Search Results (1,152)

The Medium Earth Orbit (MEO) spaceborne Synthetic Aperture Radar (SAR) has great coverage ability, which can improve maritime ship target surveillance performance significantly. However, due to the huge computational load required for imaging processing and the severe defocusing caused by ship motions, traditional ship recognition conducted in focused image domains cannot process MEO SAR data efficiently. To address this issue, a multi-level focusing-classification strategy for MEO SAR ship recognition is proposed, which is applied to the range-compressed ship data domain. Firstly, global fast coarse-focusing is conducted to compensate for sailing motion errors. Then, a coarse-classification network is designed to realize major target category classification, based on which local region image slices are extracted. Next, fine-focusing is performed to correct high-order motion errors, followed by applying fine-classification applied to the image slices to realize final ship classification. Equivalent MEO SAR ship images generated by real LEO SAR data are utilized to construct training and testing datasets. Simulated MEO SAR ship data are also used to evaluate the generalization of the whole method. The experimental results demonstrate that the proposed method can achieve high classification precision. Since only local region slices are used during the second-level processing step, the complex computations induced by fine-focusing for the full image can be avoided, thereby significantly improving overall efficiency. Full article

Co_0.95R_0.05Fe₂O₄ nanoparticles were synthesized using a sol-gel approach incorporating bio-based agents and were found to be single phases adopting a cubic Fd-3m structure. XPS shows the presence of Gd³⁺ and Ho³⁺ ions. The spin–orbit splitting of about 15.4 eV observed in Co 2p core-level spectra is an indication that Co is predominantly present as Co³⁺ state, while the satellite structures located at about 6 eV higher energies than the main lines confirm the existence of divalent Co in Co_0.95R_0.05Fe₂O₄. The positions of the Co 3s and Fe 3s main peaks obtained by curve fitting and the exchange splitting obtained values for Co 3s and Fe 3s levels point to the high Co³⁺/Co²⁺ and Fe³⁺/Fe²⁺ ratios in both samples. The saturation magnetizations are smaller for the doped samples compared to the pristine ones. For theoretical magnetization calculation, we have considered that the heavy rare earths are in octahedral sites and their magnetic moments are aligned antiparallelly with 3d transition magnetic moments. ZFC-FC curves shows that some nanoparticles remain superparamagnetic, while the rest are ferrimagnetic, ordered at room temperature, and showing interparticle interactions. The M_S/Ms ratio at room temperature is below 0.5, indicating the predominance of magnetostatic interactions. Full article

In response to the increasing demand for high-precision navigation of satellites operating in the cislunar space, this study introduces an onboard orbit determination algorithm considering both convergence and computational efficiency, referred to as the Sliding-Window Batch Processing (SWBP) algorithm. This algorithm combines the strengths of data batch processing and the sequential processing algorithm, utilizing measurement data from multiple historical and current epochs to update the orbit state of the current epoch. This algorithm facilitates rapid convergence in orbit determination, even in instances where the initial orbit error is large. The SWBP algorithm has been used to evaluate the navigation performance in the Distant Retrograde Orbit (DRO) and the Earth–Moon transfer orbit. The scenario involves a low-Earth-orbit (LEO) satellite establishing satellite-to-satellite tracking (SST) links with both a DRO satellite and an Earth–Moon transfer satellite. The LEO satellite can determine its orbit accurately by receiving GNSS signals. The experiments show that the DRO satellite achieves an orbit determination accuracy of 100 m within 100 h under an initial position error of 500 km, and the transfer orbit satellite reaches an orbit determination accuracy of 600 m within 3.5 h under an initial position error of 100 km. When the Earth–Moon transfer satellite exhibits a large initial orbital error (on the order of hundreds of kilometers) or the LEO satellite’s positional accuracy is degraded, the SWBP algorithm demonstrates superior convergence speed and precision in orbit determination compared to the Extended Kalman Filter (EKF). This confirms the proposed algorithm’s capability to handle complex orbital determination scenarios effectively. Full article

