•We propose a simple yet effective CMAFF module that can fuse the complementary information of multispectral remote sensing images with joint common-modality and differential-modality attentions.•We ...confirm the effectiveness of our cross-modality fusion attention module through extensive ablation studies.•We design a new two-stream object detection network YOLOFusion for multispectral remote sensing images and verify its performance.
Cross-modality fusing complementary information of multispectral remote sensing image pairs can improve the perception ability of detection algorithms, making them more robust and reliable for a wider range of applications, such as nighttime detection. Compared with prior methods, we think different features should be processed specifically, the modality-specific features should be retained and enhanced, while the modality-shared features should be cherry-picked from the RGB and thermal IR modalities. Following this idea, a novel and lightweight multispectral feature fusion approach with joint common-modality and differential-modality attentions are proposed, named Cross-Modality Attentive Feature Fusion (CMAFF). Given the intermediate feature maps of RGB and thermal images, our module parallel infers attention maps from two separate modalities, common- and differential-modality, then the attention maps are multiplied to the input feature map respectively for adaptive feature enhancement or selection. Extensive experiments demonstrate that our proposed approach can achieve the state-of-the-art performance at a low computation cost.
Tin halide perovskites attract incremental attention to deliver lead‐free perovskite solar cells. Nevertheless, disordered crystal growth and low defect formation energy, related to Sn(II) oxidation ...to Sn(IV), limit the efficiency and stability of solar cells. Engineering the processing from perovskite precursor solution preparation to film crystallization is crucial to tackle these issues and enable the full photovoltaic potential of tin halide perovskites. Herein, the ionic liquid n‐butylammonium acetate (BAAc) is used to tune the tin coordination with specific O…Sn chelating bonds and NH…X hydrogen bonds. The coordination between BAAc and tin enables modulation of the crystallization of the perovskite in a thin film. The resulting BAAc‐containing perovskite films are more compact and have a preferential crystal orientation. Moreover, a lower amount of Sn(IV) and related chemical defects are found for the BAAc‐containing perovskites. Tin halide perovskite solar cells processed with BAAc show a power conversion efficiency of over 10%. This value is retained after storing the devices for over 1000 h in nitrogen. This work paves the way toward a more controlled tin‐based perovskite crystallization for stable and efficient lead‐free perovskite photovoltaics.
The synergistic strategy of tuning the solution coordination and crystallization process by introducing ionic liquid is implemented to successfully fabricate pinhole‐free tin perovskite films with preferential crystal orientation, which possess improved oxidation repellency for Sn(II) and enhanced hydrophobicity. As a result, the stabilization of high‐efficiency lead‐free tin halide perovskite solar cells is achieved.
This paper describes a novel homotopy method to compute fuel-optimal trajectories starting from a time-optimal solution. The time-optimal problem is proposed to serve as a gateway for solving the ...minimum thrust problem. Homotopy is used to link the original low-thrust fuel-optimal problem with the minimum thrust problem. Two new variables are introduced in the dynamic model. The first is a logarithm of mass variable and the second is an acceleration magnitude variable. The analytic expression of the logarithm of mass co-state is solved. For the time-optimal problem, initial co-state of logarithm of mass can be expressed by thrust magnitude and transfer time. Then, the number of unknown initial co-states decreased. The effectiveness and optimality of the proposed method is validated through simulations of two rendezvous missions.
Self-organizing control for satellite clusters is a challenging and promising problem which has drawn considerable attention in the recent past. The artificial potential function method has been ...widely used for self-organizing control due to its elegant mathematical analysis and simplicity. This paper proposes a set of self-organizing control rules for satellite clusters described by artificial potential functions, so that the reconfiguration, uniform distribution and collision avoidance operations can be achieved spontaneously. It may work regardless of the failure of satellites, attendance of new members or existence of space debris, and the corresponding communication and control system are completely distributed. Particularly, the artificial potential functions are written in terms of relative orbital elements derived from T–H equations, so that the self-organizing control can reflect relative motion dynamics, and guide the satellite along a fuel-efficient trajectory. The proposed method is especially suitable for highly distributed micro-satellite clusters and large-scale clusters to track moving targets, with few fuel cost and relatively high control accuracy, and it is applicable to clusters in either deep space or near-earth space. The stability of the control was proved by Lyapunov second method, and verified by Monte Carlo simulation. Finally, the comparison between self-organizing control and fuel-optimal control was made to demonstrate the performance properties.
