A Parallel Discrete Event Simulation Engine for the Low-Earth-Orbit Satellite Constellation Networks

Low-Earth-Orbit satellite constellation networks(LEO-SCN)can provide low-cost,largescale,flexible coverage wireless communication services.High dynamics and large topological sizes characterize LEO-SCN.Protocol development and application testing of LEO-SCN are challenging to carry out in a natural environment.Simulation platforms are a more effective means of technology demonstration.Currently available simulators have a single function and limited simulation scale.There needs to be a simulator for full-featured simulation.In this paper,we apply the parallel discrete-event simulation technique to the simulation of LEO-SCN to support large-scale complex system simulation at the packet level.To solve the problem that single-process programs cannot cope with complex simulations containing numerous entities,we propose a parallel mechanism and algorithms LP-NM and LP-YAWNS for synchronization.In the experiment,we use ns-3 to verify the acceleration ratio and efficiency of the above algorithms.The results show that our proposed mechanism can provide parallel simulation engine support for the LEO-SCN.

Spacecraft potential variations of the Swarm satellites at low Earth orbital altitudes

In this study, we provide the first detailed analysis of variations in the spacecraft potential (Vs) of the three Swarm satellites, which are flying at about 400-500 km. Unlike previous studies that have investigated extreme charging events, usually with spacecraft potentials as negative as −100 V, this study is focused on variations of Swarm Vs readings, which fall within a few negative volts. The Swarm observations show that spacecraft at low Earth orbital (LEO) altitudes are charged only slightly negatively, varying between −7 V and 0 V, with the majority of recorded potentials at these altitudes clustering close to −2 V. However, a second peak of Vs data is found at −5.5 V, though the event numbers for these more-negative observations are less, by an order of magnitude, than for incidents near the −2 V peak. These two distinct Vs peaks suggest two different causes. We have thus divided the Swarm spacecraft Vs data into two categories: less-negatively charged (−5 < Vs < 0 V) and more-negatively-charged (−6.5 < Vs < −5 V). These two Vs categories exhibit different spatial and temporal distributions. The Vs observations in the first category remain relatively closer to 0 V above the magnetic equator, but become much more negative at low and middle latitudes on the day side;at high latitudes, these first-category Vs readings are relatively more-negative during local summer. Second-category Vs events cluster into two bands at the middle latitudes (between ±20°-50° magnetic latitude), but with slightly more negative readings at the South Atlantic Anomaly (SAA) region;at high latitudes, these rarer but more-negative second-category Vs events exhibit relatively more-negative values during local winter, which is opposite to the seasonal pattern seen in the first category. By comparing Vs data to the distributions of background plasma density at Swarm altitudes, we find for the first category that more-negative Vs readings are recorded at regions with higher background plasma density, while for the second category the more-negative Vs data are observed at regions with lower background plasma density. This can be explained as follows: the electron and ion fluxes incident on Swarm surface, whose differences determine the potential of Swarm, are dominated by the background “cold” plasma (due to ionization) and “hot” plasma (due to precipitated particles from magnetosphere) for the two Vs categories, respectively.

WITHDRAWN: High-Resolution Radiometer for Remote Sensing of Solar Flare Activity from Low Earth Orbit Satellites

This article has been withdrawn at the request of the author(s) and/or editor. The Publisher apologizes for any inconvenience this may cause.

Design and simulation of fault diagnosis based on NUIO/LMI for satellite attitude control systems

This paper presents a scheme of fault diagnosis for flexible satellites during orbit maneuver. The main contribution of the paper is related to the design of the nonlinear input observer which can avoid false alarm arising from the disturbance from orbit control force. The effects of orbit control force on the fault diagnosis system for satellite attitude control systems, including the disturbing torque caused by the misalignments and the model uncertainty caused by the fuel consumed, are discussed, where standard Lu- enberger observer cannot work well. Then the nonlinear unknown input observer is proposed to decouple faults from disturbance, Besides, a linear matrix inequality approach is adopted to reduce the effect of nonlinear part and model uncertainties on the observer. The numerical and semi-physical simulation demonstrates the effectiveness of the proposed observer for the fault diagnosis system of the satellite during orbit maneuver.

