Satellite Research Articles

Gravitational wave measurement in the mid-band with atom interferometers

Gravitational Waves (GWs) have been detected in the ∼ 100 Hz and nHz bands, but most of the gravitational spectrum remains unobserved. A variety of detector concepts have been proposed to expand the range of observable frequencies. In this work, we study the capability of GW detectors in the “mid-band”, the ∼ 30 mHz– 10 Hz range between LISA and LIGO, to measure the signals from and constrain the properties of ∼ 1 – 100 M ⊙ compact binaries. We focus on atom-interferometer-based detectors. We describe a Fisher matrix code, AIMforGW, which we created to evaluate their capabilities, and present numerical results for two benchmarks: terrestrial km-scale detectors, and satellite-borne detectors in medium Earth orbit. Mid-band GW detectors are particularly well-suited to pinpointing the location of GW sources on the sky. We demonstrate that a satellite-borne detector could achieve sub-degree sky localization for any detectable source with chirp mass ℳ c ≲ 50 M ⊙. We also compare different detector configurations, including different locations of terrestrial detectors and various choices of the orbit of a satellite-borne detector. As we show, a network of only two terrestrial single-baseline detectors or one single-baseline satellite-borne detector would each provide close-to-uniform sky-coverage, with signal-to-noise ratios varying by less than a factor of two across the entire sky. We hope that this work contributes to the efforts of the GW community to assess the merits of different detector proposals.

Modeling Interference From Millimeter Wave and Terahertz Bands Cross-Links in Low Earth Orbit Satellite Networks for 6G and Beyond

Modeling Interference From Millimeter Wave and Terahertz Bands Cross-Links in Low Earth Orbit Satellite Networks for 6G and Beyond

The system design of high-throughput miniature software-defined radio as a Ka/K-band communication payload for CubeSats

Ka-Band communication payloads, enabling high-rate data transmission, typically require higher power consumption, are mostly more suitable for larger satellites. This research focuses on the design of a miniature software-defined radio (MSDR) specifically tailored for Ka-band communication onboard CubeSats. The communication system aligns with the DVB-S2 standard for high transmission capacities, and incorporates the linear-step frequency modulation (LSFM) for robust transmission. Additionally, the modular software design can be easily adopted for future missions. The proposed Xilinx MPSoC-based SDR opens possibilities of broadband communication for CubeSat constellations in low Earth orbits (LEOs).

Improved GRACE-FO kinematic orbit determination with epoch-difference KBR during the GPS flex power period

The flex power technology in satellite navigation systems enhances anti-jamming capabilities but can impact the quality of GPS observations and the accuracy of low Earth orbit determination, such as GRACE Follow-On (GRACE-FO) mission. This study investigates the influence of GPS flex power on Hatch-Melbourne-Wübbena (HMW) linear combinations and GRACE-FO kinematic orbits from January 1 to September 30, 2020. Epoch-differenced K-Band Ranging (KBR) data is introduced in orbit determination during the flex power period to improve both absolute and inter-satellite relative accuracy. The analysis indicates that the influence of early flex power (before February 13, 2020) on HMW combinations and orbits is minimal, whereas the effect of later flex power (after February 14, 2020) is significant: (1) HMW combinations exhibit notable systematic discontinuities even with elevation angles greater than 50 degrees, causing the fixing rate of wide-lane ambiguities to drop from 96 % to 80 %. (2) Kinematic absolute orbits show significant deteriorations of approximately 9 mm and 4 mm in the three-dimensional direction for float and integer ambiguity resolution (FAR and IAR), while relative accuracy of FAR and IAR orbits decreases by 50 % and 46 %, respectively. However, using epoch-differenced KBR (DKBR) data, the accuracy of absolute orbits could be increased by up to 15 % and the accuracy of relative orbits could be improved by at least 69 %, which showcases a positive effect. Thus, this can be considered as an alternative method to improve the accuracy of GRACE-FO orbit during the flex power period.

