On rotation of frames and physical vectors: an exercise based on plate tectonics theory

The mathematical interaction between the simultaneous rotation of both a coordinate frame and a set of physical vectors in that frame is covered and theoretically and empirically explained. A practical example related to the secular motion of the pole determined using recent GPS results is addressed. A least-squares adjustment is introduced to determine a possible displacement of the geodetic north pole of the frame caused by plausible changes in the coordinates of the observing stations defining the frame due to the rotation of the plates on which these stations are located. Two examples of GPS networks are investigated both referred to the latest definition of the IGS08 geodetic frame. The positioning and velocities of the points were exclusively obtained using GPS data as published by the International GNSS Service (IGS). The first case comprises the complete GPS/IGS network of global stations; the second one assumes the closest GPS/IGS stations to the now discontinued International Latitude Service network. The results of this exercise hints at the possibility that the secular global rotation of the frame caused by plate rotations should be accounted for in order to rigorously determine the true absolute velocities referred to the IGS frame before the actual velocities of the rotation of the plates using GPS observations are published.     

IGS08: the IGS realization of ITRF2008

On April 17, 2011, the International GNSS Service (IGS) stopped using the IGS05 reference frame and adopted a new one, called IGS08, as the basis of its products. The latter was derived from the latest release of the International Terrestrial Reference Frame (ITRF2008). However, the simultaneous adoption of a new set of antenna phase center calibrations by the IGS required slight adaptations of ITRF2008 positions for 65 of the 232 IGS08 stations. The impact of the switch from IGS05 to IGS08 on GNSS station coordinates was twofold: in addition to a global transformation due to the frame change from ITRF2005 to ITRF2008, many station coordinates underwent small shifts due to antenna calibration updates, which need to be accounted for in any comparison or alignment of an IGS05-consistent solution to IGS08. Because the heterogeneous distribution of the IGS08 network makes it sub-optimal for the alignment of global frames, a smaller well-distributed sub-network was additionally designed and designated as the IGS08 core network. Only 2?months after their implementation, both the full IGS08 network and the IGS08 core network already strongly suffer from the loss of many reference stations. To avoid a future crisis situation, updates of IGS08 will certainly have to be considered before the next ITRF release.     

华北GPS网GAMIT计算结果与IGS站选取的关系探讨

利用GAMIT软件对1999年华北GPS网的观测数据进行了处理，在处理过程中，对IGS站的选取分为：（1）选取15个IGS站；（2）选取6个IGS站；（3）选取3个IGS站；（4）对不选IGS站的四种情况进行了计算，并对四种情况得出的结果从基线向量、测站坐标、基线的重复率和计算所得的均方根的残差nrms四个方面进行了比较，得出了以下结论：使用GAMIT软件处理GPS资料时，最好选取IGS站为区域网提供参考框架；IGS站的选取，数量上不一定最多，但空间分布上应尽量均匀；对华北GPS网，选取6个左右的IGS站即可；多期GPS资料在处理时应尽量选取相同的IGS站进行计算。     

IGS reference frames: status and future improvements

The hierarchy of reference frames used in the International GPS Service (IGS) and the procedures and rationale for realizing them are reviewed. The Conventions of the International Earth Rotation and Reference Systems Service (IERS) lag developments in the IGS in a number of important respects. Recommendations are offered for changes in the IERS Conventions to recognize geocenter motion (as already implemented by the IGS) and to enforce greater model consistency in order to achieve higher precision for combined reference frame products. Despite large improvements in the internal consistency of IGS product sets, defects remain which should be addressed in future developments. If the IGS is to remain a leader in this area, then a comprehensive, long-range strategy should be formulated and pursued to maintain and enhance the IGS reference frame, as well as to improve its delivery to users. Actions should include the official designation of a high-performance reference tracking network whose stations are expected to meet the highest standards possible.Also published in the proceedings of the workshop and symposium Celebrating a Decade of the International GPS Service, Astronomical Institute, University of Bern, Switzerland.     