A cislunar cargo spacecraft with low-thrust propulsion traveling between the Earth and the Moon is essential for sustainable, long-term manned lunar exploration. In low-thrust Earth–Moon transfer ...(LTEMT), lunar capture is the primary prerequisite for spacecraft subject to the circular restricted three-body model. Therefore, this study identifies sufficient conditions for lunar capture, which are determined by the Jacobi integral and Hill’s region. This paper proposes a guidance scheme that includes thrust direction, thrust efficiency, and a five-stage flight control sequence based on the variation of the Jacobi integral. The LTEMT problem is then converted to an initial value problem of a differential equation with three parameters. Lunar capture set theories (LCSTs), which are convenient for identifying lunar capture sets, are presented and proved according to the continuous properties of the ordinary differential equation. Finally, the solutions of the LTEMT trajectories departing from a geosynchronous orbit with an altitude of approximately 35,827 km are discussed for different thrust accelerations and cut-off values of the thrust efficiency. The robustness is analyzed assuming that navigation and switching time errors are present to demonstrate the adaptability of this method. The results reveal that the proposed guidance scheme and LCSTs can provide technical support for future cislunar cargo missions.
This paper proposes a novel electromagnetic separation system for the small spherical satellite Q-SAT, which is developed by Tsinghua University. Q-SAT was successfully launched on August 6, 2020 and ...works well now. Q-SAT requires the separation system no pollution, no impact, multi-point synchronous release and repeatable test. In order to meet these requirements, an electromagnetic separation method is proposed and the system is designed. The system consists of bearing structure, a pair of locking and release mechanism, four ejection mechanism and a drive power. The scheme of separation system can be summarized as “two locks, four ejections”. The key to system is the lock and release mechanism using ball-lock principle driven by electromagnets. The analytical relationship of the mechanism driving force is given. How to balance the contradiction between lock and release, which is a problem bring by the system, is discussed. The prototype is made. The verification tests are fully carried out including function, vibration, shock, coldness and velocity measurement. The test results show that the system satisfies the launching requirement. The reliability of the system was proven by the successful separation. Systems based on the same electromagnetic separation method could be widely used on other satellites or payloads in the future.
•A novel separation method for a small satellite based on electromagnetic mechanism is proposed.•An electromagnetic separation system is developed for the small spherical satellite Q-SAT based on the method.•The separation system has the characteristics of no impact, no pollution, low cost, repeatable test on the ground and high synchronization.•The separation system is fully verified by test and flight.
A novel satellite cluster control method by means of a set of artificial potential functions is proposed in this paper. To avoid the huge difficulty of designing target configuration for a ...large-scale satellite cluster, the proposed cluster control method relies on intersatellite flight bounds so that satellites can autonomously converge to a given boundary range, rather than a set of exact orbit configuration. The analytic relative motion bounds in terms of relative orbital elements are deduced and used as variables in artificial potential functions. The accumulated control effort is minute, that shows great potential in micro satellite cluster control. On the other hand, collision avoidance as a key requirement for satellite cluster, also gets attention in this paper. For a long-term perspective, the minimum intersatellite flight bound rather than the real-time relative distance between satellites is introduced into the repulsive artificial potential function. Satellite members can adjust their control effort in real time according to nearby satellites, which reflect the characteristic of swarm intelligence. Monte Carlo simulation reveals the superiority in fuel consumption of the proposed collision avoidance control.
