GPS-only gravity field recovery with GOCE,CHAMP, and GRACE

Gravity missions such as the Gravity field and steady-state Ocean Circulation Explorer (GOCE) are equipped with onboard Global Positioning System (GPS) receivers for precise orbit determination (POD), instrument time-tagging, and the extraction of the long wavelength part of the Earth’s gravity field. The very low orbital altitude of the GOCE satellite and the availability of dense 1 s GPS tracking data are ideal characteristics to exploit the contribution of GPS high-low Satellite-to-Satellite Tracking (hl-SST) to gravity field determination. We present gravity field solutions based on about 8 months of GOCE GPS hl-SST data from 2009 and compare the results with those obtained from the CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery And Climate Experiment (GRACE) missions. The very low orbital altitude of GOCE significantly improves gravity field recovery from GPS hl-SST data above degree 20, but not for the degrees below 20, where the quality of the spherical harmonic coefficients remains essentially unchanged. Despite the limited time span of GOCE data used, the gravity field of the Earth can be resolved up to about degree 115 using GPS data only. Empirically determined phase center variations (PCVs) of the GOCE onboard GPS helix antenna are, however, mandatory to achieve this performance.     

附加先验轨道约束的LEO几何法实时精密定轨验证分析

低轨卫星的实时精密定轨能够极大拓展其完成复杂科学任务的能力,例如实时环境监测、机动控制和卫星自主导航等.本文根据几何法实时精密定轨模型,提出了附加LEO先验轨道约束从而改善实时定轨的精度、收敛速度和稳定性的构想.分别采用广播星历、超快速星历预报部分和实时精密星历,设计了6种实时定轨方案,并利用Swarm-A,B,C星7天的观测数据进行方案验证与分析.结果表明,使用广播星历、IGU和IGC星历的方案精度递增,附加先验轨道约束能够进一步提升精度.使用IGC星历并附加标准差为1m的先验轨道约束后,在径向、切向和法向的定轨精度分别达到6.12cm,5.55cm和4.98cm.此外,附加先验轨道约束能够显著提升收敛速度,使用IGC星历平均收敛时间约为31min,附加标准差为1m的先验轨道约束后收敛仅需约4min.     

GPS single-frequency orbit determination for low Earth orbiting satellites

The determination of high-precision orbits for Low Earth Orbiting (LEO) satellites (e.g., CHAMP, GRACE, MetOp-A) is based on dual-frequency tracking data from on-board GPS (Global Positioning System) receivers. The two frequencies allow it to eliminate the first order ionosphere effects. Data screening and precise orbit determination (POD) procedures are optimized under the assumption of the availability of two frequencies.     

GRO和LRO掩星事件模拟研究

GRO（Global Navigation Satellite System Radio Occultation）和LRO（Low Earth Orbit Radio Occultation）联合组网探测地球大气是无线电掩星探测技术的主要发展方向.本文根据掩星事件的数学判据,仿真分析了LEO卫星主要轨道参数对GRO和LRO掩星事件数量和全球分布情况的影响.研究表明：卫星轨道越低GRO掩星事件越多;轨道倾角在30°和75°之间时,GRO掩星事件较多,全球覆盖率也较大;利用极轨卫星进行LRO掩星探测时,LRO掩星事件较均匀地分布在各纬度带.研究成果对GRO和LRO联合星座设计具有参考价值.     

Very low thrust station-keeping for low Earth orbiting satellites

Traditional station-keeping for Earth observation satellites with chemical thrusters generally involves maneuvers every couple months that are able to change significantly the semi-major axis and the inclination. These strategies do not scale down to very low thrust level (a few hundreds of μN) electrical thrusters. This paper presents both in-plane and out-of-plane strategies that spread corrections over very long arcs and discretize them to tiny maneuvers every couple orbits, taking into account mission-constraints on maneuvers locations. These strategies scale up to medium thrust strategies, filling the gap between propulsion technologies. The out-of-plane strategy although features a new no-deadband property and controls the full orbital momentum. All strategies allow control very close to the reference (a few hundreds meters in osculating parameters) and very low cost.     

低轨航天器弹道系数估算及热层大气模型误差分析

利用低轨（LEO）航天器在轨期间两行轨道根数（TLEs）数据,结合经验大气密度模型NRLMSISE00,反演计算得到其在轨期间的弹道系数B’,以31年B’的平均值代替弹道系数真值,分别通过标准球形目标卫星对比以及物理参数基本相同的非球形目标卫星对比,对弹道系数真值进行了检验;利用不同外形目标卫星弹道系数在不同太阳活动周内的变化规律,结合太阳和地磁活动变化,估计经验大气密度模型的误差分布. 结果表明,利用反演弹道系数31年的平均值来代替真值,其在理论值的正常误差范围内;大气密度模型误差在210～526km高度范围内存在相同的变化趋势,且模型误差随高度增加而增大;在短周期内B’变化与太阳活动指数F_10.7存在反相关性;密度模型不能有效模拟2008年出现的大气密度异常低. 以上结果表明,经验大气密度模型结果需要修正,尤其是在太阳活动峰年和谷年,此外,磁暴期间模型误差的修正对卫星定轨和轨道预报等也具有重要意义.     

