International GPS Service Research Articles

Confirmation of Arabia plate slow motion by new GPS data in Yemen

During the last 10 years, a network of about 30 GPS sites was measured in Djibouti, East Africa. Additional points were also measured in Yemen, Oman, Ethiopia, Iran, and on La Réunion island. Merged with data from the available International GPS Service permanent stations scattered on the different plates in the area (Eurasia, Anatolia, Africa, Arabia, Somalia), this unique data set provides new insight on the current deformation in the Africa‐Somalia‐Arabia triple junction area and on the Arabian plate motion. Here we show that coherent motions of points in Yemen, Bahrain, Oman, and Iran allow us to estimate a geodetically constrained angular velocity for the Arabian plate (52.59°N, 15.74°W, 0.461°/Myr in ITRF2000). This result differs significantly from earlier determinations and is based upon our vectors in Yemen. They provide new additional data and better geometry for angular velocity determination. Combined with the African and Somalian motions, this new angular velocity results in predicted spreading rates in the Red Sea and the Gulf of Aden which are 15–20% lower than those measured from oceanic magnetic anomalies and thus averaged over the last 3 Myr. With respect to Eurasia, the geodetic motion of Arabia is also about 30% slower than predicted by NUVEL‐1A. On the basis of the kinematic results presented here and on other evidence for a similar slower geodetic rate of the Indian plate, we suggest that the whole collision zone between Africa, Arabia, India on one hand and Eurasia on the other hand has slowed down in the last 3 Myr.

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.

A Neural Network Relation Of Gps Results With Continental Hydrology

This paper presents the application of a neural network methodology to historical time series of GPS data from the IGS (International GPS Service) network, based on terrestrial water storage information. Hydrology signals at the GPS sites are important for including water loading corrections in GPS data processing. However, it is quite common that a correct global water storage model may not be available for this purpose, due to lack of science data. It is therefore mostly assumed that water mass redistribution is one of the potential contributors to the seasonal variations in GPS station position results, particularly, in the vertical direction. Presently, the IERS Special Bureau for Hydrology (SBH) has archived continental water storage data from some of the latest model developments. Examples include the monthly (GRACE, NOAA CPC, NCEP/NCAR CDAS-1) and daily (NCEP/NCAR and ECMWF reanalyses) solutions. It is valuable to study the relationship between these solutions and long-term geodetic results, especially as the water storage models continue to be refined. Using neural networks offers an effective approach to correlate the non-linear input of hydrology signals and output of geodetic results by recognizing the historic patterns between them. In this study, a neural network model is developed to enable the prediction of GPS height residuals based on the input of NOAA CPC hydrology data. The model is applied to eight GPS sites with satisfactory results.

Analysis of Global Ionospheric TEC Disturbance During the Magnetic Storm in Nov. 2004

利用全球分布的GPS原始观测数据提取的电离层总电子含量(TEC)分析了2004年11月6日至12日期间全球电离层暴的形态特点与发展过程.结果表明,11月8日磁暴主相期间电离层暴以大范围的强烈正暴为主,在11月10日的恢复相,Dst又一次降到最低值前后期间,电离层再次受到很强的扰动,大范围的正暴和负暴交替出现.这次磁暴期间夏季半球的负暴更加强烈,反映出负暴偏向于在夏季半球发生的季节变化特点.另外,磁暴期间,夜晚TEC值普遍比磁暴前的平静期要低,具体是什么机制导致还需要进一步收集数据和分析.

