PROBA-3 mission and the Shadow Position Sensors: Metrology measurement concept and budget

PROBA-3 is a space mission of the European Space Agency that will test, and validate metrology and control systems for autonomous formation flying of two independent satellites. PROBA-3 will operate in a High Elliptic Orbit and when approaching the apogee at 6·10⁴ Km, the two spacecraft will align to realize a giant externally occulted coronagraph named ASPIICS, with the telescope on one satellite and the external occulter on the other one, at inter-satellite distance of 144.3 m. The formation will be maintained over 6 hrs across the apogee transit and during this time different validation operations will be performed to confirm the effectiveness of the formation flying metrology concept, the metrology control systems and algorithms, and the spacecraft manoeuvring. The observation of the Sun’s Corona in the field of view [1.08;3.0]R_Sun will represent the scientific tool to confirm the formation flying alignment. In this paper, we review the mission concept and we describe the Shadow Position Sensors (SPS), one of the metrological systems designed to provide high accuracy (sub-millimetre level) absolute and relative alignment measurement of the formation flying. The metrology algorithm developed to convert the SPS measurements in lateral and longitudinal movement estimation is also described and the measurement budget summarized.     

Application of satellite observations to study the changes of hypoxic conditions in near-bottom water in the western part of Peter the Great Bay (the Sea of Japan)

In this paper we explore the possibilities of applying satellite ocean colour (OC) observations and SST to study the changes in the conditions of hypoxia in the near-bottom water in the western part of Peter the Great Bay. Near-bottom water hypoxia occurs in water bodies with increased organic matter influx when the dissolved oxygen (DO) consumed at its oxidation is not restored. Consumption of most DO is usually attributed to the oxidation of organic matter formed as a result of increased algae growth during water eutrophication. Satellite data on indicators of phytoplankton (chlorophyll-a concentration (Chl) and fluorescence (FLH)) allow to analyze the spatial-temporal changes of this substation. Coloured dissolved organic matter (CDOM), non-algal particles (NAP) influence on satellite Chl estimates and also on near-bottom water hypoxia formation. This study analyzes daily, seasonal, and inter-annual changes in the distributions of indicators (Chl, FLH, the coefficients of light absorption by coloured detrital matter (a_CDM) and light backscattering by suspended particles (b_bp)), based on the instant satellite OC data from MODIS-Aqua. Data on the Chl, the sea surface temperature (SST) from the MODIS-Aqua, the precipitation from the TRMM satellite and the hydrometeorological stations (HMSs), the wind speed and direction from HMS “Vladivostok” are used to study the influence of hydrometeorological conditions on the Chl values. These distributions were compared with the literary information based on field observations of the hypoxia cases in the same area and with the changes in the vertical DO, Chl, temperature, salinity distribution obtained by coastal expeditions in October-November 2010 and February-March 2011. Significant interrelations within 95% confidence level between the satellite Chl, FLH values calculated at the MUMM atmospheric correction and in situ Chl values obtained in the autumn of 2010 were reached separately for the cases with winds of northern and southern directions with the correlation coefficients of 0.71, 0.48 and 0.49, 0.71, respectively. Significant dependences of Chl on SST and Chl on wind speed explained by the influence of continental runoff and water ventilation were obtained. Therefore, the changes of Chl reflect the changes of hypoxic conditions in the near-bottom water. In Amursky Bay the onset of hypoxia was at the Chl and SST values equal to 4 mg m^?3 and 13 °C (↑ – at increasing SST); near Furugelm Island it was at 1.6 mg m^?3 and 25 °C (↑), 1 mg m^?3 and 21 °C (↓). The difference in the Chl values was reflected in the hypoxia onset timings that were the beginning of June (2011), August (2013), and September (2014), respectively. The water flow from the eastern coast of Amursky Bay in early August of 2013 recorded from the OC and SST satellite imagers appeared in an additional hypoxic zone. Decreased OC characteristics in the runoff of the Razdolnaya River in August-September of 2014 were a sign of hypoxia at its mouth. Near Furugelm Island the hypoxia destruction (increase in the DO level from 1 to 4.5 ml L^?1) was observed at the Chl of 0.9 mg m^?3 and SST = 18 °C (↓). At the autumn maximum of Chl equal to 1.7 mg m^?3 and SST = 4 °C (↓) in mid-November the DO level here increased to 8 ml L^?1. In Amursky Bay, short-term destructions/weakening of hypoxia manifested themselves in sharp increases of Chl. At that, the ratio between the Chl value and the approximation level was equal to 2 and higher for SST equal to 22–25 °C (↑), to 0.9 and higher for SST equal to 5–13 °C (↓). With the water stratification destruction in temperature and the noticeable weakening of the stratification in salinity (mid-November), the hypoxia destructed (the DO level increased from 2 to 6 ml L^?1). In this case, Chl and SST were about 3 mg m^?3 and 5 °C (↓).     