Recovery of the Earth’s gravity field from formation-flying satellites: Temporal aliasing issues

Temporal and mean gravity field models derived from the twin-satellite, leader–follower mission GRACE have provided a new type of information for Earth sciences. In this contribution, we study the potential of various alternative satellite formations for gravity field determination in the post-GRACE era in a simulation environment. In particular, the effects of spherical harmonic truncation and of temporal aliasing in the processing of gravity products from such future formations are investigated.     

基于空间隔离的低轨卫星系统频谱共享方法

随着业务需求的全球性增长,传统低频波段有限的频谱资源已不能满足低轨（LEO）卫星的业务承载要求,采用Ka频段的LEO星座系统设计成为了主流。然而,采用Ka频段的LEO星座系统不可避免地会对同频段已经存在的静止轨道（GEO）卫星系统产生干扰。首先,通过基于空间隔离的干扰规避方法,采用对LEO与GEO的频谱共用系统设置隔离角来进行上下行场景的干扰消除,使该LEO系统能满足国际电信联盟（ITU）对同频段内通信干扰的约束条件;然后,通过引入GEO带的概念,分析并研究了干扰规避区域的确定方法和其对LEO星座密度的影响。最后,通过仿真得出了干扰隔离角、GEO带与LEO星座密度的关系,得到了在采用隔离角的方式进行干扰避让时满足LEO星座系统业务不中断所需要的最小卫星数量,对LEO星座设计具有一定的指导意义。      

Impact of GPS antenna phase center variations on precise orbits of the GOCE satellite

The first European Space Agency Earth explorer core mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) has been launched on March 17, 2009. The 12-channel dual-frequency Global Positioning System receiver delivers 1 Hz data and provides the basis for precise orbit determination (POD) on the few cm-level for such a very low orbiting satellite (254.9 km). As a member of the European GOCE Gravity Consortium, which is responsible for the GOCE High-level Processing Facility (HPF), the Astronomical Institute of the University of Bern (AIUB) provides the Precise Science Orbit (PSO) product for the GOCE satellite. The mission requirement for 1-dimensional POD accuracy is 2 cm. The use of in-flight determined antenna phase center variations (PCVs) is necessary to meet this requirement. The PCVs are determined from 154 days of data and the magnitude is up to 3-4 cm. The impact of the PCVs on the orbit determination is significant. The cross-track direction benefits most of the PCVs. The improvement is clearly seen in the orbit overlap analysis and in the validation with independent Satellite Laser Ranging (SLR) measurements. It is the first time that SLR could validate the cross-track component of a LEO orbit.     

近地轨道集群航天器电磁编队飞行非线性反馈控制方法

针对近地轨道集群航天器电磁编队飞行的动力学和控制问题, 提出了一种非线性反馈控制方法. 基于电磁力模型和地磁场模型, 分析了地磁场对近地轨道电磁编队的影响; 建立了集群航天器电磁编队高精度相对轨道动力学模型; 基于Lyapunov稳定性理论设计了一种非线性反馈控制律, 利用该方法对两星电磁编队维持控制进行了仿真验证. 仿真结果表明, 地磁场引起的电磁干扰力可以忽略, 但是电磁干扰力矩的影响必须考虑; 近地轨道集群航天器电磁编队是可控的, 所设计的控制方法是可行的.     

路由算法在LEO通信星座构型优化中的应用

低轨(LEO,Low Earth Orbit)通信星座的特殊功能决定了其构型优化需要综合考虑成本、覆盖和路由性能等因素.以往的研究往往忽略了路由性能,导致优化得到的构型不能很好的满足要求.首先给出了两个意义明确的星座路由性能的评价指标,然后通过将它们作为目标函数进行仿真,验证了其优化必要性和评价有效性.最后利用多目标遗传算法实现了星座的构型优化.仿真结果显示,将路由算法加入到星座的构型优化中,在仅对星座构型参数进行微调的前提下,不仅满足了成本、覆盖等基本要求,而且星座的路由性能大大提高.     

AIUB-CHAMP02S: The influence of GNSS model changes on gravity field recovery using spaceborne GPS

The gravity field model AIUB-CHAMP02S, which is based on six years of CHAMP GPS data, is presented here. The gravity field parameters were derived using a two step procedure: In a first step a kinematic trajectory of a low Earth orbiting (LEO) satellite is computed using the GPS data from the on-board receiver. In this step the orbits and clock corrections of the GPS satellites as well as the Earth rotation parameters (ERPs) are introduced as known. In the second step this kinematic orbit is represented by a gravitational force model and orbit parameters.     

基于混合粒子滤波的载波估计算法

针对粒子滤波载波估计算法的高复杂度、粒子退化及贫化问题,提出了一种基于混合粒子滤波的载波估计方法.该方法引入多阶马尔科夫模型,采用多个非零均值高斯分布的加权和来近似重要性函数的最佳选择,并根据最大后验概率准则规范粒子的迭代计算.仿真结果表明,在非高斯噪声环境下,低轨卫星通信TDMA/DEQPSK(Time Division Multiple Address/Differential Quadrature Phase Shift Keying)数据帧非合作接收载波估计时,与基于经典粒子滤波的载波估计算法相比,提高了粒子\"效率\",在误码性能相当的情况下,有效降低了计算复杂度.