Implementation and testing of the gridded Vienna Mapping Function 1 (VMF1)

The new gridded Vienna Mapping Function (VMF1) was implemented and compared to the well-established site-dependent VMF1, directly and by using precise point positioning (PPP) with International GNSS Service (IGS) Final orbits/clocks for a 1.5-year GPS data set of 11 globally distributed IGS stations. The gridded VMF1 data can be interpolated for any location and for any time after 1994, whereas the site-dependent VMF1 data are only available at selected IGS stations and only after 2004. Both gridded and site-dependent VMF1 PPP solutions agree within 1 and 2 mm for the horizontal and vertical position components, respectively, provided that respective VMF1 hydrostatic zenith path delays (ZPD) are used for hydrostatic ZPD mapping to slant delays. The total ZPD of the gridded and site-dependent VMF1 data agree with PPP ZPD solutions with RMS of 1.5 and 1.8 cm, respectively. Such precise total ZPDs could provide useful initial a priori ZPD estimates for kinematic PPP and regional static GPS solutions. The hydrostatic ZPDs of the gridded VMF1 compare with the site-dependent VMF1 ZPDs with RMS of 0.3 cm, subject to some biases and discontinuities of up to 4 cm, which are likely due to different strategies used in the generation of the site-dependent VMF1 data. The precision of gridded hydrostatic ZPD should be sufficient for accurate a priori hydrostatic ZPD mapping in all precise GPS and very long baseline interferometry (VLBI) solutions. Conversely, precise and globally distributed geodetic solutions of total ZPDs, which need to be linked to VLBI to control biases and stability, should also provide a consistent and stable reference frame for long-term and state-of-the-art numerical weather modeling.     

高纬度BDS/GPS PPP中的对流层延迟改正模型优选

由于高纬度地区气温气压值及变化率与中低纬度地区有较大差异,因此目前发布的多种对流层延迟模型在高纬度地区使用的精度会不同。为了给高纬度地区BDS/GPS用户提供更好的对流层延迟模型选择,文中采用UNB3,EGNOS和GPT2模型,以IGS发布的ZPD产品和SINEX文件作为参考,对比基于这三种对流层延迟模型计算的天顶对流层总延迟量以及精密单点定位精度,可知GPT2较UNB3和EGNOS在高纬度地区定位中有更好的精度表现。     

A real-time meteorological-based troposphere (RMT) correction with integrity bound for long baseline DGPS

This paper concentrates on the analysis of a real-time meteorological (MET)-based troposphere (RMT) model where MET data are used in real-time to provide troposphere error corrections with a bounded level of integrity for a prototype National Differential Global Positioning System-High Performance (NDGPS-HP) architecture. Toward this goal, three aspects are studied for this approach: sensitivity analysis, accuracy assessment, and integrity analysis. A Hopfield zenith delay and Chao mapping function models were chosen as a good compromise between accuracy and complexity in the integrity analysis. The sensitivity analysis results indicate that the Hopfield model is mostly sensitive to hot humid conditions, which is compounded slightly more and where some relative humidity sensors are less accurate. The accuracy assessment was performed with respect to both absolute and relative accuracy. In the absolute accuracy assessment, the comparison was made in terms of zenith troposphere delay estimation error, with respect to the International GPS Service (IGS) final troposphere zenith path delay (ZPD) product, which was used as the true ZPD. For locations where IGS stations are not available, a relative accuracy assessment was performed whereby comparisons were made in terms of GPS double difference (DD) carrier-phase troposphere correction residuals using various techniques. The accuracy assessment results indicate that the RMT has insignificant differences from the prototype National Oceanic and Atmosphere Administration (NOAA) troposphere error forecast model. An integrity analysis was performed, which presents integrity bounds for the RMT that can be applied to a NDGPS-HP architecture in which integrity requirements exist. The overriding goal of this effort was to establish a preliminary real-time troposphere error estimation model, with defined levels of integrity in its troposphere error estimation that can be included in an NDGPS-HP architecture, where integrity is a key system requirement. The conclusion is drawn that the RMT model may be well suited for a variety of users within a NDGPS-HP architecture. An erratum to this article can be found at     

