Online scheduling of distributed Earth observation satellite system under rigid communication constraints

This paper focused on online scheduling of distributed Earth observation satellite system in a dynamic environment. The objective was to maximize the total profit of the overall system by efficiently coordinating the different satellites with stochastic arrival of urgent tasks, subject to rigid communication and observation time window constraints. We formulated this problem as a single-task, single-robot, time-extended assignment problem with intra-schedule dependency using the multi-robot task assignment taxonomy and formulated the sub-problem after releasing urgent tasks in a mixed-integer linear programming model. We first described the online scheduling algorithm for a single satellite, then we proposed the modified consensus-based bundle algorithm (m-CBBA) and modified asynchronous consensus-based bundle algorithm (m-ACBBA) with synchronous and asynchronous communication, respectively. Compared with initial versions of CBBA and ACBBA, the modified versions added the communication loop prediction phases to efficiently utilize scarce communication opportunities and reduce the communication requirements. Additionally, we introduce two contract net protocol (CNP)-based algorithms for comparison, respectively SingleItem-CNP-based (SI-CNP) algorithm and Batch-CNP-based (BA-CNP) algorithm. Computational experiments indicated that both the total profit and percentage of scheduled urgent tasks achieved by the m-ACBBA and m-CBBA algorithms were much higher than those achieved by both SI-CNP and BA-CNP. Additionally, the number of communications needed by either m-ACBBA or m-CBBA algorithm was lower than that by SI-CNP. When the communication cost in the system is high, the m-CBBA algorithm is preferred because it balances the profit and the required number of communications. When the communication cost is low, the m-ACBBA algorithm is preferred because it achieves high total profit and high percentage of scheduled urgent tasks.     

Semi-annual variations of chlorophyll concentration on the Eastern Tropical Pacific coast of Mexico

An analysis of the main remotly sensed oceanographic variables was conducted to characterize the dominant drivers that modulate the spatial-temporal variability of coastal phytoplankton biomass in the northern limit of the Eastern Tropical Pacific. The phytoplankton biomass was analyzed using monthly average satellite chlorophyll (Chlo) concentration data from MODIS sensors for the period 2003–2017. Oceanographic conditions were analyzed by considering (i) Sea Surface Temperature (SST) high-resolution data from the GHRSST project, (ii) wind stress calculated with data from the zonal and meridional components of the CCMP product, (iii) climatological precipitation, and (iv) climatological river flow. Chlo time series and spatial variability were analyzed using Hovmöller diagrams and Empirical Orthogonal Functions (EOF), respectively. A strong semi-annual signal in Chlo concentration along the coast was observed: the first peak occurs in winter-spring (5.0 mg·m⁻³) and a second one in summer-autumn (6.5 mg·m⁻³). A high year-round average of Chlo concentration (3.0–15.0 mg·m⁻³) was maintained by a 10 km wide along-shore coast, with maximum values spatially associated with river mouths, of which Santiago river registered the highest Chlo values (20.0 mg·m⁻³). Surface oceanographic conditions showed a marked annual cycle with warmer conditions (30–31 °C) from July to October and colder ones (23–24 °C) from December to April; during the cold period, seasonal wind stress stimulated coastal upwelling. The EOF showed that 70% of Chlo concentration variability was controlled by the semi-annual pattern, which responded to coastal upwelling conditions during cold period, while rivers outflows influenced high Chlo concentration during the warm period. These results highlighted the importance for land-ocean interface to sustain coastal ecosystems' biological production and the major role of watersheds as sources of nutrients to maintain high biological production during warm periods in transitional tropical-subtropical zones.     

基于MHSS算法的ARAIM完好性和可用性预测

传统的接收机自主完好性监视只能提供非精密进近阶段的完好性监视,无法实现对完好性要求更加严格的飞行阶段的监视,而高级接收机自主完好性监视（AdvancedReceiverAutonomousIntegrityMonitoring,ARAIM）可以在精密进近阶段为飞机提供满足航向信标性能垂直引导（LocalizerPerformancewithVerticalguidance,LPV）的完好性监视服务。文章基于多假设分组解算法,根据目前的星座环境,结合民航不同飞行阶段的应用需求,对ARAIM算法进行仿真研究。针对水平保护级（HorizontalProtectionLevel,HPL）和垂直保护级（VerticalProtectionLevel,VPL）的指标,在不同星座组合环境下,选取南北半球不同经纬度的5个观测机场,分别仿真了HPL和VPL的变化情况,并对全球HPL和VPL平均分布趋势,以及以99%的概率满足LPV-200进近可用性指标时,ARAIM可用性的全球覆盖率进行了仿真预测分析。仿真结果表明,不论是全球定位系统和格洛纳斯双星座还是增加北斗卫星导航系统后的三星座,5个观测机场的HPL和VPL均能满足LPV-200进近对完好性指标的要求;但在全球范围内,双星座条件下,ARAIM并不能完全支持LPV-200进近对完好性监视的要求,而三星座则可大大提高ARAIM的可用性,为民航用户提供满足精密进近的所需导航服务性能。     

