主从式卫星集群系统的长期轨道保持

针对主从式卫星集群系统的空间构型长期保持问题提出一种控制算法,使得多星系统能够在给定的相对距离约束下在轨长期运行,并且消耗较少的燃料。该卫星集群系统由一颗主星和多颗从星构成,主星保持在特定轨道上,负责系统中的数据处理、任务规划及决策,从星围绕主星自由运动并实现特定功能。根据系统的特点,考虑主星与从星、从星与从星之间约束条件的异同,设计了具有较强适应性的控制逻辑与基于李雅普诺夫稳定性理论的控制器。仿真结果表明,在推力器最大推力一定的情况下,提出的控制算法能够有效地应用于多星集群飞行中,并成功地实现长期的轨道保持。     

Coordinated attitude control for flexible spacecraft formation with actuator configuration misalignment

This paper investigates the coordinated attitude control problem for flexible spacecraft formation with the consideration of actuator configuration misalignment. First, an integral-type sliding mode adaptive control law is designed to compensate the effects of flexible mode, environmental disturbance and actuator installation deviation. The basic idea of the Integral-type Sliding Mode Control (ISMC) is to design a proper sliding manifold so that the sliding mode starts from the initial time instant, and thus the robustness of the system can be guaranteed from the beginning of the process and the reaching phase is eliminated. Then, considering the nominal system of spacecraft formation based on directed topology, an attitude cooperative control strategy is developed for the nominal system with or without communication delay. The proposed control law can guarantee that for each spacecraft in the spacecraft formation, the desired attitude objective can be achieved and the attitude synchronization can be maintained with other spacecraft in the formation. Finally, simulation results are given to show the effectiveness of the proposed control algorithm.     

Optimal spacecraft formation establishment and reconfiguration propelled by the geomagnetic Lorentz force

The Lorentz force acting on an electrostatically charged spacecraft as it moves through the planetary magnetic field could be utilized as propellantless electromagnetic propulsion for orbital maneuvering, such as spacecraft formation establishment and formation reconfiguration. By assuming that the Earth’s magnetic field could be modeled as a tilted dipole located at the center of Earth that corotates with Earth, a dynamical model that describes the relative orbital motion of Lorentz spacecraft is developed. Based on the proposed dynamical model, the energy-optimal open-loop trajectories of control inputs, namely, the required specific charges of Lorentz spacecraft, for Lorentz-propelled spacecraft formation establishment or reconfiguration problems with both fixed and free final conditions constraints are derived via Gauss pseudospectral method. The effect of the magnetic dipole tilt angle on the optimal control inputs and the relative transfer trajectories for formation establishment or reconfiguration is also investigated by comparisons with the results derived from a nontilted dipole model. Furthermore, a closed-loop integral sliding mode controller is designed to guarantee the trajectory tracking in the presence of external disturbances and modeling errors. The stability of the closed-loop system is proved by a Lyapunov-based approach. Numerical simulations are presented to verify the validity of the proposed open-loop control methods and demonstrate the performance of the closed-loop controller. Also, the results indicate the dipole tilt angle should be considered when designing control strategies for Lorentz-propelled spacecraft formation establishment or reconfiguration.     

Study of satellite retrieved CO2 and CH4 concentration over India

This paper reports a study of spatial and temporal variations of columnar averaged concentration of CO₂ and CH₄ over India using SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) and Greenhouse gas Observing SATellite (GOSAT) data. Comparison of these data with the global view National Oceanic and Atmospheric Administration (NOAA) land data and also location specific flask data is made. The temporal variation in column averaged global CO₂ is similar to that over India and it is also similar to the NOAA surface flask data and global view. The variation in NOAA surface CH₄ is location dependent and its global view appears to vary seasonally in opposite phase with the column averaged CH₄ values from satellites, reflecting the limited comparability of surface and column averaged data. Over India the CO₂ maximum is in May and minimum in August/September while for CH₄ the maximum is in September and minimum in February/March. The seasonal variation of CH₄ over India is correlated with the eastern coastal rice cultivation.     

