Design of BDS-3 integrity monitoring and preliminary analysis of its performance

With the improvement in the service accuracy and expansion of the application scope of satellite navigation systems, users now have high demands for system integrity that are directly related to navigation safety. As a crucial index to measure the reliability of satellite navigation systems, integrity is the ability of the system to send an alarm when an abnormity occurs. The new-generation Beidou Navigation Satellite System (BDS-3) prioritized the upgrading of system integrity as an important objective in system construction. Because the system provides both basic navigation and satellite-based augmentation system (SBAS) services by the operational control system, BDS-3 adopts an integrated integrity monitoring and processing strategy that applies satellite autonomous integrity monitoring and ground-based integrity monitoring for both the basic navigation service and SBAS navigation service. BDS-3 also uses an improved and refined integrity parameter system to provide slow, fast and real-time integrity parameters for basic navigation, and provide SBAS-provided integrity information messages in accordance with Radio Technical Commission for Aeronautics (RTCA) specification and dual frequency, multi-constellation (DFMC) specification to support the SBAS signal frequency, single constellation operation and DFMC operation respectively. The performance of BDS-3 system integrity monitoring is preliminarily verified during on-orbit testing in different states, including normal operation, satellite clock failure and satellite ephemeris failure. The results show that satellite autonomous integrity monitoring, ground-based integrity monitoring and satellite-based augmentation all correctly work within the system. Satellite autonomous integrity monitoring can detect satellite clock failure but not satellite orbit failure. However, ground-based integrity monitoring can detect both. Moreover, the satellite-based augmentation integrity system monitors the differential range error after satellite ephemeris and clock error corrections based on user requirements. Compared to the near minute-level time-to-alert capability of ground-based integrity monitoring, satellite autonomous integrity monitoring reduces the system alert time to less than 4 s. With a combined satellite-ground monitoring strategy and the implementation of different monitoring technologies, the BDS-3 integrity of service has been considerably improved.     

3J53弹簧表面裂纹缺陷原因分析

3J53钢丝在车间缠绕成弹簧并经热处理后发现其表面存在线性缺陷。用扫描电镜对钢丝线性缺陷的形貌进行了分析,对线性缺陷区域进行了能谱分析,并利用金相显微镜观察了线性缺陷的中心部位及其周围区域的金相组织。结果表明:钢丝表面的线性缺陷呈锯齿状,沿钢丝轴向方向,为裂纹型折叠缺陷。折叠缺陷是钢丝在生产过程中形成,在绕簧和热处理过程中,裂纹型折叠缺陷发生扩展和延伸,最终形成线性缺陷。     

Preliminary analysis of the quality and positioning performance of BDS-3 global interoperable signal B1C&B2a

BeiDou-3 Navigation Satellite System (BDS-3) satellites are equipped with the new generation GNSS signals B1C and B2a, which support the interoperability with GPS and Galileo systems. In this study, the pseudo-range multipath error and carrier phase observation noise of the BDS-3 B1C and B2a signals were evaluated based on zero baseline measurements from the day of year (DOY) 113 to 116, 2020. Further, the precision and performance of the single point positioning (SPP) and precise point positioning (PPP) are assessed at 9 stations. This assessment manifests that the standard deviations (STDs) of the pseudo-range multipath error are about 0.09 ~ 0.22 m, while STDs of the carrier phase observation noise are about 0.075 mm. For the single-frequency SPP, its positioning precision is about 2.03 ~ 4.85 m and 3.29 ~ 10.73 m at the 99.99% confidence level in horizontal and vertical directions, respectively, while the dual-frequency SPP precision is about 1.92 ~ 8.02 m and 4.81 ~ 12.77 m in horizontal and vertical directions, respectively. For the daily static PPP, the convergence time is about 20 ~ 30 min, while the daily positioning precision can reach 1.38 ~ 4.42 cm and -1.31 ~ 4.34 cm in horizontal and vertical directions, respectively. In general, the quality and the SPP and PPP performance of the BDS-3 B1C&B2a signals are comparable to the GPS and Galileo.     

Finite element modelling and experimental investigation of the impact of filling different materials in copper tubes during 3D free bending process

3D free bending process, an advanced metal forming technology, has attracted much attention due to its unique geometrical flexibility and efficiency. Filling tubes with materials may effectively reduce the distortion in the cross-section area and restrain wrinkling and collapse of tube wall. In the present study, the impacts of filling different materials on the copper tubes during 3D free bending process were investigated. We have found that copper tubes filled with low melting point alloys could induce the most uniform stress distribution, which result in better formability and moderate thickness changing rates when compared to SS304 steel balls and PU rubbers. The lowest ellipticity reached to 1.467%.     