Comparing shallow landslide susceptibility maps in Northeastern Türkiye (Beşikdüzü, Trabzon): a multivariate statistical, machine learning, and physical data-based analysis

This study endeavors to assess and compare the efficacy of various modeling approaches, including statistical, machine learning, and physical-based models, in the creation of shallow landslide susceptibility maps within the Besikduzu district of Trabzon province, situated in the Black Sea Region of Türkiye. The landslide inventory data, spanning from 2000 to 2018, was acquired through meticulous field surveys and analysis of Google Earth satellite imagery. Key topographic and geologic input parameters, such as slope, aspect, topographic wetness index, stream power index, plan and profile curvature, and geologic units, were extracted from a high-resolution 10 m spatial DEM (Digital Elevation Model) and a 1:25,000 scaled digital geology map, respectively. Additionally, soil unit weight and shear strength parameters, critical for the physical-based model, were determined through field samples. To evaluate landslide susceptibility, logistic regression, random forest, and Shalstab were employed as the chosen methods. The accuracy of susceptibility maps generated by each method was assessed using the area under the curve method, yielding impressive values of 0.99 for the random forest model, 0.97 for the logistic regression model, and 0.93 for the Shalstab model. These results underscore the robust performance of all three methods, suggesting their applicability for generating shallow landslide susceptibility maps not only in the Black Sea Region but also in analogous areas with similar geological characteristics.

The engineering face-lift of the 32m dish: technical upgrades of Ghana Radio Astronomy Observatory (GRAO)

Ghana's 32-meter radio telescope, inaugurated in August 2017, was once a redundant telecommunications dish that underwent conversion. Prior to this transformation, feasibility studies were conducted to assess the dish's structural integrity, technical compatibility, and economic viability. These studies aimed to determine if the conversion project could be technically achieved given the available technology, expertise, and resources. This paper delves into the engineering considerations surrounding structural, mechanical, software, control and monitoring, radio frequency, and timing frequency reference requirements that distinguish the operation of a radio telescope from its former role as a satellite earth station. Significant components such as the azimuth bearing, sub-reflector support, cable wrap, and electrical motors underwent replacement. Additionally, a new C-band receiver, radio frequency controller, active hydrogen maser timing frequency, and software were developed. Testing protocols to meet science requirements for both single-dish observations and Very Long Baseline Interferometry (VLBI) are also discussed. The conversion process proved to be lengthy and encountered numerous unforeseen circumstances, yet it provided invaluable learning experiences for a developing country like Ghana.

Statistical Reliability Analysis of Interplanetary Spacecraft Operating at Different Interplanetary Extremity

The success of spacecraft depends on their reliability which is influenced by the space environment complexity. This study examines the reliability of interplanetary spacecraft in different regions. We focused on spacecraft in interplanetary space, specifically within the interplanetary boundary. Our analysis excluded planetary landers, probes, and satellites in Earth orbit. 131 spacecraft were studied, with data on launch mass and lifespan recorded. Non-parametric analysis was initially conducted, followed by parametric analysis using the Weibull Distribution. Results showed higher reliability for spacecraft beyond Earth and Mars extremities. Analyses were also done based on spacecraft mass categories to validate reliability effects. Various factors affecting reliability were discussed, including design, integration processes, testing methodologies, and mass constraints.

Low Earth Orbit Satellite Network Routing Algorithm Based on Graph Neural Networks and Deep Q-Network

Low Earth orbit (LEO) satellite networks are characterized by rapid topological changes, numerous network nodes and varying states of node resource constraints, which have resulted in traditional routing algorithms no longer being suitable for LEO satellite network routing. Therefore, this paper proposes an inductive learning architecture based on Graph Sample and Aggregate (GraphSAGE), which can significantly reduce the number of topology nodes to be trained, thereby reducing the computational complexity of the nodes. Then deep reinforcement learning (DRL) is employed for the continuous learning optimization of routing algorithms, and its generalization is improved by selecting GraphSAGE to construct the DRL agent. In the proposed graph neural-network-based routing optimization algorithm for LEO satellite networks, each Deep Q-Network (DQN) agent independently generates the hidden states of the nodes through the GraphSAGE model and uses them as inputs to the DRL model to make routing decisions. After a simulation and comparison, the proposed algorithm not only improves the overall network throughput, but also reduces the average end-to-end delay. The average throughput of the proposed algorithm increases by 29.47% and 18.42% compared to that of Dijkstra and the DQN, respectively. The average end-to-end delay is reduced by 39.76% and 15.29%, respectively, and can also adapt to changing topologies.

Energetic Neutral Atoms Detected in the 2022 February 15 Solar Energetic Particle Event

Energetic neutral atoms (ENAs) are expected to be produced near the Sun during large solar energetic particle (SEP) events. However, their detection by SEP instruments near 1 au has been limited. The clearest reported measurement has been from the Solar Terrestrial Relations Observatory (STEREO) during the 2006 December 5 SEP event. Additional evidence of ENAs has been found through reanalysis of observations by the Solar Anomalous and Magnetospheric Particle Explorer obtained near the equator in low Earth orbit and associated with several large X-ray flares and fast coronal mass ejections (CMEs). Here we describe another detection of ENAs from the STEREO Low Energy Telescope associated with the large 2022 February 15 SEP event. Given the timing and spectrum of the ENAs and the location of the source region (behind the east limb from STEREO’s viewpoint), these ENAs are most likely a result of acceleration by a CME-driven shock when the CME was at approximately 2–3 R S. The possibility of a postflare loop origin is considered unlikely.

