The value of ocean reflections of GPS signals to enhance satellite altimetry: data distribution and error analysis

The time and space distribution of general reflection altimetry from two satellites (senders and receivers) via the the oceans surface is examined with specific reference to GPS senders and two current receiving satellites. While a considerable enhancement of conventional altimeter coverage is possible in all configurations if the reflection signals can be used, repeating passes of these (with GPS senders) having reasonably small cycle times (days to tens of days) occur only if the receiving orbit is nearly polar. Results of an analysis of the fundamental geometry show that over a large range of reflection angles the error of recovered sea heights depends almost entirely on the errors in the delay signal and the radial error of the receiving satellite (using current estimates of GPS orbit accuracies). The most critical element is the precision of the delay measurement. Both it and the accuracy of the receiving orbit should be below the decimeter level for the technique to achieve its full potential.     

Interferometric Baseline Performance Estimations for Multistatic Synthetic Aperture Radar Configurations Derived from GRACE GPS Observations

Recent studies have demonstrated the usefulness of global positioning system (GPS) receivers for relative positioning of formation-flying satellites using dual-frequency carrier-phase observations. The accurate determination of distances or baselines between satellites flying in formation can provide significant benefits to a wide area of geodetic studies. For spaceborne radar interferometry in particular, such measurements will improve the accuracy of interferometric products such as digital elevation models (DEM) or surface deformation maps. The aim of this study is to analyze the impact of relative position errors on the interferometric baseline performance of multistatic synthetic aperture radar (SAR) satellites flying in such a formation. Based on accuracy results obtained from differential GPS (DGPS) observations between the twin gravity recovery and climate experiment (GRACE) satellites, baseline uncertainties are derived for three interferometric scenarios of a dedicated SAR mission. For cross-track interferometry in a bistatic operational mode, a mean 2D baseline error (1σ) of 1.4 mm is derived, whereas baseline estimates necessary for a monostatic acquisition mode with a 50 km along-track separation reveal a 2D uncertainty of approximately 1.7 mm. Absolute orbit solutions based on reduced dynamic orbit determination techniques using GRACE GPS code and carrier-phase data allows a repeat-pass baseline estimation with an accuracy down to 4 cm (2D 1σ). To assess the accuracy with respect to quality requirements of high-resolution DEMs, topographic height errors are derived from the estimated baseline uncertainties. Taking the monostatic pursuit flight configuration as the worst case for baseline performance, the analysis reveals that the induced low-frequency modulation (height bias) fulfills the relative vertical accuracy requirement (σ<1 m linear point-to-point error) according to the digital terrain elevation data level 3 (DTED-3) specifications for most of the baseline constellations. The use of a GPS-based reduced dynamic orbit determination technique improves the baseline performance for repeat-pass interferometry. The problem of fulfilling the DTED-3 horizontal accuracy requirements is still an issue to be investigated. DGPS can be used as an operational navigation tool for high-precision baseline estimation if a geodetic-grade dual-frequency spaceborne GPS receiver is assumed to be the primary instrument onboard the SAR satellites. The possibility of using only single-frequency receivers, however, requires further research effort.Deutsche Forschungsgemeinschaft (DFG) research fellow until Sept. 2004 at the Microwaves and Radar Institute, Deutsche Zentrum für Luft- und Raumfahrt (DLR) e.V., 82234 Weßling, Germany     

Improved relativistic transformations in GPS

For GPS satellite clocks, a nominal (hardware) frequency offset and a conventional periodic relativistic correction derived as a dot product of the satellite position and velocity vectors, are used to compensate the relativistic effects. The conventional hardware clock rate offset of 38,575.008 ns/day corresponds to a nominal orbit semi-major axis of about 26,561,400 m. For some of the GPS satellites, the departures from the nominal semi-major axis can cause an apparent clock rate up to 10 ns/day. GPS orbit perturbations, together with the earth gravity field oblateness, which is largely responsible for the orbit perturbations, cause the standard GPS relativistic transformations to depart from the rigorous relativity transformation by up to 0.2 ns/day. In addition, the conventional periodic relativistic correction exhibits periodic errors with amplitudes of about 0.1 and 0.2 ns, with periods of about 6 h and 14 days, respectively. Using an analytical integration of the gravity oblateness term (J₂), a simple analytical approximation was derived for the apparent clock rate and the 6-h periodic errors of the standard GPS gravity correction. For daily linear representations of GPS satellite clocks, the improved relativistic formula was found to agree with the precise numerical integration of the GPS relativistic effects within about 0.015 ns. For most of the Block IIR satellites, the 6-h periodical errors of the GPS conventional relativistic correction are already detectable in the recent IGS final clock combinations.     