挠性充液航天器超近程逼近段动力学建模与控制

超近程逼近过程中,服务航天器为刚柔液耦合的复杂系统,单摆等液体晃动模型不再适用,对这种工况下的航天器进行了动力学建模与控制的研究。采用虚功率原理推导了一种新的适用于三轴推力作用下的液体晃动等效模型,通过引入相对位置导引矢量与相对位置误差矢量,建立了相对轨道误差动力学模型,结合相对姿态动力学模型,得到超近程逼近段的刚柔液耦合的相对轨道姿态动力学模型。针对模型存在的不确定性和未知干扰,设计了基于滑模估计器的相对姿轨耦合控制律。通过数学仿真验证了控制律的有效性,仿真中的晃动结果与Flow-3D结果能够吻合,验证了晃动模型的合理性。     

基于观测器的平台无陀螺姿态复合控制

针对超静卫星星体平台无陀螺、载荷敏感器与星体平台执行机构非共基准安装时整星存在姿态异位控制问题,提出了一种基于观测器估计星体平台姿态的复合控制方法。首先,建立星体平台/Stewart平台/载荷的动力学模型,并获得Stewart平台作动器关节空间的等效动力学模型。针对关节空间等效模型,设计super twisting观测器,以作动器平动位移为输入,以载荷和星体平台之间的相对姿态和角速度为输出,实现星体平台姿态和角速度估计。其次,以载荷测量姿态信息为输入,设计Stewart作动器的积分滑模控制律,实现载荷高精度指向控制。以观测器估计的星体平台姿态信息为输入,设计星体平台控制器实现星体平台的稳定控制。Lyapunov稳定性分析表明所设计的观测器和控制器能够保证闭环系统渐近稳定。数学仿真结果表明：在星体平台有陀螺时,载荷能够实现0.1″指向精度;在星体平台无陀螺时,采用观测器估计星体平台姿态并进行控制,载荷亦可实现0.1″指向精度。     

局部可观测理论在INS/GPS机动对准中的应用

从研究INS/GPS(Inertial Navigation System/Global Positioning System)组合系统的姿态角误差可观测性出发,首次将局部可观测性理论应用于INS/GPS组合系统,定量地计算出各种不同机动方式的局部可观测矩阵的条件数,找到了提高姿态角误差可观测性的最佳机动方式.研究结果表明,通过载体做正弦水平机动飞行可以提高姿态角误差局部可观测性,使空中对准时间明显减少,姿态角误差大大降低.当对准时间为120 s时,东北天向姿态角误差的均值分别为12.34″,12.19″和-28.31″,它们的均方根值分别为0.97″,1.05″和0.62″.      

近距离星间相对姿轨耦合动力学建模与控制

针对在轨服务过程中近距离星间相对位置以及相对姿态精确控制的问题,考虑相对姿态与相对轨道之间的耦合作用,建立星间相对姿轨耦合动力学模型.提出编队星相对基准星进行接近绕飞的任务需求,给出相对姿态以及相对轨道的期望状态.设计相对姿态轨道联合控制算法对相对姿态和轨道进行联合控制.仿真结果表明,所设计的控制算法能够很好地抑制相对姿态与相对轨道的相互影响作用,最终实现编队星对基准星的精确指向绕飞运动.     

基于能耗与可靠性约束的动量轮可重构性评价

针对目前在控制系统的可重构性评价中较少考虑系统能耗、可靠性限制的问题,文章研究了约束条件下卫星姿态控制系统可重构性度量及评价方法,建立了含执行机构失效因子的控制系统模型,并基于可控性格兰姆矩阵得到能耗约束下可重构性度量指标;然后将可靠性指标引入到系统可重构性评价度量中,进一步缩小可重构性集合;最终形成综合两种约束的可重构性评价方法。通过对含动量轮系统的重构构型仿真,得到约束前后的可重构性对比,仿真结果符合实际分析,验证了该方法的有效性。     

电磁监测试验卫星矢量磁场探测方法

电磁监测试验卫星是中国第一颗近地轨道电磁场科学探测试验卫星, 探测空间背景磁场是其重要任务之一. 空间背景磁场探测需要在卫星平台上对空间矢量磁场进行长期稳定准确探测, 电磁监测试验卫星采用磁通门磁强计和基于CPT效应的绝对磁场校准装置(Coupled Dark State Magnetometer, CDSM)分别探测空间相对矢量磁场以及绝对标量磁场, 通过数据处理, 使最终的矢量磁场探测数据具有准确性. 这种数据处理方法在理想模型下拥有解析解, 实施过程中载荷的噪声、 准确度及稳定性影响模型的准确性, 会产生数据校准误差. 通过对在轨磁场探测的模拟确定了这种数据处理方法的性能, 验证了在载荷设计性能的基础上电磁监测试验卫星的磁场探测可以实现1 nT的准确度.