Automated daily processing of more than 1000 ground‐based GPS receivers for studying intense ionospheric storms

As the number of ground‐based and space‐based receivers tracking the Global Positioning System (GPS) satellites steadily increases, it is becoming possible to monitor changes in the ionosphere continuously and on a global scale with unprecedented accuracy and reliability. As of August 2005, there are more than 1000 globally distributed dual‐frequency GPS receivers available using publicly accessible networks including, for example, the International GPS Service and the continuously operating reference stations. To take advantage of the vast amount of GPS data, researchers use a number of techniques to estimate satellite and receiver interfrequency biases and the total electron content (TEC) of the ionosphere. Most techniques estimate vertical ionospheric structure and, simultaneously, hardware‐related biases treated as nuisance parameters. These methods often are limited to 200 GPS receivers and use a sequential least squares or Kalman filter approach. The biases are later removed from the measurements to obtain unbiased TEC. In our approach to calibrating GPS receiver and transmitter interfrequency biases we take advantage of all available GPS receivers using a new processing algorithm based on the Global Ionospheric Mapping (GIM) software developed at the Jet Propulsion Laboratory. This new capability is designed to estimate receiver biases for all stations. We solve for the instrumental biases by modeling the ionospheric delay and removing it from the observation equation using precomputed GIM maps. The precomputed GIM maps rely on 200 globally distributed GPS receivers to establish the “background” used to model the ionosphere at the remaining 800 GPS sites.

The International GPS Service tracking network: Enabling diverse studies and projects through international cooperation

Abstract The International GPS Service (IGS), formulated beginning in 1989 and formalized in 1994, was founded on the collaborative operation of approximately 30 permanent GPS stations to benefit global geodynamics. The same cooperative principles, today applied to a network of over 300 stations, still serve to maximize global benefit without unnecessary duplication of investment in global infrastructure. The scope of applications of the dataset has grown to include atmospheric, oceanographic, subdaily, and low-earth orbiter activities through working groups and pilot projects fostered within the IGS in the now traditional IGS spirit of collaboration. These activities and the IGS infrastructure are viewed as critical elements to the Global Geodetic Observing System. This presentation will review the present nature of the IGS tracking network and its ability to support new applications.

The IGGOS viewed from the Space Geodetic Services

Abstract The Space Geodetic (Technique) Services of the International Association of Geodesy, in particular the International VLBI Service for Geodesy and Astrometry (IVS), the International Laser Ranging Service (ILRS) and the International GPS Service (IGS) will make major contributions to the proposed Integrated Global Geodetic Observing System (IGGOS) through the provision of time series of parameters which describe the fundamental components of global reference frames, in particular the International Celestial Reference Frame (ICRF) and the International Terrestrial Reference Frame (ITRF), and parameters of the transformation between them (Earth Orientation Parameters (EOP's), nutation, origin and scale). The three techniques VLBI, SLR and GPS are represented by services, which in the last years have taken over the coordination of the supporting components including observation sites, data routing and management, and analysis facilities. Moreover, they have the responsibility for consolidated product delivery by timely provision of high quality time series, with temporal latency in some cases approaching near real time. The results of the different services reflect the distinct characteristics of the technique observables, which are partly similar and partly complementary. Due to the independence and different sensitivity of the techniques, systematic errors between the three techniques can be detected by appropriate combination of the results and to some extent at the level of the observables. Such a procedure will increase the reliability and the consistency of the products in satisfying the requirements of IGGOS. This paper reviews the different techniques with regard to their advantages and indicates sources of systematic errors. It will focus on requirements which have to be fulfilled for the combination of the service-dependent time series and might provide a basis for discussion on future plans in order to evolve the services themselves towards fulfilling the objectives of IGGOS.

Surface mass redistribution inversion from global GPS deformation and Gravity Recovery and Climate Experiment (GRACE) gravity data