The international global navigation satellite systems service (IGS): development and achievements

Since 21 June 1992 the International GPS Service (IGS), renamed International GNSS Service in 2005, produces and makes available uninterrupted time series of its products, in particular GPS observations from the IGS Global Network, GPS orbits, Earth orientation parameters (components x and y of polar motion, length of day) with daily time resolution, satellite and receiver clock information for each day with different latencies and accuracies, and station coordinates and velocities in weekly batches for further analysis by the IERS (International Earth Rotation and Reference Systems Service). At a later stage the IGS started exploiting its network for atmosphere monitoring, in particular for ionosphere mapping, for troposphere monitoring, and time and frequency transfer. This is why new IGS products encompass ionosphere maps and tropospheric zenith delays. This development became even more important when more and more space-missions carrying space-borne GPS for various purposes were launched. This article offers an overview for the broader scientific community of the development of the IGS and of the spectrum of topics addressed today with IGS data and products.     

求取小区域GPS网CGCS2000坐标的方法研究

本文通过联测IGS跟踪站获取南宁地区若干控制点的ITRF框架瞬时历元坐标,并根据IGS跟踪站速度内插出南宁地区地壳板块运动速度,从而将控制点ITRF坐标进行历元转换与框架转换;同时根据南宁某CORS站观测值所求板块运动速度,对相关结论进行验算。结果表明:基于准确的板块运动速度场,采用历元与框架转换方式求取CGCS2000坐标能够满足小区域GPS控制测量的精度要求。     

Evaluation of the ITRF2008 GPS vertical velocities using satellite antenna z-offsets

We develop a method to evaluate the terrestrial reference frame (TRF) scale rate error using Global Positioning System (GPS) satellite antenna phase center offset (APCO) parameters and apply it to ITRF2008. We search for the TRF in which z-APCO parameters have the smallest drift. In order to provide realistic error bars for the z-APCO drifts, we pay attention to model periodic variations and auto-correlated noise processes in the z-APCO time series. We will show that the GPS scale rate with respect to a frame is, as a first approximation, proportional to the estimated mean z-APCO trend if that frame is used to constrain station positions. Thus, an ITRF2008 scale rate error between ?0.27 and ?0.06 mm/yr depending on the GPS analysis center can be estimated, which demonstrates the high quality of the newly constructed ITRF2008. We will also demonstrate that the traditional estimates of the GPS scale rate from 7-parameter similarity transformations are consistent with our newly derived GPS scale rates with respect to ITRF2008 within two sigmas. We find using International GNSS Service (IGS) products that the traditional approach is relevant for scale rate determination even if some of the z-APCO values supplied by the IGS were not simultaneously calibrated. As the scale rate is related to the accuracy of vertical velocities, our estimates supply a conservative evaluation that can be used for error budget computation.     

利用SLR与伪距资料综合定轨

以GPS伪距为观测量对GPS35卫星进行定轨 ,然后将SLR与GPS伪距资料综合起来进行定轨 ,并将计算的轨道与IGS精密轨道进行了比较     

Accurate absolute GPS positioning through satellite clock error estimation

 An algorithm for very accurate absolute positioning through Global Positioning System (GPS) satellite clock estimation has been developed. Using International GPS Service (IGS) precise orbits and measurements, GPS clock errors were estimated at 30-s intervals. Compared to values determined by the Jet Propulsion Laboratory, the agreement was at the level of about 0.1 ns (3 cm). The clock error estimates were then applied to an absolute positioning algorithm in both static and kinematic modes. For the static case, an IGS station was selected and the coordinates were estimated every 30 s. The estimated absolute position coordinates and the known values had a mean difference of up to 18 cm with standard deviation less than 2 cm. For the kinematic case, data obtained every second from a GPS buoy were tested and the result from the absolute positioning was compared to a differential GPS (DGPS) solution. The mean differences between the coordinates estimated by the two methods are less than 40 cm and the standard deviations are less than 25 cm. It was verified that this poorer standard deviation on 1-s position results is due to the clock error interpolation from 30-s estimates with Selective Availability (SA). After SA was turned off, higher-rate clock error estimates (such as 1 s) could be obtained by a simple interpolation with negligible corruption. Therefore, the proposed absolute positioning technique can be used to within a few centimeters' precision at any rate by estimating 30-s satellite clock errors and interpolating them. Received: 16 May 2000 / Accepted: 23 October 2000     