雾霾与GPS对流层天顶延迟相关性探究

利用国际GNSS服务（International GNSS Service,IGS）提供的对流层天顶延迟（zenith path delay,ZPD）产品,研究其与雾霾的相关性,并探究了造成雾霾的“元凶”——悬浮颗粒物与气压、温度和湿度的变化关系。首先,研究了中国境内4个IGS站30 d的日平均ZPD与量化评定雾霾的空气质量指数（air quality index,AQI）的变化趋势,发现二者基本同步增大（减小）。内陆3个站点的相关系数绝对值均大于0.5,说明ZPD与表征雾霾的AQI有着较强的相关关系,雾霾对对流层延迟产生影响。其次,对1 h采样率的北京房山空气质量分指数（individual air quality index,IAQI）与ZPD进行分析,二者的变化趋势基本一致,其中PM2.5、PM10、AQI与ZPD的相关系数分别为0.504 2、0.539 1和0.555 4。同时,当AQI达到300以上重度污染时,会对ZPD产生5 cm以上差值的显著影响。最后,利用IGS的M文件探究了北京房山各IAQI与气压、温度、湿度24 h变化,一天中IAQI、气压、湿度均呈“U”变化趋势,而温度则呈现倒“U”变化,说明雾霾的形成与气压、温度、湿度相关,并利用逐步线性回归给出了概略模型。     

Progress in Carrier Phase Time Transfer

The progress of the joint Pilot Project for time transfer, formed by the International GPS Service (IGS) and the Bureal International des Poids et Mesures (BIPM), was recently reviewed. Three notable milestones were set. (1) The IGS will implement, at least in a test mode, an internally realized time scale based on an integration of combined frequency standards within the IGS network. This will eventually become the reference time scale for all IGS clock products (instead of the current GPS broadcast time). (2) A new procedure for combined receiver and satellite clock products will be implemented officially in November 2000. Receiver clocks are an entirely new product of the IGS. (3) The BIPM will coordinate an effort to calibrate all Ashtech Z12-T (and possibly other) receivers suitable for time transfer applications, either differentially or absolutely. Progress reports will be presented publicly in the spring 2001. ? 2001 John Wiley & Sons, Inc.     

Assessment of time-series of troposphere zenith delays derived from the Global Data Assimilation System numerical weather model

Troposphere zenith path delays derived from the Global Data Assimilation System (GDAS) numerical weather model (NWM) are compared with those of the International GNSS Service (IGS) solutions over a 1.5-year period at 18 globally distributed IGS stations. Meteorological parameters can be interpolated from the NWM model at any location and at any time after December 2004. The meteorological parameters extracted from the NWM model agree with in situ direct measurements at some IGS stations within 1 mbar for pressure, 3° for temperature and 13% for relative humidity. The hydrostatic and wet components of the zenith path delay (ZPD) are computed using the meteorological parameters extracted from the NWM model. The total ZPDs derived from the GDAS NWM agree with the IGS ZPD solutions at 3.0 cm RMS level with biases of up to 4.5 cm, which can be attributed to the wet ZPDs estimates from the NWM model, considering the less accurate interpolated relative humidity parameter. Based on this study, it is suggested that the availability and the precision of the GDAS NWM ZPD should be sufficient for nearly all GPS navigation solutions.

PPP估计天顶对流层延迟方法与结果分析

介绍了精密单点定位估计天顶对流层延迟的方法,利用武汉大学研发的TriP软件解算了7个IGS跟踪站的天顶对流层延迟,将其与IGS分析中心提供的天顶对流层延迟数据进行了对比分析。     

Apparent clock variations of the Block IIF-1 (SVN62) GPS satellite

The Block IIF satellites feature a new generation of high-quality rubidium clocks for time and frequency keeping and are the first GPS satellites transmitting operational navigation signals on three distinct frequencies. We investigate apparent clock offset variations for the Block IIF-1 (SVN62) spacecraft that have been identified in L1/L2 clock solutions as well as the L1/L5-minus-L1/L2 clock difference. With peak-to-peak amplitudes of 10?C40?cm, these variations are of relevance for future precision point positioning applications and ionospheric analyses. A proper characterization and understanding is required to fully benefit from the quality of the new signals and clocks. The analysis covers a period of 8?months following the routine payload activation and is based on GPS orbit and clock products generated by the CODE analysis center of the International GNSS Service (IGS) as well as triple-frequency observations collected with the CONGO network. Based on a harmonic analysis, empirical models are presented that describe the sub-daily variation of the clock offset and the inter-frequency clock difference. These contribute to a better clock predictability at timescales of several hours and enable a consistent use of L1/L2 clock products in L1/L5-based positioning.     