Demand and key technology for a LEO constellation as augmentation of satellite navigation systems

A Low Earth Orbit (LEO) constellation augmenting satellite navigation is important in the future development of Global Navigation Satellite System (GNSS). GNSS augmented by LEO constellations can improve not only the accuracy of Positioning, Navigation, and Timing (PNT), but also the consistency and reliability of secure PNT system. This paper mainly analyzes the diverse demands of different PNT users for LEO augmented GNSS, including the precision demand in real-time, the availability demand in special areas, the navigation signal enhancement demand in complex electromagnetic environments, and the integrity demand with high security. Correspondingly, the possible contributions of LEO constellations to PNT performance are analyzed from multiple aspects. A particular attention is paid to the special PNT user requirements that cannot be fulfilled with existing GNSS, such as the PNT service demand in the polar regions and the onboard GNSS orbit determination demand of some LEO satellites. The key technologies to be considered in the constellation design, function realization, and payload development of the LEO-augmented navigation system are summarized.

International Law Review Of Space Debris Mitigation Efforts

Space debris has become an increasingly pressing issue in space exploration and utilization. This article conducts a comprehensive review of aspects of international law relating to space debris mitigation efforts. This research analyzes existing legal frameworks, including relevant multilateral treaties and international legal principles related to the space environment. With a focus on the principles of shared responsibility and avoidance of damage to the space environment, the study identifies key challenges in managing space junk and reducing the risk of collisions in earth orbit. An in-depth analysis of the role of national space agencies and international organizations in implementing mitigation measures was also carried out. In the context of international cooperation, this article evaluates existing collaborative efforts and proposes new measures to improve the effectiveness of space debris mitigation. A better understanding of legal responsibility and burden-sharing in addressing these issues became a central point in the study. By analyzing recent developments in space law and international environmental regulations, this article provides insight into the direction the global community might take in addressing future space debris mitigation challenges. It is hoped that this research can provide guidance for policymakers, academics, and legal practitioners in their efforts to maintain a clean and sustainable space environment.

Highly flexible porous indium-tin-oxide coating with enhanced atomic oxygen and electrostatic discharge resistance

Polyimides (PI) are commonly used in spacecraft in low Earth orbit (LEO) due to their flexibility. However, they are susceptible to erosion caused by atomic oxygen (AO) and electrostatic discharge (ESD) induced by plasma in the harsh LEO environment. Indium-tin-oxide (ITO), owing to its excellent electrostatic and AO resistance, has emerged as the most promising inorganic protective coating for PI in LEO applications. But the inherent rigidity of ITO and the high-temperature annealing process commonly required for synthesis make it challenging to employ ITO as flexible polymer substrates. In this work, we have synthesized the porous ITO layer with a low-temperature (≤150 °C) method and combined it with layer double hydroxides (LDHs) to enhance the flexibility, AO resistance, and ESD protection of the coating. The porous ITO in combination with LDHs layers effectively improve the flexibility (after 20,000 bending cycles without failure problem), AO resistance (erosion yield down to -0.102 × 10−26 cm3/atom under an exposure dose of 5.22 × 1021 atoms/cm2) and ESD resistance properties (sheet resistance around 202 kΩ/sq). The synthesis method proposed avoids the high-temperature annealing and the residual stress effect on the coating's flexibility. This work provides an efficient approach to realizing flexible and protective coating on polymers for long-lasting spacecraft electronic devices in low Earth orbit.

Comprehensive modeling and characterization of the General-Purpose Heat Source Radioisotope Thermoelectric Generator for solar system missions

The radioisotope thermoelectric generator relies solely on radioactive decay for its energy and generates power through the thermoelectric effect. The General-Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG) represents the RTG with the largest power and highest conversion efficiency ever built, and its energy supply supported four daring interplanetary missions (Galileo, Ulysses, Cassini, New Horizons). To explore the use potential of the GPHS-RTG in the solar system, its conservative application range and performance should be determined. In this work, a comprehensive model of the GPHS-RTG was developed, and its calculated results agreed well with Lockheed Martin’s test report (temperature relative error < 1 %). The study demonstrates that within the Earth orbit, the distance of the mission area from the sun and the angle of incidence sunlight significantly affect the thermoelectric performance and hot-junctions’ temperature. Moreover, conservative application ranges and their extending methods (changing coating material, thermal loading, and operational voltage) for GPHS-RTG at different attitudes in the solar system were determined for the first time by comparing the proportion of the hot-junction over-temperature area. The temperature distribution and its cause of formation in the axial and circumferential directions of the GPHS-RTG during solar system missions were revealed in the end. This work is valuable for future RTG design and mission planning.

