Zero-difference and single-difference precise orbit determination for LEO using GPS

Various methods for precise orbit determination (POD) of low earth orbiters (LEO) are briefly intro-duced in this paper. Based on the software named SHORD-Ⅲ developed by our institute,sin-gle-difference (SD) and zero-difference (ZD) dynamic POD based on LEO carrying an on-board GPS receiver is mainly discussed. The approaches are tested using real GRACE data (November 5―25,2002) and independently validated with Satellite Laser Ranging (SLR) measurements over the same 21 days. Comparisons with the scientific orbits provided by GFZ indicate that the SD POD RMS accuracy can achieve 5,10 and 6 cm in radial,along and cross the track,and the ZD POD RMS accuracy can achieve 4,8 and 4 cm in radial,along and cross the track. SLR validation shows that SD POD accuracy is better than 8 cm in distance,and ZD POD accuracy is better than 6 cm.     

星载GPS非差精密定轨精度分析

采用一种变相的“非差”动力定轨方法，建立了相应的数学模型，编制了星栽GPS低轨卫星的定轨软件，并以此分析了定轨过程中各主要误差源对定轨的影响以及所能达到的定轨精度和预报精度。该方法以相位平滑伪距为观测值，通过对同一历元的现测值进行卫星间求差，消除了星栽GPS接收机钟差，模型简单，求解参数少。在只估计初始坐标和速度参数、考虑各种误差的综合影响时，定轨一天的径向精度约为3m，能满足一定的定轨精度要求。     

JASON-2卫星精密轨道确定：GPS，SLR和DORIS分析

本文针对GPS,SLR和DORIS3种观测手段的定轨问题进行了研究．以JASON-2卫星为例,讨论了GPS,SLR和DORIS的定轨策略、轨道精度评价和轨道叠加问题．并基于2009-01-21—25日实测数据进行了试验．试验结果表明,DORIS定轨结果要优于GPS和SLR;不同的轨道验证方法得到的GPS和SLR轨道的相对精度不同;基于SLR验证和轨道重叠验证的结果定权,综合GPS,SLR和DORIS进行轨道叠加,其叠加轨道的精度是一致的,通过与JPL轨道比较,其径向精度为2cm．     

Precise orbit determination for Jason-1 satellite using on-board GPS data with cm-level accuracy

The joint US/French Jason-1 satellite altimeter mission, launched from the Vandenberg Air Force Base on December 7, 2001, continues the time series of centimeter-level ocean topography observations as the follow-on to the highly successful T/P radar altimeter satellite. Orbit error especially the radial orbit error is a major component in the overall budget of all altimeter satellite missions, in order to continue the T/P standard of observations. Jason-1 has a radial orbit error budget requirement of 2.5 cm. In this work, two cycles (December 19, 2002 to January 7, 2003) of the Jason-1 on-board GPS data were processed using the zero-difference (ZD) dynamic precise orbit determination (POD) technique. The resulting Jason-1 orbit accuracy was assessed by comparison with the precise orbit ephemeris (POE) produced by JPL, orbit overlaps and SLR residuals. These evaluations indicate that the RMS radial accuracy is in the range of 1－2 cm.     

Precise orbit determination of Haiyang-2 using satellite laser ranging

With the successful launch and official commissioning of China’s first dynamic ocean environment satellite Haiyang-2(HY-2),China’s capabilities for oceanic environment monitoring and oceanic resource detecting have been further improved and enhanced.Precise tracking and orbit determination are not only key technical concerns in the ocean dynamic environment satellite project but also necessary conditions for carrying out related oceanic science research using observational data obtained using spaceborne instruments including radar altimeter.In this study,the current available status of international satellite laser ranging(SLR) monitoring on HY-2 was introduced.Six-months of SLR data from HY-2 were processed to obtain precise satellite orbit information using the dynamic orbit determination method.We carried out a detailed assessment of the SLR orbit accuracy by internal evaluation,comparisons with the orbit derived by the French Doppler orbitography and radio-positioning integrated by satellite(DORIS) system,and station-satellite distance validation.These assessments indicate that the three-dimensional orbital accuracy of HY-2 is about 12.5 cm,and the radial accuracy is better than 3 cm.It provides a good example of the application of international SLR monitoring and precise orbit determination in China’s earth observation satellite project.     