Precise Point Positioning Using IGS Orbit and Clock Products

The contribution details a post-processing approach that used undifferentiated dual-frequency pseudorange and carrier phase observations along with IGS procise orbit products, for stand-alone precise geodetic point positioning (static or kinematic) with cm precision. This is possible if one takes advantage of the satellite clock estimates available with the satellite coordinates in the IGS precise orbit products and models systematic effects that cause cm variations in the satelite to user range. This paper will describe the approach, summarize the adjustment procedure, and specify the earth- and space-based models that must be implementetd to achieve cm-level positioning in static mode. Furthermore, station tropospheric zenth path delays with cm precision and GPS receiver clock estimates procise to 0.1 ns are also obtained. ? 2001 John Wiley & Sons, Inc.     

Climate applications of a global, 2-hourly atmospheric precipitable water dataset derived from IGS tropospheric products

A global, 2-hourly atmospheric precipitable water (PW) dataset is produced from ground-based GPS measurements of zenith tropospheric delay (ZTD) using the International Global Navigation Satellite Systems (GNSS) Service (IGS) tropospheric products (~80–370 stations, 1997–2006) and US SuomiNet product (169 stations, 2003–2006). The climate applications of the GPS PW dataset are highlighted in this study. Firstly, the GPS PW dataset is used as a reference to validate radiosonde and atmospheric reanalysis data. Three types of systematic errors in global radiosonde PW data are quantified based on comparisons with the GPS PW data, including measurement biases for each of the fourteen radiosonde types along with their characteristics, long-term temporal inhomogeneity and diurnal sampling errors of once and twice daily radiosonde data. The comparisons between the GPS PW data and three reanalysis products, namely the NCEP-NCAR (NNR), ECMWF 40-year (ERA-40) and Japanese reanalyses (JRA), show that the elevation difference between the reanalysis grid box and the GPS station is the primary cause of the PW difference. Secondly, the PW diurnal variations are documented using the 2-hourly GPS PW dataset. The PW diurnal cycle has an annual-mean, peak-to-peak amplitude of 0.66, 0.53 and 1.11 mm for the globe, Northern Hemisphere, and Southern Hemisphere, respectively, with the time of the peak ranging from noon to late evening depending on the season and region. Preliminary analyses suggest that the PW diurnal cycle in Europe is poorly represented in the NNR and JRA products. Several recommendations are made for future improvements of IGS products for climate applications.     

Global GPS data analysis at the National Geodetic Survey

NOAA’s National Geodetic Survey (NGS) has been one of the Analysis Centers (ACs) of the International GNSS Service (IGS) since its inception in 1994. Solutions for daily GPS orbits and Earth orientation parameters are regularly contributed to the IGS Rapid and Final products, as well as solutions of weekly station positions. These solutions are combined with those of the other ACs and then the resultant IGS products are distributed to users. To perform these tasks, NGS has developed and refined the Program for the Adjustment of GPS EphemerideS (PAGES) software. Although PAGES has continuously evolved over the past 15 years, recent efforts have focused mostly on updating models and procedures to conform more closely to IGS and the International Earth Rotation Service (IERS) conventions. Details of our processing updates and demonstrations of the improvements will be provided.     

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.     

GPS卫星钟的特性与预报研究

经实验分析发现:GPS卫星钟差的预报精度与卫星的种类密切相关,最近发射的BLOCK ⅡR和BLOCKIIR-M类卫星比以往的BLOCK ⅡA类卫星要更加稳定,其卫星钟差的预报精度明显较高。直接利用IGS超快速产品和线性模型预报后6小时的卫星钟差,精度在0.5纳秒水平;但一些BLOCK ⅡA类卫星是不稳定的,通过对其预报残差的分析发现:同一颗卫星每天在相同时段用相同的模型去预报其卫星钟差,预报所得的残差呈周期性变化,并且这种周期性变化并不完全重合,还具有一定的随机性。依据这一特性本文构建了一个新的预报模型来实时预报GPS卫星钟差。该模型不仅能预报卫星钟差的总体变化趋势,还能预报残差的周期性变化以及随机项的变化,因此精度更高。预报结果均与IGS发布的最终产品相比,实验显示利用该方法实时预报GPS卫星钟差,预报精度可达0.5纳秒水平。     