Precision real-time navigation of LEO satellites using global positioning system measurements

Continued advancements in remote sensing technology along with a trend towards highly autonomous spacecraft provide a strong motivation for accurate real-time navigation of satellites in low Earth orbit (LEO). Global Navigation Satellite System (GNSS) sensors nowadays enable a continuous tracking and provide low-noise radiometric measurements onboard a user spacecraft. Following the deactivation of Selective Availability a representative real-time positioning accuracy of 10 m is presently achieved by spaceborne global positioning system (GPS) receivers on LEO satellites. This accuracy can notably be improved by use of dynamic orbit determination techniques. Besides a filtering of measurement noise and other short-term errors, these techniques enable the processing of ambiguous measurements such as carrier phase or code-carrier combinations. In this paper a reference algorithm for real-time onboard orbit determination is described and tested with GPS measurements from various ongoing space missions covering an altitude range of 400–800 km. A trade-off between modeling effort and achievable accuracy is performed, which takes into account the limitations of available onboard processors and the restricted upload capabilities. Furthermore, the benefits of different measurements types and the available real-time ephemeris products are assessed. Using GPS broadcast ephemerides a real-time position accuracy of about 0.5 m (3D rms) is feasible with dual-frequency carrier phase measurements. Slightly inferior results (0.6–1 m) are achieved with single-frequency code-carrier combinations or dual-frequency code. For further performance improvements the use of more accurate real-time GPS ephemeris products is mandatory. By way of example, it is shown that the TDRSS Augmentation Service for Satellites (TASS) offers the potential for 0.1–0.2 m real-time navigation accuracies onboard LEO satellites.     

基于星载GPS的CHAMP卫星精密简动力定轨

定轨是地球探测卫星任务顺利执行的关键。星载GPS技术提供了大量、连续的高低卫星跟踪观测,为低轨卫星精密定轨提供了技术支撑。为了确定CHAMP卫星的轨道,并分析定轨精度,利用CHAMP卫星星载GPS数据,运用零差简动力法进行精密定轨,给出了精密定轨流程。利用实际数据进行了精密定轨实验,结果与德国地学研究中心（GFZ）公布的CHAMP卫星快速轨道（RSO）进行了对比,结果显示：求解轨道可以达到厘米量级。     

Single receiver phase ambiguity resolution with GPS data

Global positioning system (GPS) data processing algorithms typically improve positioning solution accuracy by fixing double-differenced phase bias ambiguities to integer values. These “double-difference ambiguity resolution” methods usually invoke linear combinations of GPS carrier phase bias estimates from pairs of transmitters and pairs of receivers, and traditionally require simultaneous measurements from at least two receivers. However, many GPS users point position a single local receiver, based on publicly available solutions for GPS orbits and clocks. These users cannot form double differences. We present an ambiguity resolution algorithm that improves solution accuracy for single receiver point-positioning users. The algorithm processes dual- frequency GPS data from a single receiver together with wide-lane and phase bias estimates from the global network of GPS receivers that were used to generate the orbit and clock solutions for the GPS satellites. We constrain (rather than fix) linear combinations of local phase biases to improve compatibility with global phase bias estimates. For this precise point positioning, no other receiver data are required. When tested, our algorithm significantly improved repeatability of daily estimates of ground receiver positions, most notably in the east component by approximately 30% with respect to the nominal case wherein the carrier biases are estimated as real values. In this “static” test for terrestrial receiver positions, we achieved daily repeatability of 1.9, 2.1 and 6.0 mm in the east, north and vertical (ENV) components, respectively. For kinematic solutions, ENV repeatability is 7.7, 8.4, and 11.7 mm, respectively, representing improvements of 22, 8, and 14% with respect to the nominal. Results from precise orbit determination of the twin GRACE satellites demonstrated that the inter-satellite baseline accuracy improved by a factor of three, from 6 to 2 mm up to a long-term bias. Jason-2/Ocean Surface Topography Mission precise orbit determination tests results implied radial orbit accuracy significantly below the 10 mm level. Stability of time transfer, in low-Earth orbit, improved from 40 to 7 ps. We produced these results by applying this algorithm within the Jet Propulsion Laboratory’s (JPL’s) GIPSY/OASIS software package and using JPL’s orbit and clock products for the GPS constellation. These products now include a record of the wide-lane and phase bias estimates from the underlying global network of GPS stations. This implies that all GIPSY–OASIS positioning users can now benefit from this capability to perform single-receiver ambiguity resolution.     

