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.

Information gain based sensor search scheduling for low-earth orbit constellation estimation

This paper addresses the problem of sensor search scheduling in the complicated space environment faced by the low-earth orbit constellation.Several search scheduling methods based on the commonly used information gain are compared via simulations first.Then a novel search scheduling method in the scenarios of uncertainty observation is proposed based on the global Shannon information gain and beta density based uncertainty model.Simulation results indicate that the beta density model serves a good option for solving the problem of target acquisition in the complicated space environments.

A Strategy for Magnifying Vibration in High-Energy Orbits of a Bistable Oscillator at Low Excitation Levels

This work focuses on how to maintain a high-energy orbit motion of a bistable oscillator when subjected to a low level excitation. An elastic magnifier （EM） positioned between the base and the bistable oscillator is used to magnify the base vibration displacement to significantly enhance the output characteristics of the bistable oscillator. The dimensionless electromechanical equations of the bistable oscillator with an EM are derived, and the effects of the mass and stiffness ratios between the EM and the bistable oscillator on the output displacement are studied. It is shown that the jump phenomenon occurs at a lower excitation level with increasing the mass and stiffness ratios. With the comparison of the displacement trajectories and the phase portraits obtained from experiments, it is vMidated that the bistable oscillator with an EM can effectively oscillate in a high-energy orbit and can generate a superior output vibration at a low excitation level as compared with the bistable oscillator without an EM.

Atmospheric Remnants in the Low Earth Orbit Region around 200 km Altitude

Study of atmospheric remnants in the low Earth orbit region (~200 km altitude) using Global Models, with application to electric thrusters of in situ resources utilization type.

A Study on the Relationship between the Orbital Lifetime and Inclination of Low Lunar Satellites

A detailed theoretical analysis on the orbital lifetime and orbital inclination of a Low Moon-Orbiting satellite （LMOs） and the ‘stable areas＇ of long orbital lifetime are given. Numerical simulations under the real force model were carried out, which not only validate the theoretical analysis and also give some valuable results for the orbit design of the LMOs.

Re-Entry of Space Objects from Low Eccentricity Orbits

This paper deals with the re-entry predictions of the space objects from the low eccentric orbit. Any re-entering object re-enters the Earth’s atmosphere with a high orbital velocity. Due to the aerodynamic heating the object tends to break into multiple fragments which later pose a great risk hazard to the population. Here a satellite is considered as the space object for which the re-entry prediction is made. This prediction is made with a package where the trajectory path, the time of re-entry and the survival rate of the fragments is done. The prediction is done using DRAMA 2.0—ESA’s Debris Risk Assessment and Mitigation Analysis Tool suite, MATLAB and Numerical Prediction of Orbital Events software. The predicted re-entry time of OSIRIS 3U was found to be on 7th March 2019, 7:25 (UTC), whereas the actual re-entry time was on 7th March 2019, 7:03 (UTC). The trajectory path found was 51.5699 deg. (Lat), &minus;86.5738 deg. (Long.) with an altitude of 168.643 km. But the actual trajectory was 51.76 deg. (Lat), &minus;89.01deg. (Long.) with an altitude of 143.5 km.

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.

Design of low-energy transfer from lunar orbit to asteroid in the Sun-Earth-Moon system

Asteroid exploration trajectories which start from a lunar orbit are investigated in this work.It is assumed that the probe departs from lunar orbit and returns to the vicinity of Earth,then escapes from the Earth by performing a perigee maneuver.A low-energy transfer in Sun-EarthMoon system is adopted.First,the feasible region of lowenergy transfer from lunar orbit to perigee within 5 000 km height above the Earth surface in Sun-Earth-Moon system is calculated and analyzed.Three transfer types are found,i.e.,large maneuver and fast transfers,small maneuver and fast transfers,and disordered and slow transfers.Most of feasibility trajectories belong to the first two types.Then,the lowenergy trajectory leg from lunar orbit to perigee and a heliocentric trajectory leg from perigee to asteroid are patched by a perigee maneuver.The optimal full-transfer trajectory is obtained by exploiting the differential evolution algorithm.Finally,taking 4179 Toutatis asteroid as the target,some low-energy transfer trajectories are obtained and analyzed.

