LEO navigation augmentation constellation design with the multi-objective optimization approaches

Low Earth Orbit (LEO) satellite for navigation augmentation applications can significantly reduce the precise positioning convergence time and attract increasing attention recently. A few LEO Navigation Augmentation (LEO-NA) constellations have been proposed, while corresponding constellation design methodologies have not been systematically studied. The LEO-NA constellation generally consists of a huge number of LEO satellites and it strives for multiple optimization purposes. It is essentially different from the communication constellation or earth observing constellation design problem. In this study, we modeled the LEO-NA constellation design problem as a multi-objective optimization problem and solve this problem with the Multi-Objective Particle Swarm Optimization (MOPSO) algorithm. Three objectives are used to strive for the best tradeoff between the augmentation performance and deployment efficiency, namely the Position Dilution of Precision (PDOP), visible LEO satellites and the orbit altitude. A fuzzy set approach is used to select the final constellation from a set of Pareto optimal solutions given by the MOPSO algorithm. To evaluate the performance of the optimized constellation, we tested two constellations with 144 and 288 satellites and each constellation has three optimization schemes: the polar constellation, the single-layer constellation and the two-layer constellation. The results indicate that the optimized two-layer constellation achieves the best global coverage and is followed by the single-layer constellation. The MOPSO algorithm can help to improve the constellation design and is suitable for solving the LEO-NA constellation design problem.     

Performance of a two-layer LEO satellite communication network

A novel two-layer low Earth orbiting (LEO) satellite network is proposed. Comparing with a single layer global network, not only the system performance is improved but also the number of satellites can be reduced. The main ideas include (1) to raise the altitude of the original global network, and (2) to add several satellites at lower altitude to form a regional network to serve calls from the areas of heavy traffic. In order to fully utilize the benefits of the two-layer network, additional functions such as adjustable beam and dynamic channel management are added. Call blocking rate is derived. Numerical examples are provided to demonstrate several interesting phenomena.     

Reinforcement learning based dynamic distributed routing scheme for mega LEO satellite networks

Recently, mega Low Earth Orbit (LEO) Satellite Network (LSN) systems have gained more and more attention due to low latency, broadband communications and global coverage for ground users. One of the primary challenges for LSN systems with inter-satellite links is the routing strategy calculation and maintenance, due to LSN constellation scale and dynamic network topology feature. In order to seek an efficient routing strategy, a Q-learning-based dynamic distributed Routing scheme for LSNs (QRLSN) is proposed in this paper. To achieve low end-to-end delay and low network traffic overhead load in LSNs, QRLSN adopts a multi-objective optimization method to find the optimal next hop for forwarding data packets. Experimental results demonstrate that the proposed scheme can effectively discover the initial routing strategy and provide long-term Quality of Service (QoS) optimization during the routing maintenance process. In addition, comparison results demonstrate that QRLSN is superior to the virtual-topology-based shortest path routing algorithm.     

Routing in networks of intersatellite links

How to characterize and implement point-to-point (i.e., unicast) routing algorithms is discussed. The characteristics of routing algorithms are first described as well as the specificities of routing in constellations of nongeostationary (NGO) satellites equipped with intersatellite links (ISLs). Then, some of the candidate algorithms displaying the characteristics identified before are simulated and evaluated for different constellations. From the simulation results, it appears that (1) from a routing perspective, constellations are not similar to terrestrial networks, and (2) the constellation geometry has a significant impact on the routing performance.     

基于聚类的星座卫星可用性分选方法

卫星可用状况会随时间发生变化,为了实现对卫星工作情况的监控,应经常对目标卫星进行分析。通过实际侦收卫星信号确定卫星可用状况是最可靠的方法,但这种方法工作量大、周期长、效率低、实时性差。为了解决这一问题,本文提出一种运用聚类算法的卫星可用状况快速分选方法。该方法利用从互联网下载的最新轨道参数,根据卫星轨道特性方面的差别进行自动分选,具有速度快、人工干预少的优点。将这种方法用于铱星星座分选,分选正确率可达80.44%。     

基于Markov过程的卫星星座备份策略研究

如何在满足星座可用性指标的同时降低卫星星座备份成本,是当前研究的热点问题。为了选出一种兼顾星座性能与经济效益的备份策略,研究不同备份策略对某低轨卫星星座性能的影响,建立考虑降级运行的星座可用度模型,利用 Markov 状态转移过程对星座可用度模型进行求解,并基于真实低轨卫星星座在轨运行数据对采用不同备份策略的星座在不同...     