激光在天空对地观测中的应用

1960年7月世界上第一台激光器问世后，激光测距迅速兴起，不管是地面激光测距，还是激光测卫和激光测月，都为大地测量学的发展作出了重大贡献；特别是激光测卫测月成果，为我们深化对地球动态效应的认识，揭示地球的奥秘，提供了许多重要的科学数据，本文综析了值得注视的下列新近发展。.在IGEX-98国际大联测中，求定GLONASS卫星的激光轨道与微波轨道之差；.评定PRN05/06号GPS卫星星历的精度；.检核Topex/Poseidon海洋测高卫星用GPS定轨的测量误差，.用机载激光测深系统测量海水的浓度；.用EOS-ALT星载激光测距/测高系统测量地球动态参数。     

High precision orbit determination of CHAMP satellite

IntroductionCHAMP satellite was launched on July 15 ,2000 to study the earth gravity field, magneticfield, and at mosphere/ionosphere . CHAMPmission is managed by GeoForshungsZentrum(GFZ) Potsdam withinternational partners .In-cluding accelerometer ,star …     

Towards a 1 mGal accuracy and 1 min resolution altimetry gravity field

Over the past three decades, radar altimetry has made a significant contribution to marine gravity field modeling. To improve the accuracy and resolution, we propose a new twin-satellite altimetry. Such a system has several advantages. Among others, it provides (i) twice the number of samples per time epoch, and (ii) information about the cross-track surface gradient with high accuracy because most of the environmental and tidal errors will be common to the simultaneous measurements and therefore cancel out when computing the cross-track gradient computation. We describe a rigorous procedure for the deduction of the sea surface gradient at each altimeter observation point (i.e., not only at the crossovers), from the twin-satellite altimetry system. The precision of the gradient will be slightly affected by orbit errors, instrument drift, and inaccuracies in the geophysical corrections to be applied. We also demonstrate that a 1 mGal accuracy and 1 min resolution marine altimetry gravity field can be obtained if certain conditions are met. To achieve the expected goal, we recommend an orbital configuration, phasing two satellites in 4-s time delay such that the Earth rotation creates a natural baseline between the two satellites, and a 18 kHz SAR altimeter.     

Precise orbit determination for the GRACE mission using only GPS data

The GRACE (gravity recovery and climate experiment) satellites, launched in March 2002, are each equipped with a BlackJack GPS onboard receiver for precise orbit determination and gravity field recovery. Since launch, there have been significant improvements in the background force models used for satellite orbit determination, most notably the model for the geopotential. This has resulted in significant improvements to orbit accuracy for very low altitude satellites. The purpose of this paper is to investigate how well the orbits of the GRACE satellites (about 470 km in altitude) can currently be determined using only GPS data and based on the current models and methods. The orbit accuracy is assessed using a number of tests, which include analysis of orbit fits, orbit overlaps, orbit connecting points, satellite Laser ranging residuals and K-band ranging (KBR) residuals. We show that 1-cm radial orbit accuracy for the GRACE satellites has probably been achieved. These precise GRACE orbits can be used for such purposes as improving gravity recovery from the GRACE KBR data and for atmospheric profiling, and they demonstrate the quality of the background force models being used.     