Co_0.95R_0.05Fe₂O₄ nanoparticles with R = La, Pr, Nd, Sm, and Eu were synthesized via an environmentally friendly sol–gel method. The prepared samples were studied using X-ray diffraction measurements (XRD), transmission electron microscopy (TEM), X-ray photoelectron microscopy (XPS), and magnetic measurements. All compounds were found to be single phases adopting a cubic Fd-3m structure. EDS analysis confirmed the presence of Co, Fe, R, and oxygen in all cases. The XPS measurements reveal that the Co 2p core-level spectra are characteristic for Co³⁺ ions, as indicated by the 2p_3/2 and 2p_1/2 binding energies and spin–orbit splitting values. The analysis of the Fe 2p core-level spectra reveals the presence of both Fe³⁺ and Fe²⁺ ions in the investigated samples. The doped samples exhibit lower saturation magnetizations than the pristine sample. Very good agreement with the saturation magnetization values was obtained if we assumed that the light rare-earth ions occupy octahedral sites and their magnetic moments align parallel to those of the 3d transition metal ions. The ZFC-FC curves indicate that some nanoparticles remain superparamagnetic, while others exhibit ferrimagnetic ordering at room temperature, suggesting the presence of interparticle interactions. The M_r/M_s ratio at room temperature reflects the dominance of magnetostatic interactions. Full article

In order to provide a theoretical basis for the thermal analysis and management of spacecraft/payload interstellar pointing attitudes, which are used for inter-satellite communication, this paper develops an analytical method for solar heat flux under pointing attitudes. The key to solving solar heat flux is calculating the angle between the sun vector and the normal vector of the object surface. Therefore, a method for calculating the included angle is proposed. Firstly, a coordinate system was constructed based on the pointing attitude. Secondly, the angle between the coordinate axis vector and solar vector variation with a true anomaly was calculated. Finally, the reaching direct solar heat flux was obtained using an analytical method or commercial software. Based on the proposed method, the direct solar heat flux of relay satellites in commonly used lunar orbits, including Halo orbits and highly elliptic orbits, was calculated. Thermal analysis on the payload of interstellar laser communication was also conducted in this paper. The calculated temperatures of each mirror ranged from 16.6 °C to 21.2 °C. The highest temperature of the sensor was 20.9 °C, with a 2.3 °C difference from the in-orbit data. The results indicate that the external heat flux analysis method proposed in this article is realistic and reasonable. Full article

We investigate the survivability of solar system-like planetary systems during close encounters in stellar associations using a suite of 1980 N-body simulations. Each system is based on one of the possible five-planet resonant configurations proposed to represent the initial solar system architecture and is systematically scaled in both planetary mass and orbital compactness to explore the parameter space of observed exoplanetary architectures. Simulations explore a range of stellar encounter scenarios drawn from four distinct cluster environments. Our results show that system survival depends critically on the interplay between planetary mass and orbital scale: compact configurations are more resistant to external perturbations, while increased planetary mass improves resilience only up to a threshold, beyond which internal instabilities dominate. No system whose planets are twice as massive as the ones in the solar system survives stellar encounters. Systems that are at least an order of magnitude more compact than the solar system remain stable under typical encounter conditions. These findings place strong constraints on the initial architectures of planetary systems that can endure stellar-dense birth environments. Full article

We investigate magnetic instabilities toward multiple-Q states in centrosymmetric bilayer triangular-lattice systems. By focusing on the interplay between the layer-dependent Dzyaloshinskii–Moriya interaction and layer-independent bond-dependent anisotropic interaction, both of which originate from the relativistic spin-orbit coupling, we construct a low-temperature phase diagram based on an effective spin model that also includes frustrated isotropic exchange interactions. Employing simulated annealing, we reveal the stabilization of three distinct double-Q phases in the absence of an external magnetic field, each characterized by noncoplanar spin textures with spatially modulated local scalar spin chirality. Under applied magnetic fields, we identify field-induced phase transitions among single-Q, double-Q, and triple-Q states, some of which exhibit a finite net scalar spin chirality indicative of topologically nontrivial order. These findings highlight centrosymmetric systems with sublattice-dependent Dzyaloshinskii–Moriya interactions as promising platforms for realizing a variety of multiple-Q spin textures. Full article