The investigation of real-time LEO timing via GPS PPP with different RT products

Low Earth Orbit (LEO) satellites mostly utilize crystal oscillators instead of high-performance atomic clocks mounted on medium to high-orbit satellites. Therefore, keeping high-precision time synchronization between LEO and GNSS satellites is the premise of realizing the high-precision application of LEO satellites. In this contribution, GPS precise point positioning (PPP) technology was employed to release precise timing for LEO satellites with different Real-time (RT) products. GRACE-FO C and GRACE-FO D satellites and five precise RT products from different analysis centers, which are CNES, GFZ, GMV, WHU, and CAS were chosen to achieve precise timing. The results indicated that the overall trend of clock difference fluctuations presented with all real-time products is generally consistent, fluctuating within the range of ± 4 ns. Furthermore, real-time precise products from GMV and GFZ exhibit significant fluctuations during the daily initial time period. Additionally, real-time products from WHU outperform other products and demonstrate greater stability. In terms of frequency stability, the clock stability of various analysis centers is generally consistent, with short-term stability (1280 s) reaching approximately 10−13 and long-term stability (10240 s) reaching around 10−14. WHU's timing stability is notably superior to other real-time products. In the time synchronization experiment, real-time products fluctuate within the range of ± 2 ns, while IGS final products fluctuate within ± 1 ns. In terms of time synchronization stability comparison, short-term stability (1280 s) still fluctuates around 10−13, and long-term stability (10240 s) fluctuates around 10−14. WHU exhibits relatively good overall stability, while GFZ shows comparatively lower overall stability.

Blockchain-based secure communication of internet of things in space–air–ground integrated network

Under the Internet of Things (IoT) era, the cooperation of multiple service providers has facilitated the development of large-scale distributed IoT with a strong demand for reliable and real-time communications. The Space–Air–Ground Integration Network (SAGIN) can be seen as an extension of the traditional terrestrial network with open and ubiquitous connectivity that can be used to support the future IoT in the 6G era. However, the deployment of SAGIN is fraught with numerous challenges, particularly in terms of security. As an emerging technology, blockchain has received academic attention in mitigating the trust problem of SAGIN communication systems. In this paper, we introduce a decentralized data transmission framework that combines the characteristics of SAGIN and blockchain. This framework leverages the increasing computational and communication capabilities of low Earth orbit satellites. Subsequently, we provide an overview of the three-layer structure of this framework and detail the four working steps of the transmission mechanism. Simulation results show that our proposed scheme has excellent performance in terms of latency and throughput while ensuring the security of data transmission.

A Case Study of Ionospheric Storm‐Time Altitudinal Differences at Low Latitudes During the May 2021 Geomagnetic Storm

AbstractPrevious studies paid little attention to the ionospheric storm‐time altitudinal differences due to insufficiency of ionospheric measurements. In this work, multiple instrumental observations were used to investigate the ionospheric storm‐time response at low latitudes in the American and Asian‐Australian sectors during the May 2021 geomagnetic storm. The ground‐based Global Navigation Satellite Systems (GNSS) total electron content (TEC) and Low Earth Orbit (LEO) satellite topside TEC presented opposite (positive/negative) variations in the low‐latitude and equatorial region of both sectors during this storm. The electron density profiles from the Constellation Observing System for Meteorology, Ionosphere, and Climate‐2 (COSMIC‐2) and the Sanya Incoherent Scatter Radar showed a good agreement and well explained the opposite variations between GNSS TEC and LEO satellite topside TEC. The F2‐layer peak height (hmF2) and peak density (NmF2) displayed inverse variations, and the feature was present mostly in the regions between equatorial ionization anomaly crests. The combined modulation effects of the storm‐time zonal electric fields and the field‐aligned transports possibly resulted in the contrary variations of hmF2 and NmF2 in the low‐latitude and equatorial region, leading to the storm‐time altitudinal differences during this storm. Relatively, the storm‐time thermospheric composition disturbances might be a minor factor responsible for these differences.