CPLat: first operational experimental processing center for SIRGAS in Argentina

The SIRGAS permanent GPS network which is in fact the IGS network densification for the American continent, consists today of more than 200 stations covering the continent and islands. It is currently processed by the IGS RNAAC SIR centre at Deutsches Geodätisches Forschungsinstitut producing weekly free solutions relying on IGS final orbits and EOP that contribute to the ITRF through IGS. By August 2006, the SIRGAS Working Group I had accepted five proposals for experimental processing centers within the region that would collaborate with IGS RNAAC SIR. One of them, Centro de Procesamiento La Plata (CPLat) in Argentina, began processing 60 stations on October 2006. By January 2007 CPLat reached operational capability, delivering weekly free solution SINEX files, with an internal consistency of 1.5 mm average for the horizontal components, and 3 mm in the vertical. Comparisons with IGS global and IGS RNAAC SIR weekly solutions were taken as external consistency indications, showing average RMS residuals of 1.8, 2.4 and 5 mm for the north, east, and vertical component, respectively. Analysis and comparison of adjusted solution time series from CPLat and other processing centers has proved to be highly valuable for solution QC, namely detection and identification of station anomalous behavior or modelling problems. These procedures will ensure the maintenance of the performance specifications for CPLat solutions. Action is being taken in order to guarantee the continuity of this effort beyond the experimental phase.     

IGS contribution to the ITRF

We examine the contribution of the International GNSS Service (IGS) to the International Terrestrial Reference Frame (ITRF) by evaluating the quality of the incorporated solutions as well as their major role in the ITRF formation. Starting with the ITRF2005, the ITRF is constructed with input data in the form of time series of station positions (weekly for satellite techniques and daily for VLBI) and daily Earth Orientation Parameters. Analysis of time series of station positions is a fundamental first step in the ITRF elaboration, allowing to assess not only the stations behavior, but also the frame parameters and in particular the physical ones, namely the origin and the scale. As it will be seen, given the poor number and distribution of SLR and VLBI co-location sites, the IGS GPS network plays a major role by connecting these two techniques together, given their relevance for the definition of the origin and the scale of the ITRF. Time series analysis of the IGS weekly combined and other individual Analysis Center solutions indicates an internal precision (or repeatability) <2 mm in the horizontal component and <5 mm in the vertical component. Analysis of three AC weekly solutions shows generally poor agreement in origin and scale, with some indication of better agreement when the IGS started to use the absolute model of antenna phase center variations after the GPS week 1400 (November 2006).     

改进的启发式分割算法在GNSS坐标时间序列阶跃探测中的应用

在启发式分割算法的基础上,引入z检验和标准正态均一性检验（standard normal homogeneity test,SNHT）,提出了一种新的阶跃探测法,并将其应用于20个陆态网（Crustal Movement Observation Network ofChina,CMONOC）和10个国际全球卫星导航系统服务（International Global Navigation Satellite System Ser-vice,IGS）参考站近5 a的坐标时间序列。结果显示,对于IGS站东（east,E）、北（north,N）、垂直（up,U）3个方向分量已知阶跃的平均准确探测率分别为68.5%、71.3%、64.8%,且利用阶跃点前后25 d初始拟合残差的平均值之差得到修复后的坐标时间序列保持了良好的连续性。基于剔除粗差后的坐标时间序列,利用坐标时间序列分析软件（coordinate time series analysis software,CATS）估计出的速度显示:对于由已知阶跃与改进算法探测出的阶跃历元的组合方式,所有测站E、N、U方向的估计速度与CMONOC官网发布的速度的平均偏差分别为2.86 mm/a、1.14 mm/a、2.31 mm/a,明显小于由已知阶跃和肉眼判断明显阶跃历元的组合方式获得的平均速度偏差。上述结果表明,改进的启发式分割算法可应用于坐标时间序列的阶跃探测。     

