Scientific objectives and payloads of Tianwen-1, China’s first Mars exploration mission

This paper describes the scientific objectives and payloads of Tianwen-1, China’s first exploration mission to Mars. An orbiter, carrying a lander and a rover, lifted-off in July 2020 for a journey to Mars where it should arrive in February 2021. A suite of 13 scientific payloads, for in-situ and remote sensing, autonomously commanded by integrated payload controllers and mounted on the orbiter and the rover will study the magnetosphere and ionosphere of Mars and the relation with the solar wind, the atmosphere, surface and subsurface of the planet, looking at the topography, composition and structure and in particular for subsurface ice. The mission will also investigate Mars climate history. It is expected that Tianwen-1 will contribute significantly to advance our scientific knowledge of Mars.     

Early analysis of precise orbit and clock offset determination for the satellites of the global BeiDou-3 system

Eight new-generation BeiDou satellites (BeiDou-3) have been launched into Medium Earth Orbit (MEO), allowing for global coverage since March 2018, and they are equipped with new hydrogen atomic clocks and updated rubidium clocks. Firstly, we analyzed the signals for the carrier-to-noise-density ratio (C/N0) and pseudorange multipath (MP) by using international GNSS (Global Navigation Satellite System) Monitoring and Assessment System (iGMAS) station data, and found that B1C has a lower C/N0, and B2a has the same level of C/N0 as the B1I and B3I signals. For pseudorange multipath, compared with the BeiDou-2 satellites, the obvious systematic variation of MP scatters related to the elevation angle is greatly improved for the BeiDou-3 and BeiDou-3e satellites signals. For the signals of the BeiDou-3 satellites, the order of the Root Mean Square (RMS) values of multipath and noise is B3I?<?B1I?<?B2a?<?B1C. Then, the comparison of the precise orbit determination and clock offset determination for the BeiDou-2, BeiDou-3, and BeiDou-3 experimental (BeiDou-3e) satellites was done by using 10 stations from iGMAS. The 3D precision of the 24?h orbit overlap is 24.55, 25.61, and 23.35?cm for the BeiDou-3, BeiDou-3e, and BeiDou-2 satellites, respectively. BeiDou-3 satellite has a comparable precision to that of the BeiDou-2 satellite. For the precision of clock offset estimation, the Standard Deviation (STD) of the BeiDou-3 MEO satellite is 0.350?ns, which is an improvement of 0.042?ns over that of the BeiDou-2 MEO satellite. The stabilities of the BeiDou-3 and BeiDou-3e onboard clocks are better than those of BeiDou-2 by factors of 2.84 and 1.61 at an averaging time of 1000 and 10,000?s, respectively.     

RTK model and positioning performance analysis using Galileo four-frequency observations

This paper proposes a real-time kinematic (RTK) model that uses one common reference satellite for the Galileo system with four frequency observations. In the proposed model, the double-differenced (DD) pseudorange and carrier phase biases among the different frequencies are estimated as unknown parameters to recover the integer features of the DD ambiguities among the different frequencies for ambiguity resolution and precise positioning. Analysis results show that the E5a, E5b, and E5 frequencies have virtually the same performance in terms of the positioning accuracy, observation residuals, and ratio values of ambiguity resolution. However, the E1 frequency performs worse than the E5a, E5b, and E5 frequencies. The RTK results for the combination of multiple frequencies are much better than those for a single-frequency observation, the coordinates’ standard deviation is improved about 20–30%, and the ambiguity fix time is improved about 10%.     

系列化构型运载能力设计余量留取方法研究

调研了美国航天型号研制中设计余量留取的情况,结合系列化构型论证工作,对运载能力设计余量留取问题进行了初步研究,分析了关键参数对运载能力的影响,针对不同目标轨道、系列化构型的差异,探索得到了运载能力设计余量差异化留取的方法。     

脉冲星方位误差估计的两步卡尔曼滤波算法

为了克服钟差和卫星位置误差对脉冲星方位误差估计的影响，设计了两步卡尔曼滤波（TSKF）算法。首先，介绍了脉冲星方位误差估计的传统模型，并通过分析和仿真验证了钟差、卫星位置误差以及2种误差同时存在时会使脉冲星方位误差估计结果产生较大偏差。其次，在传统的估计模型中加入了钟差和卫星位置误差，并将钟差和钟差变化率增广为新的状态量，从而推导出包含2种误差的新模型，并证明了该模型的完全可观测性；根据该模型并按照两步卡尔曼滤波原理，得到了TSKF算法的步骤。最后，通过仿真表明:在钟差和卫星位置误差同时影响下，传统脉冲星方位误差估计算法偏差较大且发散；TSKF算法则能够有效隔离2种误差的影响，使赤经和赤纬误差估计达到0.2 mas之内的精度。      

非线性弹簧形态优化及可调节恒力装置设计

基于组合结构研究一种可调节恒力装置的设计方法，恒力装置由负刚度非线性弹簧和正刚度线性弹簧组合而成。采用B样条曲线描述非线性弹簧的形态，基于GA算法和非线性有限元法实现非线性弹簧的形态优化设计。根据恒力装置的工作原理，设计线性螺旋弹簧，并分析刚度/强度特性。组合结构的力学性能分析表明恒力装置可以实现输出恒力。为了实现自我保护、输出的恒力大小/方向可调，进行了恒力装置的部组件设计。本文的恒力装置设计方法为平面薄膜天线和太阳电池阵的工程研制提供了技术支撑。     

Event-triggered attitude control of spacecraft

The problem of spacecraft attitude stabilization control system with limited communication and external disturbances is investigated based on an event-triggered control scheme. In the proposed scheme, information of attitude and control torque only need to be transmitted at some discrete triggered times when a defined measurement error exceeds a state-dependent threshold. The proposed control scheme not only guarantees that spacecraft attitude control errors converge toward a small invariant set containing the origin, but also ensures that there is no accumulation of triggering instants. The performance of the proposed control scheme is demonstrated through numerical simulation.     

固体火箭冲压发动机设计点性能优化分析

针对冲压动力飞行器射程优化总体需求，提出了基于飞行器-发动机性能因子的评估方法，建立了固体火箭冲压发动机性能模型及优化设计流程，为总体与动力一体化优化设计提供了支撑。根据典型含硼贫氧推进剂性能分析结果可知，相同马赫数及余气系数下，高度对比冲的影响不明显；相同马赫数下，比冲随余气系数的增加先增加后减小；相同余气系数下，比冲随马赫数的增加而降低；飞发性能因子随马赫数、余气系数的增加先增加后降低，在一定条件下达到最大值。