用蒙特卡罗方法研究卫星内部带电问题

由高能电子引起的卫星内部带电,是危害地球同步轨道卫星主要的空间环境效应之一.内部带电的物理机制和相应的卫星设计规范研究,近年来受到国际空间界的高度重视.本文用蒙特卡罗方法,通过模拟高能电子对电介质轰击后在介质中产生电荷沉积,计算了轰击后电介质内部最大电场强度,并进而分析了电介质内部电场与电子能量、屏蔽厚度及介质厚度的关系.该结果对卫星设计有重要参考价值.     

卫星太阳电池阵的刚-柔耦合动力学

研究了卫星太阳电池阵在多点撞击过程中的刚-柔耦合动力学问题.基于Jourdain速度变分原理和单向递推组集方法,建立了柔性多体系统的动力学模型.用Hertz撞击法则建立了撞击力和撞击处的局部变形关系,分别用刚体模型和柔性体模型仿真计算了太阳电池阵的撞击力、撞击时间,揭示了变形运动和大范围运动的相互耦合作用.     

GPS实时定姿系统的设计与实现

以GPS OEM板为基本硬件,在自行研制GPS实时定姿软件的基础上,搭建了GPS实时定姿系统。该系统硬件结构简单,成本低廉。定姿软件解决了数据接收、解码、同步,模糊数快速计算,动态检校等关键技术。经多次静态、动态试验验证,系统运行稳定,动态跟踪性能良好。基线长度为1.2m时,定姿精度优于0.3°,可单独用于系统的导航,或作为系统导航设备的校准装置,具有很好的应用价值。     

基于星载AIS数据的TEC测量方法

基于星载船舶自动识别系统（AIS）,提出一种计算全球电离层电子总含量（TEC）的方法。通过在卫星上搭载两个相互垂直的线极化天线,测量AIS信号穿过电离层时的法拉第旋转角,再通过法拉第旋转角与TEC的关系估算TEC。基于天拓五号卫星的AIS数据进行了实验验证,并分析了硬件设备误差和观测参数误差对结果造成的影响。实验表明,本方法测量出的TEC值与基于全球定位系统（GPS）测量的TEC值差值平均为0.762 TECU,证明了此方法的可行性。与现有的TEC测量方法相比,该方法只需利用现有的AIS系统,无需部署地面站,可大幅提高数据更新速率。     

成像观测卫星多星联合调度模型及其列生成求解算法研究

首先介绍成像观测卫星调度问题的特点和主要约束,将成像观测卫星调度分为调度预处理、调度模型及求解、调度结果评估3个阶段。在此基础上,论述成像观测卫星调度模型,并采用列生成法,将多卫星调度问题分解为集合分割主问题和单卫星调度子问题,通过循环迭代来求解调度模型。最后,针对本文提出的算法设计一个问题实例,并给出算法计算结果。结果分析表明,本文提出的模型和算法能较好解决实际应用问题。     