Monitoring hydrological redistributions through their integrated gravitational effect is the primary aim of the Gravity Recovery and Climate Experiment (GRACE) mission. Time‐variable gravity data from GRACE can be uniquely inverted to hydrology, since mass transfers located at or near the Earth's surface are much larger on shorter timescales than those taking place within the deeper Earth and because one can remove the contribution of atmospheric masses from air pressure data. Yet it has been proposed that at larger scales this may be achieved independently by measuring and inverting the elastic loading associated with redistributing masses, e.g., with the global network of the International GPS Service (IGS). This is particularly interesting as long as GRACE monthly gravity solutions do not (yet) match the targeted baseline accuracies at the lower spherical harmonic degrees. In this contribution (1) we describe and investigate an inversion technique which can deal jointly with GPS data and monthly GRACE solutions. (2) Previous studies with GPS data have used least squares estimators and impose solution constraints through low‐degree spherical harmonic series truncation. Here we introduce a physically motivated regularization method that guarantees a stable inversion up to higher degrees, while seeking to avoid spatial aliasing. (3) We apply this technique to GPS data provided by the IGS service covering recent years. We can show that after removing the contribution ascribed to atmospheric pressure loading, estimated annual variations of continental‐scale mass redistribution exhibit pattern similar to those observed with GRACE and predicted by a global hydrology model, although systematic differences appear to be present. (4) We compute what the relative contribution of GRACE and GPS would be in a joint inversion: Using current error estimates, GPS could contribute with up to 60% to degree 2 till 4 spherical harmonic coefficients and up to 30% for higher‐degree coefficients.

Evaluation of solar radio bursts' effect on GPS receiver signal tracking within International GPS Service network

The direct interference from solar radio bursts (SRB) has not usually been considered as a potential threat to global positioning system (GPS) signal tracking, since the flux densities of most bursts are below 40,000 solar flux units (sfu), a threat threshold to GPS L1 frequency proposed by Klobuchar et al. (1999). Recent analysis indicated that a much lower threshold should be adopted for codeless or semicodeless dual‐frequency GPS receivers. In this investigation, severe signal corruptions were found at dayside International GPS Service GPS receiver stations during a large solar radio burst that accompanied the super flare of 28 October 2003. Almost no GPS L2 signals were tracked during the solar flux peak time for areas near the subsolar point. Correlation analysis was performed between the rate of loss of lock on GPS L2 frequency and solar radio flux density at different bands, and a correlation index as high as 0.75 is revealed in the 1415 MHz solar radiation band, which is located between the two GPS operating frequencies L2 (1227.60 MHz) and L1 (1575.42 MHz). The correlation analysis indicates that GPS signal losses of lock were primarily caused by microwave in‐band interference and that the threat threshold of SRB effects on the GPS system should be re‐evaluated, since the flux density of the burst at 1415 MHz was just 4,000–12,000 sfu, which is far below the previously proposed threat threshold. The signal‐tracking performance of different types of GPS receivers during such a super flare event is also presented.

Comparison of polar motion with oceanic and atmospheric angular momentum time series for 2-day to Chandler periods

During a 4-year period starting in July 1996 and using intervals ranging from 3 days to 4 years, four precise polar motion (PM) series have been compared to excitation by atmospheric angular momentum (AAM) augmented with oceanic angular momentum (OAM) data. The first three series (C03, C04 and Bulletin A) are multi-technique combinations generated by the International Earth Rotation and Reference Systems Service (IERS) and the fourth combined series (IGS00P02) is produced by the International GPS Service (IGS) using only GPS data. The IGS PM compared the best with the combined excitations of atmosphere and oceans (AAM+OAM) at all intervals, showing high overall correlation of 0.8–0.9. Even for the interval of only three days, the IGS PM gave a significant correlation of about 0.6. Moreover, during the interval of February 1999 – July 2000, which should be representative of the current precision of the IGS PM, a significant correlation (>0.4) extended to periods as short as 2.2 days and 2.5 days for the x p and y p PM components, respectively. When using the IERS Bulletin B (C04) PM and an interval of almost 6 years, starting in November 1994, the combined OAM+AAM accounted for practically all the annual, semi-annual and Chandler wobble (CW) PM signals. When only AAM was used, either the US National Centers for Environment Prediction reanalysis data, which were used throughout this study, or the Japanese Meteorological Agency data, two large and well-resolved amplitude peaks of about 0.1 mas/day, remained at the retrograde annual and CW periods.