CubeSats provide a low-cost, convenient, and effective way of acquiring remote sensing data, and have great potential for remote sensing object detection. Although deep learning-based models have ...achieved excellent performance in object detection, they suffer from the problem of numerous parameters, making them difficult to deploy on CubeSats with limited memory and computational power. Existing approaches attempt to prune redundant parameters, but this inevitably causes a degradation in detection accuracy. In this paper, the novel Context-aware Dense Feature Distillation (CDFD) is proposed, guiding a small student network to integrate features extracted from multi-teacher networks to train a lightweight and superior detector for onboard remote sensing object detection. Specifically, a Contextual Feature Generation Module (CFGM) is designed to rebuild the non-local relationships between different pixels and transfer them from teacher to student, thus guiding students to extract rich contextual features to assist in remote sensing object detection. In addition, an Adaptive Dense Multi-teacher Distillation (ADMD) strategy is proposed, which performs adaptive weighted loss fusion of students with multiple well-trained teachers, guiding students to integrate the learning of helpful knowledge from multiple teachers. Extensive experiments were conducted on two large-scale remote sensing object detection datasets with various network structures; the results demonstrate that the trained lightweight network achieves auspicious performance. Our approach also shows good generality for existing state-of-the-art remote sensing object detectors. Furthermore, by experimenting on large general object datasets, we demonstrate that our approach is equally practical for general object detection distillation.
An intelligent, person-following nanosatellite is under development for in-cabin astronaut assistance in the China Space Station. It is named the Intelligent Formation Personal Satellite (IFPS). The ...satellite weighs 2.0 kg and is shaped as a sphere of diameter 230 mm. Fans and MEMS flywheels are used for its position and attitude maneuvering as inside the Space Station cabin it is a weightless and standard atmospheric pressure environment. The RGB-D camera and IMU based visual-inertial SLAM method is used to support its localization and navigation. The on-board information processing and computing hardware primarily consists of an embedded AI microprocessor and an FPGA. The satellite is designed to fly autonomously and follow the designated astronaut to offer immediate assistance. Thus, efficient and robust astronaut visual tracking is the most important prerequisite for supporting its basic person-following operating mode. We achieved this by further improving our previously proposed tracking algorithm that consists of a deep convolution neural network (DCNN)-based detection module and a probabilistic-model-based tracking module. The DCNN in the detection module was further improved through optimizations of lightweight network architecture design, parameters model compression and inference acceleration. While maintaining the originally high detection accuracy, the DCNN was optimized significantly in terms of memory, computation and power consumption to quite meet the engineering constraints in the development of IFPS. The complete pipeline of the astronaut visual tracking algorithm was also designed and implemented in the embedded AI microprocessor for online tracking application. Experimental results demonstrated the effectiveness of the proposed efficient and robust astronaut detection and tracking algorithm.
•An in-cabin nanosatellite for astronaut assistance is designed. Deep-learning-based astronaut visual tracking algorithm is proposed for supporting its astronaut-following operating mode.•An efficient DCNN was developed through lightweight network architecture design, parameters model compression and inference acceleration.•The DCNN provides high detection accuracy, and its memory requirements, computation burden and power consumption are engineering affordable.•The deep-learning-based tracking algorithm runs efficiently on embedded hardware. Robust detection and tracking performance is verified by experiments.
Abstract
Using poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as hole conductor, a series of inverted planar CH
3
NH
3
PbI
3−x
Cl
x
perovskite solar cells (PSCs) were fabricated ...based on perovskite annealed by an improved time-temperature dependent (TTD) procedure in a flowing nitrogen atmosphere for different time. Only after an optimum annealing time, an optimized power conversion efficiency of 14.36% could be achieved. To understand their performance dependence on annealing time, an
in situ
real-time synchrotron-based grazing incidence X-ray diffraction (GIXRD) was used to monitor a step-by-step gradual structure transformation from distinct mainly organic-inorganic hybrid materials into highly ordered CH
3
NH
3
PbI
3
crystal during annealing. However, a re-crystallization process of perovskite crystal was observed for the first time during such an annealing procedure, which helps to enhance the perovskite crystallization and preferential orientations. The present GIXRD findings could well explain the drops of the open circuit voltage (V
oc
) and the fill factor (FF) during the ramping of temperature as well as the optimized power conversion efficiency achieved after an optimum annealing time. Thus, the present study not only illustrates clearly the decisive roles of post-annealing in the formation of solution-processed perovskite to better understand its formation mechanism, but also demonstrates the crucial dependences of device performance on the perovskite microstructure in PSCs.
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