Orbit and attitude control of spacecraft formation flying

Formation flying is a novel concept of distributing the functionality of large spacecraft among several smaller, less expensive, cooperative satellites. Some applications require that a controllable satellite keeps relative position and attitude to observe a specific surface of another satellite among the cluster. Specially, the target space vehicle is malfunctioning. The present paper focuses on the problem that how to control a chaser satellite to fly around an out-of-work target satellite closely in earth orbit and to track a specific surface. Relative attitude and first approximate relative orbital dynamics equations are presented. Control strategy is derived based on feedback linearization and Lyapunov theory of stability. Further, considering the uncertainty of inertia, an adaptive control method is developed to obtain the correct inertial ratio. The numerical simulation is given to verify the validity of proposed control scheme.

Precise orbit determination for low Earth orbit satellites using GNSS:Observations,models,and methods

Spaceborne global navigation satellite system(GNSS)has significantly revolutionized the development of autonomous orbit determination techniques for low Earth orbit satellites for decades.Using a state-of-the-art combination of GNSS observations and satellite dynamics,the absolute orbit determination for a single satellite reached a precision of 1 cm.Relative orbit determination(i.e.,precise baseline determination)for the dual satellites reached a precision of 1 mm.This paper reviews the recent advancements in GNSS products,observation processing,satellite gravitational and non-gravitational force modeling,and precise orbit determination methods.These key aspects have increased the precision of the orbit determination to fulfill the requirements of various scientific objectives.Finally,recommendations are made to further investigate multi-GNSS combinations,satellite high-fidelity geometric models,geometric offset calibration,and comprehensive orbit determination strategies for satellite constellations.

Low Altitude Satellite Constellation for Futuristic Aerial-Ground Communications

This paper discusses the significance and prospects of low altitude small satellite aerial vehicles to ensure smooth aerial-ground communications for next-generation broadband networks.To achieve the generic goals of fifthgeneration and beyondwireless networks,the existing aerial network architecture needs to be revisited.The detailed architecture of low altitude aerial networks and the challenges in resource management have been illustrated in this paper.Moreover,we have studied the coordination between promising communication technologies and low altitude aerial networks to provide robust network coverage.We talk about the techniques that can ensure userfriendly control and monitoring of the low altitude aerial networks to bring forth wireless broadband connectivity to a new dimension.In the end,we highlight the future research directions of aerial-ground communications in terms of access technologies,machine learning,compressed sensing,and quantum communications.

Delay-optimal multi-satellite collaborative computation offloading supported by OISL in LEO satellite network

By deploying the ubiquitous and reliable coverage of low Earth orbit(LEO)satellite networks using optical inter satel-lite link(OISL),computation offloading services can be provided for any users without proximal servers,while the resource limita-tion of both computation and storage on satellites is the impor-tant factor affecting the maximum task completion time.In this paper,we study a delay-optimal multi-satellite collaborative computation offloading scheme that allows satellites to actively migrate tasks among themselves by employing the high-speed OISLs,such that tasks with long queuing delay will be served as quickly as possible by utilizing idle computation resources in the neighborhood.To satisfy the delay requirement of delay-sensi-tive task,we first propose a deadline-aware task scheduling scheme in which a priority model is constructed to sort the order of tasks being served based on its deadline,and then a delay-optimal collaborative offloading scheme is derived such that the tasks which cannot be completed locally can be migrated to other idle satellites.Simulation results demonstrate the effective-ness of our multi-satellite collaborative computation offloading strategy in reducing task complement time and improving resource utilization of the LEO satellite network.