Geodetic techniques for time and frequency comparisons using GPS phase and code measurements

We review the development and status of GPS geodetic methods for high-precision global time and frequency comparisons. A comprehensive view is taken, including hardware effects in the transmitting satellites and tracking receiver stations, data analysis and interpretation, and comparisons with independent results. Other GPS techniques rely on single-frequency data and/or assume cancellation of most systematic errors using differences between simultaneous observations. By applying the full observation modelling of modern geodesy to dual-frequency observations of GPS carrier phase and pseudorange, the precision of timing comparisons can be improved from the level of several nanoseconds to near 100 ps. For an averaging interval of one day, we infer a limiting Allan deviation of about 1.4 × 10−15 for the GPS geodetic technique. The accuracy of time comparisons is set by the ability to calibrate the absolute instrumental delays through the GPS receiver and antenna chain, currently about 3 ns. Geodetic clock measurements are available for most of the major timing laboratories, as well as for many other tracking stations and the satellites, via the routine products of the International GPS Service.

Improving carrier-phase ambiguity resolution in global GPS network solutions

Integer carrier-phase ambiguity resolution is one of the critical issues for precise GPS applications in geodesy and geodynamics. To resolve as many integer ambiguities as possible, the ‘most-easy-to-fix’ double-difference ambiguities have to be defined. For this purpose, several strategies are implemented in existing GPS software packages, such as choosing the ambiguities according to the baseline length or the variances of the estimated real-valued ambiguities. Although their efficiencies are demonstrated in practice, it is proven in this paper that they do not reflect all effects of varying data quality, because they are based on theoretical considerations of GPS data processing. Therefore, a new approach is presented, which selects the double-difference ambiguities according to their probability of being fixed to the nearest integer. The probability is computed from estimates and variances of wide-lane and narrow-lane ambiguities. Together with an optimized ambiguity fixing procedure, the new approach is implemented in the routine data processing for the International GPS Service (IGS) at GeoForschungsZentrum (GFZ) Potsdam. Within a sub-network of about 90 IGS stations, it is demonstrated that more than 97% of the independent ambiguities are fixed correctly compared to 75% by a commonly used method, and that the additionally fixed ambiguities improve the repeatability of the station coordinates by 10–26% in regions with sparse site distribution.

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.

Performance Testing of Time Comparison Using GPS-Smoothed P3 Code and IGS Ephemerides

In this paper, the characteristics of time transfer using global positioning system (GPS)-smoothed P3 code and International GPS Service (IGS) precise ephemerides are studied. Recently, GPS P3 code has been used in International Atomic Time (TAI) computation as a substitute for GPS C/A code; however, this observation still has large variations when satellites are at low elevation angles. For acquiring more stable time comparison results, we adopt a carrier smoothing technique to reduce this noise. The IGS precise ephemerides are also used to reduce satellite orbital errors in order to obtain better results. To test our program for use in international time transfer, we processed GPS data between various laboratories using the smoothed P3 code and IGS ephemerides. We then compared these results to those obtained using unsmoothed P3 code and broadcast ephemerides. This test shows that using the smoothed P3 code and IGS ephemerides can reduce the uncertainty of GPS time comparison.

Study of ionospheric response to the 4B flare on 28 October 2003 using International GPS Service network data

