a priori probabilities using the Bayesian inference of a Poisson process model given in the literature. This method and a novel way to accumulate the service history are used to analyze the effects of satellite block and age. Results show that while the type and age of satellite are clearly drivers in the likelihood of a fault; such effects may be accounted for without significant impact on the broadcast values. This article also addresses the probability of unscheduled satellite outages using the new method. Finally, this article gives a high-level scheme of how the validity of the ISM parameters may be assured. [ABSTRACT FROM AUTHOR]

Research on LEO inter-satellite laser communication based on wavelength division multiplexing inter-satellite links and its routing and wavelength assignment (RWA) technology is an important way to solve the problem of low-latency, high-capacity, high-speed transmission, and low-cost on-orbit real-time routing of free-space optical communications. Since both the transmitting and receiving sides of the inter-satellite communication are high-speed moving satellites, the network topology in the whole inter-satellite communication process is time-varying. Therefore, based on this particularity, we propose the SpacePro constellation and combined the classical NeLS constellation for mathematical modeling, and the network topology and inter-satellite links are predicted and analyzed in their entire cycles. The RWA under the super satellite nodes with computation and storage functions algorithm (SCSA) is proposed and its effect is compared and analyzed in the wavelength resource requirement and connectivity. The results showed that the SCSA algorithm can save up to 50% in wavelength requirements, and the SpacePro constellation can achieve the same network connectivity effect with fewer on-board processors and wavelengths than NeLS. Satellite optical network node connectivity and wavelength demand are a trade-off problem. Studying the RWA problem in this scenario has a very critical reference for how the future satellite optical network performs wavelength usage configuration to help improve the performance of networks. [ABSTRACT FROM AUTHOR]

This article introduces a chip-scale ultralow-power atomic clock (ULPAC) in the microwave frequency region. A new suspended quantum package architecture along with a fully integrated frequency probing and locking loop implemented in CMOS technology results in a compact package and ultralow-power consumption. In addition, dedicated low-noise magnetic field and temperature control loops are incorporated for isolating and mitigating the internal and external factors affecting the frequency stability. The output frequency of a crystal oscillator is continuously compensated and stabilized by locking to the peak of a coherent population trapping signal, thereby inheriting the superior frequency stability of the atomic resonance. The proposed ULPAC system achieves a frequency stability of $2.2 \times 10^{-12}$ at an averaging time of 105 s, while consuming 59.9 mW. The frequency probing and locking circuits implemented in a standard 65-nm CMOS process node occupy an area of 2.55 mm2. The prototype of this atomic clock occupies a volume of 15 cm3. [ABSTRACT FROM AUTHOR]

A method is investigated to use secondary electrons to touchlessly sense the electrostatic potential of an object in geosynchronous orbit or deep space. This method involves a positively charged servicing craft, which directs a high-energy electron beam at the object of interest such that low-energy secondary electrons are emitted from the surface. The low-energy electrons emitted by the target are accelerated toward the servicing craft and arrive with an energy equal to the potential difference between the two crafts. The servicing craft measures the electron energy spectrum and, knowing its own potential, then infers the potential of the target. Depending on the application, photoelectrons could similarly be used to infer the target potential without the need for an active electron beam. Though it is possible to measure potential by directly contacting a surface, remote measurement offers significant advantages and supports missions which must operate in close proximity without making physical contact. Several missions have been proposed that use interactions between charged objects to create useful forces and torques, including electrostatic detumbling and reorbiting of debris, Coulomb formations, and virtual structures. Remote measurement of potential would benefit these missions by enabling feedback control of the active charging. Other applications include mitigation of arcing during rendezvous, docking, and proximity operations for future servicing or salvaging missions. [ABSTRACT FROM AUTHOR]