空间失稳目标线阵成像畸变分析与三维重建

针对线阵扫描型激光雷达在非合作失稳航天器三维模型重建过程中，存在运动目标线扫数据畸变且目标因失稳导致运动特性难以准确估计问题，提出一种带畸变自修正式三维重建方法，并进行畸变容忍度临界值分析。该方法基于测量系统的角精度与测距精度设计畸变容忍度临界值判定条件，首先根据实际工况与判定条件进行比对，判定是否对线扫数据进行初始畸变恢复；其次，对相邻扫描带畸变点云依次进行配准获取目标运动增量、解算目标瞬时运动参数；然后对目标运动参数进行自修正式处理，去除特征突变点干扰；同时依次恢复点云到初始几何位置，完成初始目标三维重建；最后，通过系统进一步迭代，位姿增量自收敛直至最终满足重建精度要求。仿真结果表明，该方法在缺失非合作目标先验信息的情况下，能够有效避免因线扫描导致的畸变问题，并较为快速地实现满足精度要求的目标三维重建，有较强的鲁棒性。     

Beidou wide-area augmentation system clock error correction and performance verification

On December 27, 2018, the Beidou-3 System (BDS-3) has completed the deployment of 18 Medium Earth Orbit (MEO) satellites combined as a space constellation. In addition to the augmentation information for the new system signals B1C and B2a, the BDS-3 is compatible with the three augmentation information broadcast by the BDS-2 system for B1I and B3I signals: equivalent clock error correction, User Differential Ranging Error (UDRE) and Grid Ionosphere Vertical Error (GIVE). In this paper, the observation data of Beidou monitoring network are used to analyze the pseudo-range observation quality of the smooth transition signals B1I and B3I of BDS-2 and BDS-3. At the same time, the relationship between the equivalent clock error correction and the prediction error of the satellite clock is analyzed by using the Two-Way Satellite Time and Frequency Transfer (TWSTFT) data. The results show that the correlation between the equivalent clock error correction and the monitored clock error by using the TWSTFT data is greater than 60%. We calculate the UDRE by using the equivalent clock error correction. The results show that the satellite equivalent clock error correction can improve the accuracy of User Equivalent Range Error (UERE) by about 50%. This paper also compares the positioning accuracy of the BDS-2 satellites with the BDS-2 satellites combined BDS-3 satellites. The results show that the three-dimensional positioning accuracy is improved by about 30% after the BDS-3 satellites are added.     

3GPP中TFCI编译码研究

在3GPP（第三代合作项目）协议中,为了提高传输格式组合指示（TFCI）的可靠性,对其采取了基于Reed-Muller码的超码编码。与经典RM编码相比,不但增加了码距,提高了纠错能力,而且可以利用快速哈达玛变换（FHT）提高译码速度。文章对协议中的编译码算法进行了研究,并对其纠错能力进行了仿真。     

固体火箭发动机Al_2O_3凝结及颗粒破碎的数值模拟

Al2O3凝结对固体火箭发动机Al颗粒的燃烧效率及燃气流动有很大影响。结合拉格朗日方法,建立Al2O3凝结模型,分析了在Al2O3烟雾凝结及颗粒自身破碎作用下,不同初始直径Al颗粒的燃烧效率及燃烧室流场的变化规律。计算结果与同条件下的测试数据有较好的吻合,颗粒分布符合相关实验现象。结果表明,小颗粒燃烧后,流场温度及Al2O3烟雾分布均匀;随颗粒初始直径的增加,径向出现明显分层现象;在发动机出口,小颗粒燃烧效率较高,颗粒中Al2O3质量分数较大,但破碎程度较小;随初始直径增加,颗粒燃烧效率逐渐降低,颗粒中仍含有大量未燃烧的Al,破碎程度提高,颗粒数目急剧增加。     

真空中多孔氮化硅的高温弯曲强度

研究了高温真空环境下多孔(气孔率>50%)氯化硅陶瓷的弯曲强度,并进行了初步分析.结果表明,多孔氮化硅陶瓷在真空中的高温强度随温度上升而降低,由于没有氧化作用,晶界玻璃相在高温下的软化成为影响其强度的主要因素.     

三维标准件库创建技术研究

在飞机设计过程中,需要大量使用标准件,而在标准件的建模过程中,需要反复查阅手册以确定零件的外形尺寸,耗费了大量的时间。本文阐述了在CATIA中建立标准件模型和参数库的方法,并使用CAA在VC6.0的开发环境下对CATIA标准件库的开发技术进行了研究,建立了一个基于CATIA的三维标准件库框架,解决了标准件的重复造型工作,提高了生产效率。