A photoionization method for estimating BLR “size” in quasars

We describe an alternate way to estimate Broad Line Region (BLR) radii for type-1 AGN based on determination of physical conditions in the BLR under the assumption that the line emitting gas is photoionized by a central continuum source. We derive “diagnostic” intensity ratios involving UV lines Aliiiλ$\lambda$1860, Siiii]λ$\lambda$1892 and Civλ$\lambda$1549 which enable us to compute the ionizing photon flux, and hence BLR radius from the ionization parameter definition. We compare our estimates of BLR radii with values independently obtained from reverberation monitoring of Hβ$\beta$ and, in a few cases, of C ivλ$\lambda$1549. We analyze the interpretation of the photoionization estimates in the 4D eigenvector 1 context, and discuss in some detail the case of 3C 390.3. For this object we are able to provide not only the ionizing photon flux, but also an estimate of density and ionization parameter from the measured diagnostic ratios. We also compare black hole masses obtained from this method with values derived from widely-applied correlations between mass, line broadening and luminosity. Good agreement is found for both radius and black hole mass comparisons.     

GNSS carrier phase ambiguity resolution based on integrity restriction in ambiguity domain

Carrier phase ambiguity resolution of Global Navigation Satellite System (GNSS) is a key technology for high-precision navigation and positioning, and it is a challenge for applications which require both high accuracy and high integrity. This paper proposes efficient ambiguity resolution methods based on integrity restriction using Fixed Failure rate Ratio Test (FF-RT) and Doubly Non-central F-distribution Ratio Test (DNF-RT), and derives the related processing models and numerical algorithms compared with the traditional Ratio Test (RT) method. Firstly, the integer ambiguity resolution and validation procedures, especially the Least squares AMBiguity Decorrelation Adjustment (LAMBDA) estimation and RT validation are analyzed. Then the quality evaluation using success rate, the FF-RT method using Integer Aperture (IA) estimation and the NDF-RT method are proposed. Lastly, the simulation and analysis for LAMBDA using RT, FF-RT and DNF-RT methods are performed. Simulation results show that in case of unbiased scenario FF-RT and DNF-RT have similar performances, which are significantly better than RT. In case of biased scenario it is difficult for FF-RT to predict the biased success rate thus it should not be used for bias detection, while DNF-RT can detect biases in most cases except for the biases are approximate or equal to integer, which has the important benefit for early detection of potential threat to the position solution.     

Design and performances of laser retro-reflector arrays for Beidou navigation satellites and SLR observations

Beidou is the regional satellite navigation system in China, consisting of three kinds of orbiting satellites, MEO, GEO and IGSO, with the orbital altitudes of 21500–36000 km. For improving the accuracy of satellites orbit determination, calibrating microwave measuring techniques and providing better navigation service, all Beidou satellites are equipped with laser retro-reflector arrays (LRAs) to implement high precision laser ranging. The paper presents the design of LRAs for Beidou navigation satellites and the method of inclined installation of LRAs for GEO satellites to increase the effective reflective areas for the regional ground stations. By using the SLR system, the observations for Beidou satellites demonstrated a precision of centimeters. The performances of these LRAs on Beidou satellites are very excellent.     

卫星编队飞行相对位置自适应协同控制

针对卫星编队飞行相对位置协同控制问题,基于编队卫星相对运动非线性动力学方程和一致性理论设计了两种自适应协同控制器。首先,在卫星质量不确定和星间信息交互存在通信时延的条件下,设计了一种全状态反馈自适应协同控制器,并证明了该控制策略对空间摄动力的鲁棒性。其次,进一步考虑速度信息不可测的条件下,采用滤波器设计了一种无速度反馈的自适应协同控制器。最后,以编队构型重构为例对两种自适应协同控制器进行了仿真校验。仿真结果表明：两种自适应协同控制器均可有效应用于卫星编队飞行相对位置的协同控制,能够保证编队卫星对各自期望轨迹跟踪的同时暂态保持编队构型的稳定,具有较高的控制精度。     

卫星通信频谱监测中的一种形态学滤波预处理方法

针对卫星信号监测中监视频带宽、背景噪声非平坦造成的信号漏检和误检的问题,结合卫星通信信号的特点提出了一种频谱校正的新方法。该方法将接收的频谱信号视为一维灰度图像,通过改进的形态学滤波检测法,估算出信号中非平坦的背景噪声基底,利用估计的噪声基底完成对原频谱信号的修正,可提高信号检测的准确性。仿真结果表明,该方法降低了信号带宽对传统形态学滤波中结构元素长度选择的影响,计算量小,能高效地实现非平坦频谱信号的底噪校正。     

多星与上面级非对称分离技术研究

对多星与上面级非对称分离技术进行了研究。针对上面级多星非对称分离存在较大偏心的特点,通过优化调整,使用不同大小弹簧力的匹配抵消上面级转动的影响,减小了因卫星相对上面级偏心布置导致的分离力对卫星和上面级姿态的干扰。根据总体偏差量的概率模型,用蒙特卡洛仿真法统计出卫星姿态角和角速度的概率分布规律。