In this paper, we address a problem of designing satellite constellations to deal with a sequence of Earth revisit missions in the presence of limited resource capabilities. In order to fulfill the periodic coverage requirements, we take the repeating ground-track orbit into account and propose a semi-analytical method. After analyzing the coverage capacity of the resource for a random spot target on Earth’s surface, we define and calculate the feasible longitude interval of ascending node of the orbit. Subsequently, a mixed integer linear programming (MILP) model and an improved 0/1 programming model are formulated with coverage constraints that considerably reduce the size of the search space. We demonstrate that the new method significantly improves the optimization efficiency and verify the robustness of the solution. [ABSTRACT FROM PUBLISHER]

With the rapid development of low Earth orbit (LEO) constellation satellite networks and on-board processing, in-network content caching becomes feasible, which can further improve the system performance. In order to minimize user terminals content access delay, a novel caching algorithm by optimizing content placement in LEO satellite constellation networks is proposed in this letter. The formulated problem is proved to be NP-complete (NPC) by reducing it to the 2-disjoint set cover problem, which is a known NPC problem. This letter considers the interactions among distributed satellites for making individual content decisions. Then, this letter adopts an exchange-stable matching (ESM) algorithm based on many-to-many matching game with externalities. Simulation results show that the ESM algorithm can improve the system performance in terms of content access delay. [ABSTRACT FROM PUBLISHER]

The new generation of space-based services includes large-scale, integrated, and distributed informational systems for which traditional system engineering approaches show some limits in delivering the “big picture.” Missing the view of the full range of design options, or prematurely translating the perceived stakeholders needs into design requirements, is often a consequence of insufficient regard to the end-users priorities. Objective of the present research is to bring to light the gaps extant between what system architects prioritize, and the preferences of potential system users. To this purpose, the proposed method aims at incorporating lifecycle properties (–ilities) in the concept design phase, by submitting attributes of these properties for the evaluation of two stakeholders representative groups. The case study refers to the integration of environmental measurements, coming from a global-navigation-satellite-systems-based remote sensing satellite constellation, as complementary data to the traditional weather-forecasting service, resulting in a new system of systems. The method runs through an interview-based quality function deployment process and collaborative sessions of teams of stakeholders. The strength of the formulation relies on the ability to treat a quantitative measure of the gaps extant between system desired capabilities as perceived by architects, and real end-user needs. The method can be potentially tested in a concurrent design environment as a complementary tool for eliciting requirements and suggesting the areas where investments and resources should be preferably allocated. Results can be used by researchers as pieces of knowledge to be further investigated, and by practitioners in development projects, taking into account that they are preliminary findings. [ABSTRACT FROM PUBLISHER]

During the construction of satellite constellation, communication relay service could be provided by a partially constructed constellation through buffering data and forwarding it at a later time. However, this will introduce extra end-to-end delay during the construction period. We propose a scheme to determine an optimal construction process, which is used to insert satellites into appropriate positions in each stage of construction. Simulation results show that our scheme can reduce the average end-to-end delay by 70.1% compared to the traditional scheme. [ABSTRACT FROM PUBLISHER]

This paper presents a novel solution to the problem of autonomous task allocation for a self-organising satellite constellation in Earth orbit. The method allows satellites to cluster themselves above targets on the Earth?s surface. This is achieved using coupled selection equations (CSEs), a dynamical systems approach to combinatorial optimisation whose solution tends asymptotically towards a Boolean matrix describing the pairings of satellites and targets, which solves the relevant assignment problems. Satellite manoeuvres are actuated by an artificial potential field method which incorporates the CSE output. Three demonstrations of the method?s efficacy are given, first with equal numbers of satellites and targets, then with a satellite surplus, including agent failures, and finally with a fractionated constellation. Finally, a large constellation of 100 satellites is simulated to demonstrate the utility of the method in future swarm mission scenarios. The method provides efficient solutions with quick convergence, is robust to satellite failures, and hence appears suitable for distributed, on-board autonomy. [ABSTRACT FROM AUTHOR]