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。

Demonstration on the indexes design of gravity satellite orbit parameters in the low-low satellite-to-satellite tracking mode

Combining with the exigent demand of the development of satellite gravimetry system in China, aiming at the determination of technical indexes of gravity satellite orbit parameters, on the basis of the numerical experiments and results analysis, the design indexes of gravity satellite orbit height, inter-satellite range and the orbit inclination are analyzed and calculated, and the issues towards twin gravity satellites such as coherence requirement of the orbit semi-major axes, control requirement of the pitch angle and time interval requirement to keep twin satellites formation in mobility are diseussed. Results show that the satellite orbit height is 400 km to 500 km, the inter-satellite range is about 220 km, the satellite orbit inclination is between polar orbit and sun-synchronous orbit, the semi-major axes difference of twin satellites orbit is within ± 70. 146 m, the pitch angle of twin satellites is about 0.9 degree, and the time interval to keep twin satellites formation in mobility is 7 days to 15 days.

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%。

面向低轨卫星的低资源大带宽自适应成形滤波设计

由于低轨卫星通信过程中信号电平动态变化较大,为了提高信道利用率系统需要根据通信链路情况自动控制传输速率。为保证系统通信不中断,满足实时通信的需求,提出了一种面向低轨卫星的低资源大带宽新型成形滤波器设计方法,该方法采用并行多相级联积分梳状(Cascade Integrator Comb, CIC)滤波器的方式设计成形滤波器,能够实现几百kHz至几百MHz大带宽信号的成形,解决高、中、低速信号自适应连续成形滤波问题。相比传统成形滤波器的高速信号处理能力提高了8倍,能节省乘法器资源95%以上,适应低轨卫星通信场景下星载处理使用。仿真结果表明该滤波器对信号的成形效果和带外抑制情况与传统滤波器几乎一致。

利用遗传算法实现低倾角卫星星座优化设计

针对热点区域的多重覆盖问题,采用回归圆轨道的共星下点轨迹星座方案,提出基于遗传算法求解单颗卫星轨道参数、利用简单解析公式求解星座轨道参数的方法。首先,建立了回归轨道热点区域多重覆盖模型;其次,以卫星一个回归周期内覆盖时间最长为性能指标,首颗卫星轨道根数利用遗传算法可优化求取;最后,根据星下点轨迹重合这一约束条件设计星座构型,并利用各卫星间几何关系求解各卫星的轨道根数。理论分析和仿真结果表明,该方法简单且易于实现,设计结果正确,可实现回归周期内同一卫星星下点轨迹多次覆盖目标,星座使用卫星少、覆盖频率高,具有较好的工程应用价值。

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%。

双机械臂布局下的电推进位保策略

对于地球同步轨道的电推进卫星,可采用双机械臂式电推布局进行轨道维持控制.本文研究了双机械臂式电推布局的自主位保控制策略,针对南北各一台机械臂,每台机械臂分别配置一台电推力器的布局方式,通过每天1~2次点火,以尽量少的点火指向变动实现东西/南北联合位保控制.在双侧机械臂、电推力器正常情况下研究具有解析形式的在轨位保算法.研究基于单侧矢量调节机构和该侧电推力器的故障位保策略,给出衡量不同矢量机构能力和推力比冲下的位保效率衡量方法.仿真结果表明,正常/故障情况下的位保控制精度均能够满足同步轨道位保控制要求.本算法为解析形式,计算量较小,可以实现多星共轨和定点捕获/轨位漂移,便于量上自主实施.

基于DVB-S2X协议的低轨卫星通信性能仿真分析

介绍了DVB-S2X标准,重点分析了其中高阶调制技术的基本原理和星座映射,在进行星地链路分析的基础上,构建了DVB-S2X标准低轨卫星通信仿真模型,针对DVB-S2X标准中16APSK、32APSK、64APSK调制信号开展仿真试验,以误码率和丢帧率为指标进行通信性能分析。

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。