Survivability model of LEO satellite constellation based on GERT with limited backup resources

Survivability is used to evaluate the ability of the satellite to complete the mission after failure,while the duration of maintaining performance is often ignored.An effective backup strategy can restore the constellation performance timely,and maintain good network communication performance in case of satellite failure.From the perspective of network utility,the low Earth orbit(LEO)satellite constellation survivable graphical eva-luation and review technology(GERT)network with backup satel-lites is constructed.A network utility transfer function algorithm based on moment generating function and Mason formula is proposed,the network survivability evaluation models of on-orbit backup strategy and ground backup strategy are established.The survivable GERT model can deduce the expected mainte-nance time of LEO satellite constellation under different fault states and the network utility generated during the state mainte-nance period.The case analysis shows that the proposed surviv-able GERT model can consider the satellite failure rate,backup satellite replacement rate,maneuver control replacement ability and life requirement,and effectively determine the optimal sur-vivable backup strategy for LEO satellite constellation with limi-ted resources according to the expected network utility.

GNSS rapid precise point positioning enhanced by low Earth orbit satellites

The Low Earth Orbit(LEO)satellites can be used to efectively speed up Precise Point Positioning(PPP)convergence.In this study,180 LEO satellites with a global distribution are simulated to evaluate their contribution to the PPP convergence.LEO satellites can give more redundant observations and improve satellite geometric distributions,particularly for a single Global Navigation Satellite System(GNSS).The convergence speed of the PPP foat solution using the Global Positioning System(GPS,G)or BeiDou Navigation Satellite System(BDS,C)single system as well as the G/C/Galileo navigation satellite system(Galileo,E)/GLObal NAvigation Satellite System(GLONASS,R)combined system with LEO satellites added is improved by 90.0%,91.0%,and 90.7%,respectively,with respect to the system without LEO satellites added.We introduced LEO observations to assist GNSS in PPP-AR(Ambiguity Resolution)and PPP-RTK(Real Time Kinematic).The success fx rate of a single system is signifcantly improved,and the Time-To-First-Fix(TTFF)of G and G/C/E is reduced by 86.4%and 82.8%,respectively,for the PPP-AR solution.We analyzed the positioning performance of LEO satellite assisted G/C/E PPP-RTK in the reference networks of diferent scales,namely diferent atmospheric delay interpolation accuracies.The success fx rate of the G/C/E combined system is improved from 86.8 to 94.9%,and the TTFF is reduced by 36.8%,with the addition of LEO satellites in the 57 km reference network.In the 110 km reference network,the success fx rate of the G/C/E combined system is improved from 64.0 to 88.6%,and the TTFF is reduced by 32.1%.GNSS PPP-RTK with adding the LEO satellites in the reference networks of diferent scales shows obvious improvement because the atmospheric correlation decreases with increasing distance from the reference networks.

Beam Position and Beam Hopping Design for LEO Satellite Communications

The numbers of beam positions(BPs)and time slots for beam hopping(BH)dominate the latency of LEO satellite communications.Aiming at minimizing the number of BPs subject to a predefined requirement on the radius of BP,a low-complexity user density-based BP design scheme is proposed,where the original problem is decomposed into two subproblems,with the first one to find the sparsest user and the second one to determine the corresponding best BP.In particular,for the second subproblem,a user selection and smallest BP radius algorithm is proposed,where the nearby users are sequentially selected until the constraint of the given BP radius is no longer satisfied.These two subproblems are iteratively solved until all the users are selected.To further reduce the BP radius,a duplicated user removal algorithm is proposed to decrease the number of the users covered by two or more BPs.Aiming at minimizing the number of time slots subject to the no co-channel interference(CCI)constraint and the traffic demand constraint,a low-complexity CCI-free BH design scheme is proposed,where the BPs having difficulty in satisfying the constraints are considered to be illuminated in priory.Simulation results verify the effectiveness of the proposed schemes.