Estimation of phase center corrections for GLONASS-M satellite antennas

Driven by the comprehensive modernization of the GLONASS space segment and the increased global availability of GLONASS-capable ground stations, an updated set of satellite-specific antenna phase center corrections for the current GLONASS-M constellation is determined by processing 84 weeks of dual-frequency data collected between January 2008 and August 2009 by a worldwide network of 227 GPS-only and 115 combined GPS/GLONASS tracking stations. The analysis is performed according to a rigorous combined multi-system processing scheme providing full consistency between the GPS and the GLONASS system. The solution is aligned to a realization of the International Terrestrial Reference Frame 2005. The estimated antenna parameters are compared with the model values currently used within the International GNSS Service (IGS). It is shown that the z-offset estimates are on average 7 cm smaller than the corresponding IGS model values and that the block-specific mean value perfectly agrees with the nominal GLONASS-M z-offset provided by the satellite manufacturer. The existence of azimuth-dependent phase center variations is investigated and uncertainties in the horizontal offset estimates due to mathematical correlations and yaw-attitude modeling problems during eclipse seasons are addressed. Finally, it is demonstrated that the orbit quality benefits from the updated GLONASS-M antenna phase center model and that a consistent set of satellite antenna z-offsets for GPS and GLONASS is imperative to obtain consistent GPS- and GLONASS-derived station heights.     

Geocenter variations derived from a combined processing of LEO- and ground-based GPS observations

GNSS observations provided by the global tracking network of the International GNSS Service (IGS, Dow et al. in J Geod 83(3):191–198, 2009) play an important role in the realization of a unique terrestrial reference frame that is accurate enough to allow a detailed monitoring of the Earth’s system. Combining these ground-based data with GPS observations tracked by high-quality dual-frequency receivers on-board low earth orbiters (LEOs) is a promising way to further improve the realization of the terrestrial reference frame and the estimation of geocenter coordinates, GPS satellite orbits and Earth rotation parameters. To assess the scope of the improvement on the geocenter coordinates, we processed a network of 53 globally distributed and stable IGS stations together with four LEOs (GRACE-A, GRACE-B, OSTM/Jason-2 and GOCE) over a time interval of 3 years (2010–2012). To ensure fully consistent solutions, the zero-difference phase observations of the ground stations and LEOs were processed in a common least-squares adjustment, estimating all the relevant parameters such as GPS and LEO orbits, station coordinates, Earth rotation parameters and geocenter motion. We present the significant impact of the individual LEO and a combination of all four LEOs on the geocenter coordinates. The formal errors are reduced by around 20% due to the inclusion of one LEO into the ground-only solution, while in a solution with four LEOs LEO-specific characteristics are significantly reduced. We compare the derived geocenter coordinates w.r.t. LAGEOS results and external solutions based on GPS and SLR data. We found good agreement in the amplitudes of all components; however, the phases in x- and z-direction do not agree well.     

Quality of reprocessed GPS satellite orbits

High-precision Global Positioning System (GPS) satellite orbits are one of the core products of the International GNSS Service (IGS). Since the establishment of the IGS in 1994, the quality and consistency of the IGS orbits has steadily been improved by advances in the modeling of GPS observations. However, due to these model improvements and reference frame changes, the time series of operational orbits are inhomogeneous and inconsistent. This problem can only be overcome by a complete reprocessing starting with the raw observation data. The quality of reprocessed GPS satellite orbits for the time period 1994–2005 will be assessed in this paper. Orbit fits show that the internal consistency of the orbits could be improved by a factor of about two in the early years. Comparisons with the operational IGS orbits show clear discontinuities whenever the reference frame was changed by the IGS. The independent validation with Satellite Laser Ranging (SLR) residuals shows an improvement of up to 30% whereas a systematic bias of 5 cm still persists.     