Using the GPS data from as many as 114 GPS stations of the International GPS Service for Geodynamics (IGS), the morphological features of the ionospheric total electron content (TEC) variations on the sunlit hemisphere during the 4B solar flare on 28 October 2003 is studied. It is found that the strongest sudden increase of TEC (SITEC) happened during the flare, and the magnitudes of SITEC vary at regions with different local solar zenith angle (SZA). In the northern hemisphere, the TEC enhancement is approximately symmetrical to the local noon, and its value is usually greater than 14 TECU (1 TECU = 1016/m2) if the SZA is less than 60°. On the whole, as the SZA increases, the value of TEC enhancement in the northern hemisphere decreases. It is worth mentioning that even in the regions of SZA between 90° and 100°, the SITEC was still seen from the temporal TEC curves. Using a photochemical model, the electron production rate over the sunlit boundary region is calculated and some obvious features of SITEC over this region are analyzed. In the polar region, the effect of this flare on the ionosphere exceeds the effect of the ionospheric scintillations and it seems that the ionosphere in the northern polar region responses more sensitively to this flare. In the end, superimposed on the curves of the rate of TEC change, there are some small disturbances (spikes) synchronously appearing on all curves and thus indicating an existence of similar structures in the EUV band of the flare.

High-Accuracy DGPS and Precise Point Positioning Based on Ohio CORS Network

In the precise point positioning (PPP) module of the Multi Purpose GPS Processing Software (MPGPSTM), developed at The Ohio State University, International GPS Service (IGS) products are tested along with the MPGPSTM-derived ionosphere models to provide instantaneous, kinematic, and static positioning. In this paper, we present an analysis of the accuracy of PPP during high and low daily total electron content (TEC) levels when 1 h of GPS data, either single- or dual-frequency, is available. The primary objective of the tests presented here is to verify the feasibility of both decimeter-level absolute GPS positioning in kinematic and static modes using the PPP approach, and submeter positioning using instantaneous PPP and differential GPS (DGPS). The multistation PPP and DGPS solutions are simulated in postprocessing using algorithms designed for real-time application. Static observations are used in the kinematic processing mode to evaluate the positioning accuracy obtained with both PPP and DGPS.

Validating ocean tide loading models using GPS

Ocean tides cause periodic deformations of the Earth’s surface, also referred to as ocean tide loading (OTL). Tide-induced displacements of the Earth’s crust relying on OTL models are usually taken into account in GPS (Global Positioning System) data analyses. On the other hand, it is also possible to validate OTL models using GPS analyses. The following simple approach is used to validate OTL models. Based on a particular model, instantaneous corrections of the site coordinates due to OTL are computed. Site-specific scale factors, f, for these corrections are estimated in a standard least-squares adjustment process of GPS observations together with other relevant parameters. A resulting value of f close to unity indicates a good agreement of the model with the actual site displacements. Such scale factors are computed for about 140 globally distributed IGS (International GPS Service) tracking sites. Three OTL models derived from the ocean tide models FES95.2.1, FES99, and GOT00.2 are analyzed. As expected, the most reliable factors are estimated for sites with a large loading effect. In general, the scaling factors have a value close to unity and no significant differences between the three ocean tide models could be observed. It is found that the validation approach is easy to apply. Without requiring much additional effort for a global and self-consistent GPS data analysis, it allows detection of general model misfits on the basis of a large number of globally distributed sites. For detailed validation studies on OTL models, the simultaneous estimation of amplitudes and phases for the main contributing partial tides within a GPS parameter adjustment process would provide more detailed answers.

