利用LEO星群反演地球重力场低阶带谐项变化的交叠点法

新一代重力卫星计划主要依靠高精度星载加速度计(ACC)、星间测距系统(KBR)或星载梯度计(SGG)进行地球重力场探测。搭载高精度星载GPS接收机的低轨卫星(low earth orbit,LEO)可以相对较低成本获取LEO卫星的精密轨道。卫星精密轨道一方面服务于LEO的主任务(如遥感、气象等);另一方面可以将这类LEO卫星和星座综合起来,构成LEO星群LEO星群精密轨道数据包含的丰富地球重力场信息为获取地球重力场的时变信息提供可能。本文给出一种利用LEO星群精密轨道数据反演地球重力场低阶带谐系数时变信息的实用方法——交叠点法,该方法可有效消弱非保守力等因素对重力场反演的影响。然后,以COSMIC(constellation observing system for meteorology ionosphere&climate)为实例分析LEO星群交叠点的覆盖特性,径向轨道精度对交叠点法的影响。最后进行低阶带谐系数(C02和C03)时变信号的模拟计算,并对结果进行分析。

LEO星群探测地球重力场的性能分析

探索如何利用低成本的低轨卫星(low Earth orbit,LEO)星群的精密轨道资料探测地球重力场,并以COSMIC(constellation observing system for meteorology ionosphere & climate)为研究实例初步分析了其覆盖性和方法的可行性。

A multi-path routing algorithm of LEO satellite networks based on an improved ant colony system

Geography rectangle is used to reduce signaling overhead of the LEO satellite networks.Moreover,a multi-path routing algorithm based on an improved ant colony system(MPRA-AC) is proposed.Matrix indicating the importance of the link between satellites is introduced into MPRA-AC in order to find the optimal path more quickly.Simulation results show that MPRA-AC reduces the number of iterations to achieve a satisfactory solution.At the same time,the packet delivery ratio of LEO satellite networks when running MPRA-AC and DSR-LSN(dynamic source routing algorithm for LEO satellite networks) is compared.The packet delivery ratio is about 7.9%lower when running DSR-LSN.Moreover,because of the mechanism of active load balancing of MPRA-AC,simulation results show that MPRA-AC outperforms DSR-LSN in link utilization when data packets are transmitted in the networks.

Packet Cache-Forward Method Based on Improved Bayesian Outlier Detection for Mobile Handover in Satellite Networks

In this paper, we propose a Packet Cache-Forward(PCF) method based on improved Bayesian outlier detection to eliminate out-of-order packets caused by transmission path drastically degradation during handover events in the moving satellite networks, for improving the performance of TCP. The proposed method uses an access node satellite to cache all received packets in a short time when handover occurs and forward them out in order. To calculate the cache time accurately, this paper establishes the Bayesian based mixture model for detecting delay outliers of the entire handover scheme. In view of the outliers' misjudgment, an updated classification threshold and the sliding window has been suggested to correct category collections and model parameters for the purpose of quickly identifying exact compensation delay in the varied network load statuses. Simulation shows that, comparing to average processing delay detection method, the average accuracy rate was scaled up by about 4.0%, and there is about 5.5% cut in error rate in the meantime. It also behaves well even though testing with big dataset. Benefiting from the advantage of the proposed scheme in terms of performance, comparing to conventional independent handover and network controlled synchronizedhandover in simulated LEO satellite networks, the proposed independent handover with PCF eliminates packet out-of-order issue to get better improvement on congestion window. Eventually the average delay decreases more than 70% and TCP performance has improved more than 300%.