Continuous low thrust is widely used in orbit transfer maneuvers. If the unknown maneuvers are not correctly compensated, the orbiting accuracy will be seriously affected. We propose a rapid method for pre-identifying thrust acceleration based on single-arc orbit determination in order to determine the orbit of non-cooperative continuous low-thrust maneuvering spacecraft. The single-arc orbit determination results of two ground-based radar observations with a certain time interval are used to inversely determine the direction and magnitude of acceleration of the spacecraft under continuous thrust based on their relationship with satellite orbit parameters. The solution error is relatively small when using this method, even over a short period of time when data are sparse. The results can then be applied to the orbital adjustment of a satellite. The results show that when the satellite climbs with maximum tangential acceleration, the interval between the two radar observations is greater than 7 h, and the proposed method can rapidly pre-identify tangential thrust acceleration with a solution error of less than 5%. When the satellite adjusts the orbital plane with the maximum normal acceleration, the average relative measurement error of the normal acceleration is about 20% when the time interval between two observations is 24 h. The longer the observation interval and the greater the thrust acceleration, the smaller the relative error. The calculation results can be used as the initial value for precision orbit determination of continuous low-thrust maneuvering spacecraft. Full article

Dye-sensitized solar cells (DSSCs) have emerged as a promising technology for converting sunlight into electricity at a low cost; however, it is still necessary to find a photostable, low-cost, and efficient photosensitizer. In this sense, the natural product bixin (Dye 1) has previously been reported as a potential photosensitizer. Thus, the present work reports the full synthesis of diester and diacid hybrids (Dyes 2 and 3, respectively, with corresponding yields of 93% and 52%) using the natural product bixin as a starting material and 1,3,4-oxadiazole ring as a connected point. The hydrolysis step of Dye 2 aims to obtain Dye 3 with a structural capacity to anchor the titanium dioxide (TiO₂) nanofilms via the carboxylic acid group. Both compounds (Dyes 1 and 3) can be adsorbed via pseudo-first order on the surface of TiO₂ nanofilms, reaching saturation after 10 and 6 min of exposure in an organic solution (1 × 10⁻⁵ M), respectively, with adsorption kinetics of the semisynthetic compound almost twofold higher than the natural product. Contrary to expectations, Dye 3 had spectral behavior similar to Dye 1, but with better frontier molecular orbital (FMO) parameters, indicating that Dye 3 will probably behave very similarly or have slightly better photovoltaic performance than Dye 1 in future DSSC measurements. Full article

To ensure the on-orbit safety of crewed spacecraft and avoid the threat of constellations such as Starlink to manned spacecraft, the industry has started to research equipping phased array radars for situational awareness of collision threat. In order to enhance the resource allocation capability of the space station’s protection radar system, this paper proposes a task scheduling method based on time shifting constraints and pulse interleaving. The time shifting constraint is designed to minimize the deviation between the actual execution and the desired execution time of the task, and it is negatively correlated with the threat degree of the target. Pulse interleaving is intended to utilize the idle time between the transmitted pulse and the received pulse of a task to perform other tasks, thereby improving the utilization of radar resources. Through computer simulation under typical parameters, our proposed method reduces the average time shifting ratio by about 60% compared to traditional task scheduling methods, and the scheduling success ratio is also higher than that of traditional scheduling methods. This demonstrates the effectiveness of the proposed method in enhancing scheduling efficiency and overall system performance. Full article

The electron-impact ionization and partial ionization cross sections are reported for few silicon-chlorine molecules using semi-empirical methods. The partial ionization cross sections are determined using a modified version of the binary-encounter-Bethe model. In this approach, the binary-encounter-Bethe model is modified through a two-step process, namely, transforming the binding energies of the occupied orbitals and introducing a scaling factor. The scaling can be done using either the mass spectrometry data or experimental values of cross sections. It correctly adjusts the scaling term of the BEB model so that the order of magnitude of resulting partial ionization cross sections is the same as that of experimental values. Further, the use of the experimental value of ionization and appearance energy values ensures that the cross sections have a correct threshold. This further mitigates the dependence of cross sections on energy at low values. The role of the scaling factor and the behavior of branching ratios is also examined at different energies. The species whose partial ionization cross sections are reported are highly relevant in plasma processing. However, the proposed model can be extended to any multi-centerd molecular structures comprising a large number of atoms or electrons, except in cases where resonance effects or additional ionization channels become significant. The mass spectrometry data is of utmost importance in computing partial ionization cross sections in order to obtain reliable results. Full article