Precise orbit determination for geostationary satellites with multiple tracking techniques

A new precise orbit determination (POD) strategy based on the combination of satellite laser ranging (SLR) and C-band transfer ranging for geostationary satellites (GEO) is presented.Two approaches to calibrate ranging biases of the C-band ranging system are proposed,namely the two tracking system co-location comparison and the combined POD method,with calibration accuracies estimated to be 0.5 ns and 1 ns respectively.Using data from a C-band tracking network in China,POD experiments indicate that meter-le...     

基于国家测绘局GPS连续运行参考站的定轨

为了解决采用地面GPS连续运行参考站进行高精度定轨问题,采用轨道积分方法,利用国家测绘局8个GPS连续运行参考站和9个中国周边IGS连续运行参考站,设计了多种天数的区域性定轨计算方案,研究了轨道确定的理论、步骤和参数设置规律.结果表明:3天轨道解的平均精度为0.19 m,5天轨道解的平均精度为O.21 m,7天轨道解的平均精度为O.28 m,3天解平均定轨精度最高,优于0.2 m,证明了并非随着地面数据天数的增加而精度随之增加,可以用最少的数据量获得最优的定轨结果.定轨试验计算的是区域性轨道,与全球轨道相比,存在一定的偏差.该成果对我国定轨研究特别是未来的广域增强系统轨道预测具有一定的指导意义和参考价值.     

GEO导航卫星多种观测资料联合精密定轨

针对跟踪站少、观测几何条件差以及轨道机动后GEO导航卫星精密定轨问题,提出了用激光观测数据解算无线电测距离观测数据设备时延、用CODE模型参数和多频载波相位数据进行电离层延迟精确修正的精密定轨和轨道快速恢复的处理体制.通过在轨实测数据实验证明,利用激光标定的距离观测数据组合设备时延精度优于1ns;基于3站观测数据GEO卫星精密定轨结果,RMS为0.25m;24h数据定轨结果重叠12小轨道径向互差0.55m,位置互差约1.62m;预报12h轨道重叠弧段互差为径向3.63m,位置互差8.51m;定轨结果与激光比对残差约0.10m,预报2h轨道比对残差约0.18m,预报24h轨道比对残差约2.04m.GEO卫星轨道机动后2～3h,动力学定轨结果能够恢复到与激光比对残差小于1m、与精密定轨结果比对位置互差约30m的水平.试验及分析结果表明,所提出的GEO精度定轨技术方案可靠、稳定.     

LEO卫星轨道参数对GPS掩星数量和分布的影响

利用低地球轨道LEO（LowEarth Orbit)卫星所载GPS接收机探测地球大气的分层结构是上世纪80年代后期才发展起来的新技术，不同的LEO卫星，将使发生的掩星事件数量和分布有所差别，在一定的大气折射指数分布模型和现有GPS星座的假设条件下，利用射线追踪法模拟了掩星事件，并讨论了LEO卫星的轨道倾角和高度对掩星事件发生的数量和分布的影响。其结论对实际发射LEO卫星时的参数选取具有参考价值。     

GPS接收机原始观测量的模拟实现

研究了ＧＰＳ信号这一类准正弦信号的最大似然参数估计方法，对估计方差的Ｃｒａｍｅｒ Ｒａｏ界进行了分析，采用计算机软件手段获得ＧＰＳ接收机伪距，多普勒频移观测量。为模拟分析ＧＰＳ导航算法，伪距差分ＧＰＳ等提供了客观依据。实验结果表明，利用该软件手段进行ＧＰＳ系统分析和处理是精确直观的，简便有效。     

Reference station network based RTK systems-concepts and progress

The limitation of single base "real-time kinematic"(RTK) techniques is the distance between base receiver and the rover receiver due to distance-dependent biases, namely orbit bias,ionosphere bias and troposphere bias. Techniques have been developed to overcome this distance dependence using a network of GPS reference stations spread over a wide geographic area. Because the measurement biases will be modelled and corrected for, the positioning accuracy will be almost independent of the inter-receiver distance.Since the mid-1990s investigators have been investigating the optimal means of processing reference receiver data, and then providing generally referred to as Network-RTK. In 1993 the International Association of Geodesy (IAG) established a Special Study Group on"Wide Area Modelling for Precise Satellite Positioning" . This paperfocusses on the progress made during the last few years in designing Network-RTK architectures and the associated data processing algorithms and issues. Although many university investigators have been researching the fundamental challenges in functional and stochastic modelling, currently there is only one commercially available Network-RTK product, the Trimble VRS. However, with the use of the Internet as the primary data communication link, it is predicted that various sites around the world, over the next few years.     