Galileo卫星星钟故障前后信号质量分析

星载原子钟是卫星导航系统的核心器件,是影响导航信号质量的重要因素。2016年11月,Galileo系统利用一箭四星发射升空4颗新一代Galileo卫星。根据观测显示,截至2017年7月,仅能接收到其中1颗卫星发射的导航信号。在2017年1月,欧洲航天局通告称多颗卫星原子钟大规模故障。针对此,40m 大口径高增益天线系统设备对不同批次和星钟故障情况的3颗Galileo卫星E5频点信号进行了数据采集处理,对各信号分量的功率谱、星座图、码片波形、相关损失、相关峰、S曲线偏差等信号质量指标进行了分析评估。结果表明,此次星载原子钟的大面积故障并未影响Galileo卫星信号的可用性,足够的星载备份钟避免了信号质量的严重恶化。     

Sensitivity analysis of dual-satellite geolocation

An analytic solution is presented for the linearized estimation problem arising from the dual-satellite geolocation of the source of a narrowband signal using time- and frequency-difference-of-arrival (TDOA and FDOA, respectively) observations and an altitude constraint. This solution is used to analyze the covariance matrix of the resultant source location estimate, along with the sensitivity of its mean to errors in the presumed source altitude and velocity, and the measured or predicted positions and velocities of the satellites. Following the practice of differential Global Positioning System, the technique of differential calibration is used to reduce or eliminate various sources of solution bias which would otherwise require tight, and consequently expensive, individual calibration     

Global communication using a constellation of low Earth meridianorbits

The concept of meridian orbits is briefly reviewed. It is shown that, if a satellite in the meridian orbit makes an odd number (>1) of revolutions per day, then the satellite passes over the same set of meridians twice a day. Satellites in such orbits pass over the same portion of the sky twice a day and every day. This enables a user to adopt a programmed mode of tracking, thereby avoiding a computational facility for orbit prediction, look angle generation, and auto tracking. A constellation of 38 or more satellites placed in a 1200-km altitude circular orbit is favorable for global communications due to various factors. It is shown that appropriate phasing in right ascension of the ascending node and mean anomaly results in a constellation wherein each satellite appears over the user's horizon one satellite after another. Visibility and coverage plots are provided to verify the continuous coverage     

光纤通道交换网络WRR实时调度算法分析

 随着综合航电系统的发展,1553B总线已经不能满足发展的需要,光纤通道交换式网络作为新一代高性能网络通信协议被引入到航电系统中,其性能指标与整个航电系统的总体指标有着密切的关系.为了解决系统消息实时发送的问题,研究了光纤通道交换网络实时条件下加权轮询调度算法的通信性能,利用随机Petri网对光纤通道交换网络的调度过程进行了建模,通过对模型的仿真和分析,得到了数据传输量及传输时限变化时系统网络负载、延迟时间、超时消息所占比例等重要性能指标,并分析了调度算法的性能.仿真结果证明光纤通道交换式网络具有高速稳定性,加权轮询调度算法能够满足消息传输的实时性、公平性要求.     

Satellite Navigation Systems for Land Vehicles

Today's motorists often must confirm their route by referring to a roadmap. Looking at a map while driving creates a poor traffic situation, one even more acute for drivers of emergency service vehicles. To address this problem, development engineers at the General Motors Technical Cener and Delco Electronics integrated an experimental GPS receiver into a GM Buick Park Avenue. The receiver is designed to acquire and sequentially track signals from four satellites. The vehicle's precise latitude, longitude, and altitude are determined and presented on a color cathode ray tube (CRT) map display in the car's instrument panel.     