Delay-tolerant networks (DTNs) have become a promising architecture for wireless sensor systems in challenged communication environments where traditional solutions based on persistent connectivity either fail or show serious weaknesses. As a result, different routing schemes have been investigated that take into account the time-evolving nature of the network topology. Among them, contact graph routing has been proposed for space environments with predictable connectivity. In order to evaluate routing decisions, DTN nodes need to know the contact plan in advance, which comprises all communication links among nodes that will be available in the future. Since not all potential contacts can belong to the contact plan, its design requires analyzing conflicting contacts in order to select those that meet an overall goal. In this paper, we consider the design of contact plans that can maximize fairness requirements while still maximizing the overall capacity as well. To this end, we propose to formalize the problem by means of an optimization model and evaluate its performance in terms of different fairness metrics. Since this model can be computationally intractable for a large number of contacts, we also propose to tackle it as a matching problem, resulting in algorithms of polynomial complexity, and compare these results with those of the original model. We show that fairness can be properly modeled to design contact plans and that efficient algorithms do exist to compute these plans quite accurately while also improving overall network routing metrics for a proposed case study. [ABSTRACT FROM PUBLISHER]

This paper introduces a new approach in the design of optimal satellite constellations comprising few satellites and characterized by symmetric distribution. Typical examples are missions requiring persistent observation of sites or regions for intelligence or science. To minimize the number of satellites, the mathematical necklace theory is applied to the two-dimensional Lattice Flower Constellation framework. The necklace theory identifies in the phasing space all satellite subsets characterized by symmetric distributions. Mathematically, these subsets are parameterized by necklaces, identifying the actual satellite locations in the first orbital plane and a shifting parameter governing phasing between subsequent orbital planes. This article specifically targets the emerging importance of small satellite formations and constellations. [ABSTRACT FROM AUTHOR]

The passive nature of power distribution networks has been changing to an active one in recent years as the number of small-scale Distributed Generators (DGs) connected to them rises. The consensus of recent research is that current slow central network control based upon Supervisory Control and Data Acquisition (SCADA) systems is no longer sufficient and Distributed Network Operators (DNOs) wish to adopt novel management mechanisms coupled with advanced communication infrastructures to meet the emerging control challenges. In this paper, we address this issue from the communication perspective by exploiting the effectiveness of using a Low Earth Orbit (LEO) satellite network as the key component of the underlying communication infrastructure to support a recently suggested active network management solution. The key factors that would affect the communication performance over satellite links are discussed and an analytical LEO network model is presented. The delivery performance of several major data services for supporting the management solution is evaluated against a wide range of satellite link delay and loss conditions under both normal and emergency traffic scenarios through extensive simulation experiments. Our investigation demonstrates encouraging results which suggests that a LEO network can be a viable communication solution for managing the next-generation power energy networks. [ABSTRACT FROM PUBLISHER]

In this paper, the design of a minisatellite FLOwer constellation (FC), deploying millimeter-wave (MMW) scanning RADiometers, namely, FLORAD, and devoted to tropospheric observations, is analyzed and discussed. The FLORAD mission is aimed at the retrieval of thermal and hydrological properties of the troposphere, specifically temperature profile, water-vapor profile, cloud liquid content, and rainfall and snowfall rate. The goal of frequent revisit time at regional scale, coupled with quasi-global coverage and relatively high spatial resolution, is here called pseudogeostationary scale and implemented through a FC of three minisatellites in elliptical orbits. FCs are built on compatible (resonant) orbits and can offer several degrees of freedom in their design. The payload MMW channels for tropospheric retrieval were selected following the ranking based on a reduced-entropy method between 90 and 230 GHz. Various configurations of the MMW radiometer multiband channels are investigated, pointing out the tradeoff between performances and complexity within the constraint of minisatellite platform. Statistical inversion schemes are employed to quantify the overall accuracy of the selected MMW radiometer configurations. [ABSTRACT FROM AUTHOR]