Near-field surface displacement and permanent deformation induced by the Alaska Mw 7.5 earthquake determined by high-rate real-time ambiguity-fixed PPP solutions

High-rate GPS data from the United States continuously operating reference stations in the Alaska region are processed using the recently developed precise point positioning （PPP） technique. The traditional PPP technique does not fix ambiguities into their integers because these ambiguities do not have an integer nature when data from a single receiver, as well as precise orbit and clock corrections, are used. Additional corrections, i.e., uncalibrated phase delay （UPD）, are needed to fix integer ambiguities and consequently improve positioning accuracy. This study proposes a methodology to compute for wide-lane and L2 （the second L-band frequency） UPDs using the geometry-based model and subsequently applies these parameters to the computation for ambiguity-fixed solutions. The instantaneous displacements of near-field sites, as well as the permanent deformations after the earthquake, are therefore obtained for the January 5, 2013, Alaska earthquake. The real-time performance of PPP solutions are assessed by considering realistic data latency and data interval of corrections. Ambiguity-fixed solutions are compared with ambiguity-float ones. The comparison shows that the positioning accuracy can be improved significantly when the ambiguities are fixed correctly. The solutions using the real-time corrections are also compared with those using post-processing corrections, i.e., Center for Orbit Determination in Europe final orbit and clock. Although the accuracy is somehow degraded because of the data latency and data interval, the real-time results are satisfactory for use in monitoring the small-scale deformation （1-2 cm） caused by the Alaska earthquake. In addition, the kinematic ambiguity-fixed PPP solutions for 7 days around the earthquake are calculated to obtain permanent pre- and post-earthquake deformations. The deformations computed from real-time and post-processing corrections do not appear to be significantly different.     

Application of adaptive filtering by selecting the parameter weight factor in precise kinematic GPS positioning

A new method of adaptive filtering by selecting the parameter weight factor is introduced in this paper. Based on the theoretical analysis and research, this method is firstly used for precise kinematic GPS positioning with single frequency double-differenced (DD) carrier phase observations. Two numerical examples are given to illustrate the feasibility and validity of the method. The preliminary results show that precise positioning results can be obtained by using only the GPS single frequency DD carrier phase measurements for short baselines with the new method.     

海上动态GPS大气可降水量信息反演

基于精密单点定位(PPP)技术,对海洋上空大气可降水量(PWV)信息反演方法进行研究。采用随机游走过程估计方法,动态模拟船载GPS接收机天顶对流层湿延迟(ZWD)在时间和空间尺度上的随机变化。利用渤海湾船载动态GPS测量数据和同步气象观测数据,结合海洋动态环境,以渤海湾MM5模式积分水汽为参考值,详细分析不同随机过程噪声约束和卫星截至高度角等因素对PWV提取精度的影响。结果表明,选取槡2～5mm/h的随机过程噪声约束以及7°～10°的卫星截至高度角,海上船载动态PWV反演结果与MM5模式积分水汽基本一致,其偏差的绝对值均小于3 mm,均方根误差优于1.2 mm。     

Assessment of stochastic models for GPS measurements with different types of receivers

The stochastic model plays an important role in parameter estimation. The optimal estimator in the sense of least squares can only be obtained by using the correct stochastic model and consequently guarantees the precise positioning in GPS applications. In this contribution, the GPS measurements, collected by different types of geodetic dual-frequency receiver pairs on ultra-short baselines with a sampling interval of 1 s, are used to address their stochastic models, which include the variances of all observation types, the relationship between the observation accuracy and its elevation angle, the time correlation, as well as the correlation between observation types. The results show that the commonly used stochastic model with the assumption that all the raw GPS measurements are independent with the same variance does not meet the need for precise positioning and the elevation-dependent weight model cannot work well for different receiver and observation types. The time correlation and cross correlation are significant as well. It is therefore concluded that the stochastic model is much associated with the receiver and observation types and should be specified for the receiver and observation types.     