飞行器控制的伪线性系统方法——第一部分：综述与问题

首先将非线性控制方法归纳成三类：基于李雅普诺夫泛函的设计方法、基于最优控制思想的设计方法和以线性为主导的设计方法,并对此三类方法进行了简略的综述。然后进一步在此类划分的框架下概述了飞行器控制的非线性方法,并引入了伪线性系统的概念。最后介绍了卫星姿态与轨道控制、飞行器制导与控制中的六类典型飞行器控制问题的二阶伪线性系统描述。     

Accuracy and consistency of different global ionospheric maps released by IGS ionosphere associate analysis centers

Due to the differences of ionospheric modeling methods and selected tracking stations, the accuracy and consistency of Global Ionospheric Maps (GIMs) released by Ionosphere Associate Analysis Centers (IAACs) are different. In this study, we evaluate and analyze in detail the accuracy and consistency of GIMs final products provided by six IAACs from three different aspects. Firstly, the comparison of these GIMs shows that the mean bias (MEAN) is related to the modeling methods of various IAACs. The variation trend of the standard deviation (STD) is consistent with the solar activities, and accompanied by certain seasonal and annual periodic variations. The MEAN between IGS and each center is about −1.3 to 1.0 TECU, and the STD is about 1.4–2.5 TECU. Secondly, the validation with GPS TEC shows that the STD of CODE is the smallest at various latitudes, and the STD is about 0.7–4.5 TECU. Thirdly, The validation with the Jason2 VTEC shows that the STD between Jason2 and IAACs is about 4.4–5.2 TECU. In addition, the STD between Jason2 and six GIMs in the areas with more tracking stations is better than that of the regions with fewer tracking stations in different latitude regions. Regardless of whether the tracking stations are more or less, the MEAN and STD in high solar activity are larger than in low solar activity.     

Multi-GNSS contributions to differential code biases determination and regional ionospheric modeling in China

Due to the limited number and uneven distribution globally of Beidou Satellite System (BDS) stations, the contributions of BDS to global ionosphere modeling is still not significant. In order to give a more realistic evaluation of the ability for BDS in ionosphere monitoring and multi-GNSS contributions to the performance of Differential Code Biases (DCBs) determination and ionosphere modeling, we select 22 stations from Crustal Movement Observation Network of China (CMONOC) to assess the result of regional ionospheric model and DCBs estimates over China where the visible satellites and monitoring stations for BDS are comparable to those of GPS/GLONASS. Note that all the 22 stations can track the dual- and triple-frequency GPS, GLONASS, and BDS observations. In this study, seven solutions, i.e., GPS-only (G), GLONASS-only (R), BDS-only (C), GPS + BDS (GC), GPS + GLONASS (GR), GLONASS + BDS (RC), GPS + GLONASS + BDS (GRC), are used to test the regional ionosphere modeling over the experimental area. Moreover, the performances of them using single-frequency precise point positioning (SF-PPP) method are presented. The experimental results indicate that BDS has the same ionospheric monitoring capability as GPS and GLONASS. Meanwhile, multi-GNSS observations can significantly improve the accuracy of the regional ionospheric models compared with that of GPS-only or GLONASS-only or BDS-only, especially over the edge of the tested region which the accuracy of the model is improved by reducing the RMS of the maximum differences from 5–15 to 2–3 TECu. For satellite DCBs estimates of different systems, the accuracy of them can be improved significantly after combining different system observations, which is improved by reducing the STD of GPS satellite DCB from 0.243 to 0.213, 0.172, and 0.165 ns after adding R, C, and RC observations respectively, with an increment of about 12.3%, 29.4%, and 32.2%. The STD of GLONASS satellite DCB improved from 0.353 to 0.304, 0.271, and 0.243 ns after adding G, C, and GC observations, respectively. The STD of BDS satellite DCB reduced from 0.265 to 0.237, 0.237 and 0.229 ns with the addition of G, R and GR systems respectively, and increased by 10.6%, 10.4%, and 13.6%. From the experimental positioning result, it can be seen that the regional ionospheric models with multi-GNSS observations are better than that with a single satellite system model.