Characterization of Relativistic Electron Precipitation Events Observed by the CALET Experiment Using Self‐Organizing‐Maps

AbstractRelativistic electron precipitation (REP) is a relatively high‐latitude phenomenon where high‐energy electrons trapped in the outer radiation belt are lost into the Earth’s atmosphere. REP events observed at low Earth orbit show varying temporal profiles and global distributions. While the precipitation origin has been attributed to specific wave modes or scattering sources, the sorting of REP events by type or driver remains an unsolved challenge. In this study, we analyze the temporal profile of relativistic electron precipitation events observed by the CALorimetric Electron Telescope (CALET) experiment on board the International Space Station. We use an unsupervised machine learning technique called Self‐Organizing‐Maps (SOM) to automatically detect and then classify relativistic electron events observed by the two scintillator layers at the top of the apparatus, sensitive to electrons with energies &gt;1.5 MeV and &gt;3.4 MeV, respectively. We calculate the power spectral density (PSD) of the count rates observed by both sensors and use them as an input for the SOM. The SOM technique groups the PSDs by their similarity, resulting in a classification of relativistic electron events by the periodicity of the observed precipitation. We investigate the L‐shell and magnetic local time distribution of the resulting classification, and energy spectral index associated with the observations. Clear precipitation patterns are observed and compared to past precipitation categorization attempts as well as known distributions of various scattering mechanisms. The classification reveals features through the sorting of the variability of the rapid precipitation, allowing the identification of different precipitation populations with varying properties.

Deep Learning for Time Series Classification and Extrinsic Regression: A Current Survey

Time Series Classification and Extrinsic Regression are important and challenging machine learning tasks. Deep learning has revolutionized natural language processing and computer vision and holds great promise in other fields such as time series analysis where the relevant features must often be abstracted from the raw data but are not known a priori. This article surveys the current state of the art in the fast-moving field of deep learning for time series classification and extrinsic regression. We review different network architectures and training methods used for these tasks and discuss the challenges and opportunities when applying deep learning to time series data. We also summarize two critical applications of time series classification and extrinsic regression, human activity recognition and satellite earth observation.

The Dynamical Equations of the Restricted Three-Body Problem with Poynting-Robertson Drag Force and Variable Masses

The restricted three-body problem (R3BP) is a formulation which defines the motion of a passively gravitating test particle having infinitesimal mass and moving in the gravitational environment of two bodies, called primaries. The R3BP is still an exciting and active research field that has been getting attention of scientists and astronomers because of its applications in dynamics of the solar and stellar systems, lunar theory, and artificial satellites. The equations of motion are usually the starting point in the investigations of the dynamical predictions of the infinitesimal mass. Therefore, in this paper, we examine the derivations of the dynamical equations of the R3BP with Poynting-Robertson (P-R) Drag force and variable masses. In this model formulation, both primaries are assumed to vary their masses under the combined Mestschersky law (CML) and they move in the frame of the GyldenMestschersky equation (GME). Further, the bigger primary is assumed to be emitting radiation force, which is a component of the radiation pressure and the P-R drag. The non- autonomous dynamical equations of the model are derived and converted into the autonomized equations with constant coefficients using the Mestschersky transformation (MT), the CML, the particular solutions of the GMP, and a transformation for the time dependent velocity of light. We observed that the P-R drag of the bigger primary depends on the mass parameter, radiation pressure, velocity of light and the mass variation constant . The derived systems of equations with variable and constant coefficients can be used to model the long-term motion of satellites and planets in binary systems.

Biphase routing scheme for optimal throughput in large-scale optical satellite networks

In the large-scale optical satellite network (LS-OSN), hundreds to thousands of low Earth orbit (LEO) satellites will be interconnected via laser links, offering global coverage characterized by high throughput and low latency. LS-OSNs present an attractive strategy to cultivate a comprehensively connected, intelligent world. However, the dynamic nature of the satellites, as they orbit the Earth, results in frequent changes in the LS-OSN topology. Thus, there is a pressing need for efficient routing algorithms that not only cater to massive traffic demands but also swiftly adapt to these constant topological changes. Traditional routing algorithms for services with specific bandwidth requirements often compromise on either computational speed or throughput efficiency. In response, this study introduces a routing scheme based on flow optimization and decomposition (FOND). This seeks to shorten the computation time while preserving optimal network throughput. Expanding upon the FOND scheme, we further devised two heuristic algorithms: the flow-based greedy path (FGP) and the flow-based greedy width (FGW). Simulation results from a 288-satellite constellation network indicate that both the FGP and FGW outpace contemporary methods in terms of the routing computation time while maintaining a consistent throughput equal to 100% of the network capacity. Notably, the FGP has exhibited an impressive capability, reducing the routing computation time to 0.23% compared to the baseline incremental-widest-path (IWP) algorithm, which operates on Dijkstra’s algorithm principles.