GPS接收机在探月高速再入返回中的应用

在我国探月工程3期高速再入返回飞行任务中,惯性导航单元（IMU）为主要的导航敏感器.针对惯性器件误差累积特性引起测量误差恶化问题,提出了一种适用于高速再入返回场景、轻小型化、快速定位的GPS接收机,可与IMU数据融合实现轨道误差修正.飞行验证结果表明:GPS接收机在轨道高度4 911.3 km处实现首次定位,飞行过程中有效定位时间达16 min,两次穿越大气层中"黑障"区域.精度评估结果表明,GPS接收机位置均方根误差为3.67 m:速度均方根误差为0.19 m/s.该GPS接收机满足探月高速再入返回飞行任务要求,也可应用于中低轨道微小卫星和深空再入返回航天器.      

GPS／BDS自动校时系统的研究

给出了GPS／BDS自动校时系统的设计方案,FPGA通过GPS／BDS授时接收机对接收到的时间信息进行处理,完成自动校时和计时功能．本系统的最大的优点是采用FPGA作为精确计时的控制芯片,采用GPS／BDS授时接收机作为高安全性、高可靠性的标准时钟源．     

GPS receiver performance inspection by wavelet transform

As a powerful analysis tool and the result of contemporary mathematics development, wavelet transform has shown its promising application potentials through the research in the paper. Three aspects regarding GPS receiver performance is tackled: cycle slip detection, receiver noise analysis and receiver channel bias inspection. Wavelet decomposition for double differential observation has demonstrated that this multi-level transform can reveal cycle slips as small as 0.5 cycles without any pre-adjustment processes or satellite orbit information, it can therefore be regarded as a 'geometry free' method. Based on property assumption of receiver noise, signal of noise serial is obtained at the high frequency scale in wavelet decomposition layers. This kind of noise influence on GPSb aseline result can be effectively eliminated by reconstruction process during wavelet reconstruction. Through observed data analysis, the transform has detected a kind of receiver channel bias that has not been completely removed by processing unit of GPS receiver during clock offset resetting operation. Thus the wavelet approach can be employed as a kind of system diagnosis in a generalized point of view.     

基于GPS和陀螺的航天器相对状态自主确定

根据GPS和陀螺的不同特点,将两者结合起来,设计了一种基于GPS和陀螺的相对状态自主确定算法.采用修正罗德里格参数(MRPs)作为姿态描述参数,通过建立相对轨道和相对姿态运动方程以及GPS和陀螺观测方程,运用unscented卡尔曼滤波方法(UKF)设计了双星编队相对状态自主确定算法,仿真结果验证了方法的可行性和有效性.     

An analysis of single point positioning with real-time internet-based precise GPS data

Single Point Positioning (SPP) is currently capable of providing position accuracy of several meters. To obtain a better accuracy, the Differential GPS (DGPS) method must be applied. For large-scale applications such as aerial survey and mapping, however,the requirement of a base station(s) in conventional DGPS often become problematic in practice due to the increased operational cost and complexity. Recently a concept of Global Differential GPS (GDGPS) has attracted increasing interests among the GPS communities. GDGPS has the same user's implementation as SPP,but its accuracy is augmented by the globally or regionally distributed precise GPS data currently including precise satellite orbit and clock corrections. The major advantage of GDGPS lies in two aspects: system simplicity at the user's end, and globally consistent positioning accuracy. This paper presents GDGPS positioning results using the precise GPS data generated by the Natural Resources Canada (NRCan). NRCan's precise data can be retrieved real-time from Internet base on Virtual Private Network (VPN) and Multicast technology. The packet delay and Packet Loss Rate (PLR) of multicasting over Intemet will first be investigated.The total latency of precise GPS data as well as the position accuracyof GDGPS will then be analyzed. The numerical results have shown that a meter to half-meter level accuracy is obtainable based on epoch-by-epoch data processing. With phase-smoothed code observations, the positioning accuracy can be further improved.