Survivable communication concept via multiple low Earth-orbitingsatellites

An overview of a reliable cross-link communication system using multiple low-Earth-orbit satellites is presented, covering the overall network concept, data packet routing, communication techniques including antenna subsystems, communication parameter tradeoffs, signal processing, and the overall radio design concept definition. The critical issue addressed is the maximum cross-link distance coverage obtained under stressed conditions by utilizing an optimal coding technique for a given prime power of the satellite     

星座碰撞规避的迭代学习构型保持方法CSCD

针对低轨卫星星座运行中地球引力摄动的周期特性,基于迭代学习控制(ILC)方法,提出了星座碰撞规避的迭代学习构型保持方法。该方法由反馈控制和ILC两部分构成,分别抑制卫星运行过程中的非周期摄动和周期摄动对构型保持精度的影响,进而在地球非球形引力摄动未知条件下,通过相对构型的精确保持实现对星座卫星碰撞的有效规避。仿真结果表明,在地球J摄动影响下,与传统反馈控制相比,ILC方法以更小的控制输入实现了轨道保持精度的显著提升,进而在星座卫星轨道高度相近的情形下显著降低了碰撞风险,且控制器可在保证收敛性能的前提下,实现启动时间的灵活选择。     

A Concept of Position Fixing by a Passive Mode Navigation Satellite System

A position fix in a passive mode using satellites usually necessitates an expensive computer or lengthy hand calculation. This is the largest drawback of passive navigation and it would be more desirable if the user could find his position by a mere glance at a chart and table, as one uses Loran. The first step toward this goal is to use a synchronous satellite because it simplifies the problem. The next step is to find the position of the user by a Loran type of chart, which is universal, and correct this apparent position by looking at a special table which is made according to the amount of perturbation of both the satellite and the user's position. An example of the position fix along the route between Yokohama to Hawaii is shown. The concept can be extended to orbiting satellites due to the rules which govern the motion of satellites, if the fix accuracy is in the order of 2 to 5 miles. This method should be more accurate than the common sextant and more practical due to the fact that the satellite can be used at any time and in any weather. As a total system, it will be better than Omega because it could provide additional navigation information such as communication or traffic control by using satellites.     

钟滑动辅助GNSS定位算法研究

针对GNSS卫星星座几何形状不好或者观测卫星数量不足,无法为导航解算和自主完好性监测提供足够观测量的问题,提出采用GNSS接收机时钟观测值进行辅助定位,提高定位精度,实现自主完好性监测。通过对接收机时钟建模,引入本地钟的钟差方程、辅助接收机观测方程,从而建立了一套完整的钟滑动辅助定位算法。通过对采集的真实数据作计算机仿真,证明了该算法的可行性和有效性。结果表明,在卫星星座几何形状较差或者某颗卫星出现故障时,利用钟差辅助算法能够有效地排除故障星,得到较好的定位结果。     

BDS接收机星间载波相位偏差

BDS(BeiDou satellite navigation System,北斗卫星导航系统)的MEO/IGSO (Medium Earth Orbit/Inclined Geosynchronous Satellite Orbit,中圆地球轨道/倾斜地球同步轨道)卫星在D1导航电文上调制了NH(Neumann-Hoffman,奈曼-霍夫曼)二次编码,而在GEO(Geostationary Earth Orbit,地球静止轨道)卫星上未使用.利用北斗MEO/IGSO和GEO卫星进行差分定位时,采用不同NH码符号映射规则的接收机之间会出现半周载波相位偏差的问题,严重影响RTK (Real-Time Kinematic,实时动态差分)应用.通过分析半周载波相位偏差的形成机理,从单差、双差、三差应用角度分别研究了半周载波相位偏差的影响.分析结果表明,接收机NH码符号映射规则与卫星不一致时,接收机直接用乘法器对NH码解调将存在反相,也即引入了半周载波相位偏差;在满足特定条件下,单差和双差应用时会存在半周载波相位偏差的问题;三差应用时不存在半周载波相位偏差的问题.最后,提出了需在北斗接口控制文件中明确卫星NH码调制或映射规则的修订建议.     