Pseudo-Stochastic Orbit Modeling Techniques for Low-Earth Orbiters

The Earth’s non-spherical mass distribution and atmospheric drag cause the strongest perturbations on very low-Earth orbiting satellites (LEOs). Models of gravitational and non-gravitational accelerations are utilized in dynamic precise orbit determination (POD) with GPS data, but it is also possible to derive LEO positions based on GPS precise point positioning without dynamical information. We use the reduced-dynamic technique for LEO POD, which combines the geometric strength of the GPS observations with the force models, and investigate the performance of different pseudo-stochastic orbit parametrizations, such as instantaneous velocity changes (pulses), piecewise constant accelerations, and continuous piecewise linear accelerations. The estimation of such empirical orbit parameters in a standard least-squares adjustment process of GPS observations, together with other relevant parameters, strives for the highest precision in the computation of LEO trajectories. We used the procedures for the CHAMP satellite and found that the orbits may be validated by means of independent SLR measurements at the level of 3.2 cm RMS. Validations with independent accelerometer data revealed correlations at the level of 95% in the along-track direction. As expected, the empirical parameters compensate to a certain extent for deficiencies in the dynamic models. We analyzed the capability of pseudo-stochastic parameters for deriving information about the mismodeled part of the force field and found evidence that the resulting orbits may be used to recover force field parameters, if the number of pseudo-stochastic parameters is large enough. Results based on simulations showed a significantly better performance of acceleration-based orbits for gravity field recovery than for pulse-based orbits, with a quality comparable to a direct estimation if unconstrained accelerations are set up every 30 s.     

CHAMP卫星cm级精密定轨

在卫星定位导航数据综合处理软件（PANDA软件）的基础上，解算了2002年年积日126～131d CHAMP卫星的精密轨道，并通过与GFZ精密轨道的比较、GPS观测值的验后残差和SLR观测值检验等3种方式进行了轨道精度的评估。结果显示，本文的轨道精度在径向为4～5cm，切向和法向为6～8cm。     

星载GPS伪距多路径误差与观测噪声对自主定轨的影响分析

对搭载美国BlackJack接收机的CHAMP/GRACE-A/Jason-2卫星和搭载国产接收机的HY2A/ZY3/TH1卫星的星载GPS数据的伪距多路径误差与观测噪声进行了研究,重点分析了国产接收机伪距多路径误差的变化特性,并研究了多路径误差与观测噪声对星载GPS自主定轨的影响。结果表明:国产接收机的C/A码与P1码伪距观测精度要整体差于美国的BlackJack接收机,而P2码伪距观测精度要整体优于BlackJack接收机;国产接收机P1码伪距受多路径效应影响较大,其多路径误差随高度角减小存在单调递增的变化趋势,其中HY2A、ZY3与TH1卫星的多路径误差最大分别可达3.6 m、1.8 m与0.7 m;这种单调递增变化的多路径误差会导致星载GPS自主定轨位置结果在径向与切向产生系统性偏差。     

GPS卫星广播星历轨道误差突变性分析

针对GPS卫星播发的广播星历存在误差突变的问题进行了有关研究。利用切比雪夫多项式,分别以不同的拟合时段计算卫星轨道坐标,然后分别与对应历元的IGS精密星历所提供的GPS卫星的坐标进行比较,发现了某些GPS卫星广播星历轨道误差变化的规律。这对如何削弱广播星历的轨道误差,提高导航与定位精度是十分有益的。     

全球导航卫星系统的新进展

本文综合介绍了于 2 0 0 4年 9月 2 1日至 2 4日在美国加州举行的“全球导航卫星系统” 2 0 0 4年年会(GNSS2 0 0 4 )会议的主要议题 ,并对其中我们可能关切的方面进行了重点介绍。①美国的GPS连续运行站网(CORS)。CORS由美国大地测量局 (NGS)主持运行。用户可以通过NGS网络 ,获得用户的GPS待定点相邻的CORS站 (三个以上 )的GPS相应载波相位和码距 ,以支持用户的GPS准实时或后处理定位。NGS也可以为用户通过网络提供GPS定位计算服务 ,这一服务可以在用户提供待定点的观测资料后的几个小时内完成 ,称为NGS的在线GPS定位服务。CORS目前在美国已有 5 0 0余个站。②GPS系统的进展。GPSⅡR型卫星从体形和功能方面都比较优秀 ,使GPS卫星在轨的位置误差显著降低 ,测距精度提高近一倍 ,目前GPSⅡR型卫星截止至 2 0 0 4年 1月 1日时有 9颗在轨。③利用L1 ,L2频道的GPS空基增强系统 (WAAS)。在美国大部分地区WAAS系统的水平精度可达 1～ 2m ,垂直精度可达 2～ 3m。④GPS信号的重构。美国已发展了一种高度逼真的和适应各种情况的虚拟GPS信号系统 ,这种虚拟发射装置可以是陆基的 ,空基的 ,或者星基的。GPS接收机可以利用这一虚拟的GPS信号进行精密定位。⑤Galileo卫星导航系统运行的准备工作。欧洲空间局已经重新和?     