Real-time clock offset prediction with an improved model

The GPS orbit precision of the IGS ultra-rapid predicted (IGU-P) products has been remarkably improved since 2007. However, the satellite clock offsets of the IGU-P products have not shown sufficient high-quality prediction to achieve sub-decimeter precision in real-time precise point positioning (RTPPP), being at the level of 1–3 ns (30–90 cm) RMS in recent years. An improved prediction model for satellite clocks is proposed in order to enhance the precision of predicted clock offsets. First, the proposed prediction model adds a few cyclic terms to absorb the periodic effects, and a time adaptive function is used to adjust the weight of the observation in the prediction model. Second, initial deviations of the predictions are reduced by using a recomputed constant term. The simulation results have shown that the proposed prediction model can give a better performance than the IGU-P clock products and can achieve precision better than 0.55 ns (16.5 cm) in real-time predictions. In addition, the RTPPP method was chosen to test the efficiency of the new model for real-time static and kinematic positioning. The numerical examples using the data set of 140 IGS stations show that the static RTPPP precision based on the proposed clock model has been improved about 22.8 and 41.5 % in the east and height components compared to the IGU-P clock products, while the precisions in the north components are the equal. The kinematic example using three IGS stations shows that the kinematic RTPPP precision based on the proposed clock model has improved about 30, 72 and 44 % in the east, north and height components.     

The activities of the Ionosphere Working Group of the International GPS Service (IGS)

This article is based on a position paper presented at the IGS Network, Data and Analysis Center Workshop 2002 in Ottawa, Canada, 8–11 April 2002, and introduces the IGS Ionosphere Working Group (Iono_WG). Detailed information about the IGS in general can be found on the IGS Central Bureau Web page: http://igscb.jpl.nasa.gov. The Iono_WG commenced working in June 1998. The working group's main activity currently is the routine production of ionosphere Total Electron Content (TEC) maps with a 2-h time resolution and daily sets of GPS satellite and receiver hardware differential code bias (DCB) values. The TEC maps and DCB sets are derived from GPS dual-frequency tracking data recorded with the global IGS tracking network. In the medium- and long-term, the working group intends to refine algorithms for the mapping of ionospheric parameters from GPS measurements and to realize near–real–time availability of IGS ionosphere products. The paper will give an overview of the Iono_WG activities that include a summary of activities since its establishment, achievements and future plans. Electronic Publication     

Use of IGS products in TAI applications

The Bureau International des Poids et Mesures (BIPM) is in charge of producing International Atomic Time TAI. In this aim, it uses clock data from more than 60 laboratories spread worldwide. For two decades, GPS has been an essential tool to link these clocks, and products from the International GNSS Service (IGS) have been used to improve the quality of these time links since its creation in the early 1990s. This paper reviews the various interactions between the IGS and time activities at the BIPM, and shows that TAI has greatly benefited from IGS products so that their availability is now an essential need for the quality of TAI links. On the other hand, IGS has also benefited from introducing time laboratories equipped with highly stable clocks in its network of stations. In the future, similar products will be needed for an ensemble of satellite systems, starting with GLONASS and GALILEO. It will be a major challenge to the IGS to obtain a consistent set of products, particularly for what concerns satellite clocks and inter-system bias values.     

IGS Earth Rotation Parameters

Since its official start in January 1994, the International GPS Service (IGS) has been distributing, as part of its product combination, two distinct Earth rotation parameter (ERP) series: the IGS Rapid series and the IGS Final series. Initially, the IGS Rapid ERP values were interpolations of the International Earth Rotation Service (IERS) Bulletin A, whereas the IGS Final ERP series was based on the IERS Bulletin B. Since June 1996, the IGS has been generating its own Final ERP series consistent with the IGS combined orbit products and based on weighted means of individual IGS analysis center (AC) solutions. At first, only the polar motion (PM) coordinates and their rates were combined. Length of Day (LOD) and Universal Time (UT) solutions, also based on separate weighted mean combinations, followed in March 1997. Currently, the IGS Rapid and Final combinations are produced and made available within 17 hours and 11 days, respectively, after the last observation. Both IGS and the best AC series are consistent and precise at the 0.1-milliarcsecond (mas) level for PM and at about 30 μs for LOD. Biases in some AC solutions may exceed these consistency levels. Comparisons of both IGS ERP series with external standards, such as the IERS multitechnique Bulletins and atmospheric angular momentum series, confirm the estimated precisions. ? 1999 John Wiley & Sons, Inc.     

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.     

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.