基于多目标粒子群算法的导航星座优化设计

导航星座的设计涉及诸多优化变量的选取,优化设计的目的是选取合适的优化变量使导航星座最大程度地满足人们需求。提出了将导航性能和卫星生产成本作为目标对导航星座进行多目标优化设计的研究方案,导航星座基本构型为中轨道(MEO)与地球静止轨道(GEO)卫星组成的混合星座,MEO卫星用于全球导航,GEO卫星用于增强星座对中国及周边地区的导航性能。探讨了MEO和GEO的轨道设计思路。阐述了星座导航性能与卫星生产成本的计算方法,并选取定位精度因子(PDOP)作为导航性能指标。介绍了基本粒子群算法和多目标优化的概念,提出了改进的多目标粒子群算法(MOPSO),给出了该算法的计算步骤和测试结果。讨论了导航星座多目标优化设计的数学模型,列举了优化设计变量的定义域,采用MOPSO算法对导航星座进行了多目标优化设计,通过分析优化设计结果,说明了导航星座多目标优化设计方案的可行性。     

海面低飞情况下机载航管二次雷达覆盖研究

二次雷达较之一次雷达有许多优点，使其在军用和民用领域得到广泛应用。但当装有航管二次雷达的飞机在海面低飞时，会出现常见的多径效应现象。文章详细分析了飞机机体和海面对航管二次雷达方向图的影响，并利用HFSS软件对飞机在海面低飞时航管二次雷达方向图进行仿真。结果表明，海面影响航管二次雷达的方向图并导致根据其分叉变形，根据平面对天线方向图的影响及其计算公式，对不同飞行高度的方向图进行了计算，得出了在海面低飞时的最佳飞行高度。     

Using open networking standards over MIL-STD-1553 networks

Over the past several decades, the MIL-STD-1553 networking technology has found use in a number of military and aerospace platforms, including applications on aircraft, ships, tanks, missiles, satellites, and even the International Space Station. In developing software applications for these platforms, the use of modern, open networking standards such as TCP/IP is often preferable. The Internet Protocol (IP) provides communications routing, and the Transmission Control Protocol (TCP) provides reliable delivery to the application level. Furthermore, higher-level protocols such as the HyperText Transfer Protocol (HTTP), the File Transfer Protocol (FTP), etc. can be utilized in a TCP/IP environment. Though these open communications standards are preferable for many situations, the MIL-STD-1553B standard does not immediately lend itself to TCP/IP communications. One of the reasons for this is the fundamental difference between the MIL-STD-1553B networking standard, which relies on a bus controller to control communications and other data link layer networking protocols such as IEEE 8023 (Ethernet) which are Carrier Sense Multiple Access (CSMA) networks, and are thus decentralized. Despite differences in MIL-STD-1553B networking and more traditional data link layer networking protocols, there is nothing fundamentally preventing IP communication over a 1553 network. We have implemented a method of encapsulating IP datagrams within MDL-STD-1553B data messages that allows for transparent use of Internet Protocol (IP) APIs at the application level. Our system allows traditional 1553 messages to also be transported over the network, and even allows traditional messages to take a higher transmission priority over IP traffic. We analyze the advantages of such a system and the performance level we achieved with our implementation of this concept.     

低轨星座/惯导紧组合导航技术研究CSCD

现有低轨(LEO)卫星导航研究主要以低轨星座独立导航定位和增强全球卫星导航系统(GNSS)导航定位为主,对低轨卫星和惯性导航系统(INS)组合导航技术研究较少。本文面向应用较小规模低轨星座资源实现米级定位精度的需求,提出了一种低轨星座/惯导紧组合导航方法,系统性地分析了不同规模低轨星座、不同精度级别惯导器件以及不同导航信号播发频度下组合导航定位的性能,并利用构建的仿真试验系统进行了低轨星座/惯导紧组合导航方法的仿真试验验证。试验结果表明,相较于低轨星座独立导航,低轨星座/惯导紧组合导航在星座不满足四重覆盖时仍能达到米级定位精度,并且在低轨星座规模较小和导航信号播发频度较低时,惯导测量精度对组合导航定位精度影响明显。研究结果表明,在利用低轨卫星进行导航时,通过引入惯性观测辅助低轨卫星导航,可有效提高导航效能和精度,为低轨星座和导航信号播发方式设计带来更多的选择。