Resource allocation for uplink grant-free access in beam-hopping based LEO satellite systems

The low earth orbit(LEO) satellite system provides a promising solution for the global coverage of Internet of Things(IoT) services.Confronted with the sporadic uplink transmission from massive IoT terminals, this work investigates the grant-free access scheme and resource allocation algorithm for the beam-hopping(BH) based LEO satellite systems.To improve the packet success rate, the time slots are pre-allocated to each cell according to the number of terrestrial terminals and the probability of packet arrival.When the packets arrive, the terrestrial terminals perform contention-free or contention-based grant-free access with packet repetition in the time slots allocated to their cells.The analytical expression of the packet collision probability for the grant-free access scheme is derived to provide reference for the resource allocation.To reduce the computational complexity, a heuristic resource allocation algorithm is proposed to minimize the maximum cell packet collision probability in the system.Simulation results show that the proposed resource allocation scheme achieves lower packet collision probability and higher resource utilization ratio when compared with the uniform resource allocation scheme.

低地球轨道卫星边缘计算场景中任务卸载与资源分配联合优化算法

针对低地球轨道(LEO)卫星边缘计算场景中地面用户计算任务的卸载需求,提出联合卸载与资源分配优化(JORAO)算法。考虑到LEO卫星的有限覆盖时间,以最小化所有地面用户的平均服务时延为目标,联合优化卸载策略、LEO卫星的通信和计算资源分配。将任务卸载与资源分配的联合优化问题分解为卸载决策和资源分配子问题,使用交替优化方法,获得原始优化问题的次优解。对于任务卸载决策子问题,将其建模为联盟博弈模型,当博弈达到纳什均衡时,获得最小化平均服务时延的地面用户卸载策略;对于资源分配子问题,使用拉格朗日乘子法获得最优的通信和计算资源分配结果。此外,还证明了所提算法的收敛性和稳定性。仿真结果表明,所提算法具有良好的收敛性,能显著降低地面用户的平均服务时延和提高任务卸载成功率。

手机直连低轨卫星的应用场景及业务需求分析

手机直连卫星是低轨(low earth orbiting,LEO)星座系统的重要应用方向之一。概述了手机直连卫星的主要技术路线、应用场景及业务类型;分析了低轨星座的建设成本、承载能力、运营收入;提出了我国手机直连卫星通信的主要应用应以广覆盖、海量连接为主,而卫星大流量宽带上网应用存在商业闭环的风险;最后探讨了结合应用需求的技术发展建议。

HOI延迟对LEO卫星简化动力学POD的影响

通常使用无电离层(IF)线性组合(LC)消除低地球轨道(LEO)卫星简化动力学精密定轨(POD)一阶电离层延迟误差,忽略了高阶电离层(HOI)延迟误差。随着LEO卫星POD技术的发展,计算不同轨道高度的HOI延迟并探索其变化已成为进一步提高POD精度的重要手段。首先,使用国际参考电离层-2016(IRI-2016)和国际地磁参考场第13代(IGRF-13)模型,计算电离层穿刺点(IPP)位置和地磁场强度。其次,使用平滑星载GNSS数据计算电离层斜路径总电子含量(STEC)。然后,分别计算GOCE、GRACE-A和SWARM-A/B卫星的二阶和三阶电离层延迟。最后,评估了HOI延迟对LEO卫星重叠轨道分析、卫星激光测距(SLR)检核和精密科学轨道(PSO)比较结果的影响。实验结果表明:HOI延迟对LEO卫星简化动力学POD的影响大约在毫米至厘米的数量级上;HOI延迟对LEO卫星简化动力学POD外符合精度的影响分别达到0.92,0.22,0.21和0.18 mm;随着LEO卫星轨道高度的增加,HOI延迟对LEO卫星简化动力学POD的影响减小。

东方红系列通信卫星控制技术发展综述

自1975年至今,我国已成功发展了以东方红系列为主的多型通信卫星平台,累计发射卫星70余颗,为我国国民经济和国防建设做出了重大贡献.概述我国东方红系列通信卫星平台的发展历程,梳理通信卫星控制系统发展脉络及主要特点,重点从姿态控制、轨道控制、信息体系架构和自主健康管理4个方面阐述了通信卫星控制技术主要进展.在此基础上,结合空间任务需求,对通信卫星控制技术的未来发展方向进行分析和展望,助推未来创新发展.