Evaluation of pre-CHAMP gravity models `GRIM5-S1' and `GRIM5-C1' with satellite crossover altimetry

 The new GFZ/GRGS gravity field models GRIM5-S1 and GRIM5-C1, currently used as initial models for the CHAMP mission, have been compared with other recent models (JGM 3, EGM 96) for radial orbit accuracy (by means of latitude lumped coefficients) in computations on altimetry satellite orbits. The bases for accuracy judgements are multi-year averages of crossover sea height differences from Geosat and ERS 1/2 missions. This radially sensitive data is fully independent of the data used to develop these gravity models. There is good agreement between the observed differences in all of the world's oceans and projections of the same errors from the scaled covariance matrix of their harmonic geopotential coefficients. It was found that the tentative scale factor of five for the formal standard deviations of the harmonic coefficients of the new GRIM fields is justified, i.e. the accuracy estimates, provided together with the GRIM geopotential coefficients, are realistic. Received: 20 February 2001 / Accepted: 24 October 2001     

GPS/GALILEO组合系统可见卫星与GDOP的区域和时序分析

卫星星座方案的选择对导航定位精度具有很大影响。本文根据GALILEO系统的设计轨道参数模拟得到的GALILEO系统的卫星位置,分别计算了GPS系统、GALILEO系统和GPS/GALILEO组合系统在不同卫星截止高度角的情况下,全国范围内可见卫星和GDOP值的分布情况,并选择了北京、武汉和乌鲁木齐三个城市,连续观测24小时,分析了各城市的可见卫星和GDOP值随时间的变化规律。     

GPS纯几何法在低轨卫星定轨中的应用

在近地低轨卫星上安装GPS接收机,并同时能捕获到四颗GPS卫星的话,我们就可以直接利用GPS观测值来组成观测方程,解算被求卫星的轨道位置。但由于GPS卫星是为地面上的用户设计的,其主要是满足地面用户的导航定位要求,再加上近地卫星的高动态性、高速度性,有时还不能同时捕获到四颗GPS卫星,这都给近地卫星的定轨带来了不确定的因素。本文主要对这一想法进行了试验,并分析了定轨的精度。     

北斗混合星座分层约束下的精密定轨方法

针对北斗导航卫星系统首创的GEO+IGSO+MEO混合星座设计,本文研究了根据不同星座,采取不同约束条件和数据处理策略的北斗卫星精密定轨方法,提出了一种针对北斗系统混合星座的分层约束精密定轨方案。该方案首先将北斗卫星分为非GEO(IGSO/MEO)和GEO两部分进行解算,利用GPS解算的公共参数对北斗IGSO/MEO精密定轨形成有效约束,然后固定GPS和北斗IGSO/MEO解算结果,最后单独对北斗GEO卫星进行强约束下的轨道解算。利用实测数据进行了精密定轨试验,试验结果表明:采用本文提出的方法,北斗GEO卫星和非GEO卫星三维重叠弧段轨道精度分别为0.688 m和0.042 m,比传统方法分别提高了54.2%和72.4%。另外,采用激光测距检核和测站坐标静态精密单点定位的方法对轨道精度进行了验证,激光检核精度提高了44.3%,测站坐标在水平和高程方向上精度分别平均提升了21.5%和20.7%。     

Rapid orbit determination of LEO satellites using IGS clock and ephemeris products

Different types of GPS clock and orbit data provided by the International GPS Service (IGS) have been used to assess the accuracy of rapid orbit determination for satellites in low Earth orbit (LEO) using spaceborne GPS measurements. To avoid the need for reference measurements from ground-based reference receivers, the analysis is based on an undifferenced processing of GPS code and carrier-phase measurements. Special attention is therefore given to the quality of GPS clock data that directly affects the resulting orbit determination accuracy. Interpolation of clock data from the available 15 min grid points is identified as a limiting factor in the use of IGS ultra-rapid ephemerides. Despite this restriction, a 10-cm orbit determination accuracy can be obtained with these products data as demonstrated for the GRACE-B spacecraft during selected data arcs between 2002 and 2004. This performance may be compared with a 5-cm orbit determination accuracy achievable with IGS rapid and final products using 5 min clock samples. For improved accuracy, high-rate (30 s) clock solutions are recommended that are presently only available from individual IGS centers. Likewise, a reduced latency and more frequent updates of IGS ultra-rapid ephemerides are desirable to meet the requirements of upcoming satellite missions for near real-time and precise orbit determination.