Quality assessment of GPS reprocessed terrestrial reference frame

The International GNSS Service (IGS) contributes to the construction of the International Terrestrial Reference Frame (ITRF) by submitting time series of station positions and Earth Rotation Parameters (ERP). For the first time, its submission to the ITRF2008 construction is based on a combination of entirely reprocessed GPS solutions delivered by 11 Analysis Centers (ACs). We analyze the IGS submission and four of the individual AC contributions in terms of the GNSS frame origin and scale, station position repeatability and time series seasonal variations. We show here that the GPS Terrestrial Reference Frame (TRF) origin is consistent with Satellite laser Ranging (SLR) at the centimeter level with a drift lower than 1 mm/year. Although the scale drift compared to Very Long baseline Interferometry (VLBI) and SLR mean scale is smaller than 0.4 mm/year, we think that it would be premature to use that information in the ITRF scale definition due to its strong dependence on the GPS satellite and ground antenna phase center variations. The new position time series also show a better repeatability compared to past IGS combined products and their annual variations are shown to be more consistent with loading models. The comparison of GPS station positions and velocities to those of VLBI via local ties in co-located sites demonstrates that the IGS reprocessed solution submitted to the ITRF2008 is more reliable and precise than any of the past submissions. However, we show that some of the remaining inconsistencies between GPS and VLBI positioning may be caused by uncalibrated GNSS radomes.     

COMPASS地球参考框架的建立和维持

探讨了COMPAAS地球参考框架所对应的地球参考系统的定义、地球参考框架的建立和维持方案,并选取我国现有的COMPASS跟踪站和IGS跟踪站数据进行了一系列的仿真验证。分析了跟踪站分布对地球参考框架的影响以及多分析中心组合对参考框架周解稳定性和可靠性的影响。试验结果表明,全球均匀分布的跟踪站在20个左右时就能基本满足建立地球参考框架的需求,为了保障地球参考框架的长期稳定性和精度,全球均匀分布的跟踪站应为30个左右;引入多个分析中心组合处理生成最终解不但可以探测出单个数据处理中心所引入的粗差,也可进一步增强周解的稳定性和可靠性。     

南宁CORS系统基准站地心大地坐标精确测定的方法与实践

采用地心坐标系已成为国际测量界的总趋势,而利用GPS连续运行参考站建立地心坐标框架最为普遍。利用GAM IT软件,采用有基准算法对南宁市全球导航连续运行参考站网络系统(NNCORS)的南宁基准站NANN的观测资料进行了归算,结果表明该基准站观测资料的质量是可靠的,并获得了该站在ITRF2000中毫米级精度的地心大地坐标。     

利用空间大地测量数据探测地球膨胀效应

地球自转服务局(IERS)采用多种高精度的空间探测技术综合解算得到的国际地球参考框架(ITRF)是国际上公认的精度高、稳定性好的参考框架。为了研究地球的膨胀或收缩效应,本文采用ITRF2000的站坐标和速度,利用Delaunay算法生成的三角网逼近地球形体,计算出了地球的体积变化。     

Assessment of ECMWF-derived tropospheric delay models within the EUREF Permanent Network

The Global Positioning System (GPS) observations from the EUREF Permanent Network (EPN) are routinely analyzed by the EPN analysis centers using a tropospheric delay modeling based on standard pressure values, the Niell Mapping Functions (NMF), a cutoff angle of 3° and down-weighting of low elevation observations. We investigate the impact on EPN station heights and Zenith Total Delay (ZTD) estimates when changing to improved models recommended in the updated 2003 International Earth Rotation and Reference Systems Service (IERS) Conventions, which are the Vienna Mapping Functions 1 (VMF1) and zenith hydrostatic delays derived from numerical weather models, or the empirical Global Mapping Functions (GMF) and the empirical Global Pressure and Temperature (GPT) model. A 1-year Global Positioning System (GPS) data set of 50 regionally distributed EPN/IGS (International GNSS Service) stations is processed. The GPS analysis with cutoff elevation angles of 3, 5, and 10° revealed that changing to the new recommended models introduces biases in station heights in the northern part of Europe by 2–3 mm if the cutoff is lower than 5°. However, since large weather changes at synoptic time scales are not accounted for in the empirical models, repeatability of height and ZTD time series are improved with the use of a priori Zenith Hydrostatic Delays (ZHDs) derived from numerical weather models and VMF1. With a cutoff angle of 3°, the repeatability of station heights in the northern part of Europe is improved by 3–4 mm.