广播星历旋转误差对低轨星载BDS-3/GPS实时精密定轨影响分析

基于LT-01A卫星星载BDS-3/GPS观测值进行了星载实时精密定轨研究,并重点分析了广播星历旋转误差对实时定轨精度的影响。通过赫尔默特转换评估了所选时段内GPS和BDS-3广播星历轨道旋转误差,显示BDS-3广播星历旋转误差可达-8.7 mas,平均量级较GPS大约2.5倍。BDS-3广播星历经旋转改正后,轨道切向、法向均方根(RMS)误差从25 cm左右提升至10 cm量级,提升幅度超过50%。因此,基于星载BDS-3以及BDS-3/GPS联合的实时定轨精度受BDS-3星历旋转误差影响严重,且主要作用于切向和法向。经过旋转改正后,单独BDS-3实时定轨在切向、法向、径向RMS分别为21.0 cm、10.7 cm及11.2 cm,其切向和法向精度比改正前分别提升15.0%和31.8%;BDS-3与GPS联合定轨进一步提升切向精度至19.4 cm。得益于BDS-3广播星历较高的精度,单BDS-3以及BDS-3/GPS联合的实时定轨在旋转改正前的三维RMS分别为31.9 cm和29.2 cm,较单GPS实时定轨分别提升9.1%和16.8%;添加旋转改正后,其定轨精度分别提升至26.7 cm和25.0 cm,较单GPS实时定轨分别提升22.6%和27.5%。

星载6G核心网架构与网元功能设计及验证

为全面提升卫星的在轨服务能力,将6G核心网与卫星融合。针对6G核心网在低轨卫星部署的迫切需求,设计星载6G核心网体系架构,包括分布式架构、离线自治、网元智能化等功能。对星载6G核心网主要网元进行了优化设计,包括接入与移动性管理、会话管理、分布式服务注册与发现等。通过在轨部署和仿真试验,验证了所提星载6G核心网体系架构的有效性。仿真结果表明,集中式星载5G核心网网元之间的平均通信时延为109.3 ms,而分布式星载6G核心网的平均通信时延为60.3 ms,相较而言降低了44.8%;此外,5G集中式服务注册和服务发现的平均时延分别为40.53 ms和40.04 ms,而6G分布式服务注册和服务发现的平均时延分别为35.18 ms和34.91 ms,分别降低13.2%和12.8%。

基于功能链的北斗三号卫星控制技术

为适应卫星研制技术和规模空前发展,在卫星高可靠的同时做到低成本,将传统的分系统设计方法按特点和学科进行了重新梳理,提出国内首创的功能链设计理念和方法,并成功应用于北斗三号首发试验星、组网星及备份星中,实现了北斗三号卫星的快速批产组网.阐述基于功能链的设计理念,重点介绍基于功能链的北斗三号卫星姿态及轨道等控制技术,在轨运行结果表明,卫星实现了连续可靠的工作,全面适应ka及激光星间链路的高控制精度要求以及导航卫星对高连续性的要求.

面向用户需求的低轨卫星资源分配算法

低轨(LEO)卫星多波束通信场景下,传统固定资源分配算法无法满足不同用户对信道容量的差异需求。以适应用户需求分配为主要目标,建立联合信道分配、带宽分配和功率分配的最小供需差优化模型,并引入图样分割多址接入技术(PDMA)提升信道资源的利用率。针对该模型的非凸特性,通过Q-learning算法学习资源分配最优策略为每个用户分配适合的信道容量,并引入奖励阈值进一步改进算法,加快算法的收敛,且使算法达到收敛时供需差异更小。仿真结果表明,改进后的算法收敛速度约是改进前的3.33倍:改进算法能满足更大的用户需求,比改进前Qlearning算法提升14%,是传统固定算法的2.14倍。