Evaluation of GPS Precipitable Water over Canada and the IGS Network

Abstract Precipitable water (PW) derived from the GPS zenith tropospheric delay (ZTD) is evaluated (as a first step toward variational data assimilation) through comparison with that of collocated radiosondes (RS_PW), operational analyses, and 6-h forecasts (from the Canadian Global Environmental Multiscale model) of the Canadian Meteorological Centre. Two sources of ZTD data are considered: 1) final ZTD (over Canada), computed by the Geodetic Survey Division (GSD) of Natural Resources Canada, and 2) final ZTD (distributed globally), obtained from the International GPS Service (IGS). The mean GSD GPS–derived PW (GPS_PW) is 14.9 mm (reflecting the relatively cold Canadian climate), whereas that of the IGS dataset is 20.8 mm. Intercomparison statistics [correlation, standard deviation (SD), and bias] between GPS_PW and RS_PW are, respectively, 0.97, 2.04 mm, and 1.35 mm for the GSD data and 0.98, 2.6 mm, and 0.67 mm for the IGS data. Comparisons of GPS_PW with 6-h forecast PW (TRIAL_PW) show slightly lower correlations and a higher SD. The increase in SD is greater for the IGS data, which is not surprising, because in regions such as the Tropics and subtropics, moisture forecasts are of a lower quality and the RS observation network is sparse. From a three-way intercomparison (IGS GPS_PW, RS_PW, and TRIAL_PW) of the SD statistics, it is found that GPS_PW has the lowest estimated PW error (≈1 mm) for PW in the 5–30-mm range. For PW greater than 30 mm, the RS_PW estimated error is ≈2 mm, and that of GPS_PW is ≈2.5 mm. The TRIAL_PW estimated error increases with PW, reaching 5.5 mm in the 40–55-mm PW range. These intercomparison results indicate that GPS_PW should be a useful source of humidity information for NWP applications.

Validation of GNSS orbits using SLR observations

Abstract Satellite Laser Ranging (SLR) observations allow for a completely independent validation of orbits derived using microwave measurements as provided by the International GPS Service (IGS). The orbit validation is based on the difference between the observed range (SLR measurements) and the computed range (assuming that the satellite positions are know from the IGS). The resulting range residuals are primarily an indicator for the radial accuracy of the microwave orbits. This validation method is well-known and several validation results have been published in the last 10 years. The IGS analysis center CODE (Center for Orbit Determination in Europe) provides on a routine base daily range residuals for all GNSS satellites observed by both, SLR and microwave techniques. We present results of our recent range residual analysis for two GPS satellites (PRN G05, G06) and three GLONASS satellites (PRN R03, R22, R24). Microwave orbits provided by IGS and CODE are used as well as SLR normal point observations of 13 globally distributed sites. The resulting range residual time series of about 3.3 years, starting in January 2001, for GPS satellites and of about 10 months, starting in June 2003, for GLONASS satellites are analyzed. The comparison of the range residuals with previous analysis results shows the impact of several improvements, i.e., the increasing quality of the microwave orbit as well as the use of new laser retroreflector arrays on GLONASS satellites launched after 1995. The validation of the CODE orbits shows a standard deviation of 2.7 cm for the GPS satellites and of 4.9 cm for the GLONASS satellites. A bias of about −5.8 cm for GPS satellites and of −2.3 cm for GLONASS satellites between the SLR measurements and the microwave orbits was determined. The cause of these biases is not yet fully understood. Pass-specific systematics in the range residuals were found, but it was not possible to assign them to station- or satellite-specific error sources. Deficiencies in microwave orbit modeling might probably cause pass-specific patterns.

The International GPS Service: Celebrating the 10th anniversary and looking to the next decade

After 10 years as a service of the International Association of Geodesy, the International GPS Service IGS is preparing for the future use of multiple integrated global navigation satellite systems: GPS and its modernisation, Galileo and GLONASS. Since 1994, the IGS produces GPS data and products at the highest level of precision and accuracy available anywhere: it provides GPS orbits with 3–5 cm accuracy, sub-centimetre station positions and velocities, and station and satellite clocks at the sub-nanosecond level for users world-wide. A similar suite of data and products is available for GLONASS, demonstrating the ability of the IGS to incorporate observations from other GNSSs, such as Galileo. IGS affirms its interest to contribute to the development and applications of the Galileo programme and other GNSS. IGS currently consists of over 200 actively contributing organizations in more than 80 countries and a global network of more than 350 stations. The working groups and pilot projects of the IGS demonstrate IGS involvement in applications related to the precise global reference frame, timing, ionosphere, atmospheric water vapour, low Earth orbiter precise orbit determination (LEO POD), sea level change measurements, real-time GPS applications, and GNSS developments.