The three-tiered microbial food chain without maintenance under leachate recirculation is the subject of a mathematical seven-dimensional dynamical system that is proposed in this work. This model captures the complex interactions between chlorophenol degraders, phenol degraders, and methanogens in the presence of hydrogen inhibition. The implementation allows for investigation of how hydrogen levels affect the overall system dynamics and phenol production. There is a thorough qualitative analysis provided. A stability analysis of equilibrium points is performed. It is demonstrated that the persistence of the three bacteria is correlated with the existence of the positive equilibrium point, assuming some monotonicity properties on the growth rates. Asymptotic coexistence is satisfied, although periodic orbit possibilities are not ruled out. In order to decrease the amount of organic materials within the reactor, we suggest an optimal strategy on the rate of leachate recirculation in the second stage. Lastly, we offer a few numerical investigations that support and strengthen the theoretical conclusions. Full article

Magnetic levitation enables the confinement and melting of conductive metals using alternating magnetic fields, eliminating the need for a crucible or other contact supports. This makes the technology particularly suitable for applications where container use is impractical, such as preventing contamination between the melt and the crucible, handling high-purity materials, or facilitating in-orbit operations. For a given coil design and load, selecting the appropriate feeding parameters, such as the current and frequency, is crucial to ensure the correct operation of the device. This study investigates the optimal current and frequency values required to levitate an aluminum billet using a proposed longitudinal electromagnetic levitator, which represents an initial prototype of a more complex system for automated material manipulation. The analysis was conducted through 2D and 3D finite element method (FEM) simulations, assessing the equilibrium position and stability with respect to translations and rotations under various operating conditions. The study identifies an operating configuration that ensures vertical stability while minimizing excessive heating, in order to obtain a sufficiently long confinement time before the melting point is reached. A fully coupled 2D thermal simulation was then performed to assess the billet’s heating rate under the selected operating conditions. Finally, an experiment was conducted on a prototype to confirm billet levitation. Full article

Although lithium-ion batteries are considered an ideal postulant for renewable energy harvesting, storage and applications, these batteries show promising performance; however, at the same time, these harvesting devices suffer from some major limitations, including scarce lithium resources, high cost, toxicity and safety concerns. Potassium ion batteries (PIBs) can be proven a favorable alternative to metal ion batteries because of their widespread potassium reserves, low costs and enhanced protection against sparks. In this study, DFT simulations were employed using the B3LYP/6-311⁺⁺g(d p) method to explore the application of graphene and its doped variants (N,B,P-graphene) as potential anode materials for PIBs. Various key parameters such as adsorption energy, Gibbs free energy, molecular orbital energies, non-covalent interactions, cell voltage, electron density distribution and density of states were computed as a means to evaluate the suitability of materials for PIB applications. Among the four structures, nitrogen- and phosphorus-doped graphene exhibited negative Gibbs free energy values of −0.020056 and −0.021117 hartree, indicating the thermodynamic favorability of charge transfer processes. Doping graphene with nitrogen and phosphorus decreases the HOMO-LUMO gap energy, facilitating efficient ion storage and charge transport. The doping of nitrogen and phosphorus increases the cell voltage from −1.05 V to 0.54 V and 0.57 V, respectively, while boron doping decreases the cell voltage. The cell voltage produced by graphene and its doped variants in potassium ion batteries has the following order: P-graphene (0.57 V) > N-graphene (0.54 V) > graphene (−1.05 V) > B-graphene (−1.54 V). This study illustrates how nitrogen- and phosphorus-doped graphene can be used as a propitious anode electrode for PIBs. Full article

The Moon has become the focus of renewed interest for numerous space agencies and private companies worldwide. In the coming years, various scientific and commercial missions are planned, with a particular emphasis on exploring the South Pole. These missions include orbiters, landers, as well as both static and mobile rovers. For all these operations, continuous and accurate position knowledge is essential. This paper evaluates the performance of a navigation system designed for a lunar rover using the future satellite navigation infrastructure. It highlights the critical role of integrating multiple information sources, including a Digital Elevation Model (DEM) of the lunar surface and a high-precision Inertial Measurement Unit (IMU). The results demonstrate that a comprehensive suite of instruments enables highly accurate and reliable navigation for a mobile rover. While standalone satellite navigation, due to the reduced number of available sources, offers navigation accuracy of the orders of tens of meters, the addition of the DEM lowers the error at 5 m level; the IMU further improve by roughly 40% the performance on horizontal positioning. Full article