The poster presents activities at the Astronomical Institute of the University of Bern (AIUB) in the field of precise orbit determination (POD) for Low Earth Orbiters (LEOs) using the GPS. They are currently focused on the two GRACE satellites and range from general studies about reduced-dynamic and kinematic precise orbit determination based on zero- and double difference observations to the implementation and testing of the POD procedures in the framework of the High-Level Processing Facility for ESA´s upcoming GOCE mission.

CODE provides consistent GNSS orbits for GPS and GLONASS satellites since May 2003. When extending the consistent GNSS processing on the receiver resp. satellite clock corrections intersystem and in the case of GLONASS interfrequency time biases have to be considered when analyzing the pseudorange observations.

In this contribution we discuss the completeness of a potential GNSS satellite clock product including GLONASS. The handling of the GLONASS interfrequency code biases in the analysis has a direct impact on the results, on how to combine the clock corrections, and how to use these products by the users community.

Analogue problems are expected if the upcoming European GALILEO system will be included into the combined multi-system GNSS analysis.

The processing of GNSS data from low Earth orbiters typically involves a change of the data sample interval from 5 minutes down to 30 seconds, leading to an increase in the number of processed observations and estimated clocks by one order of magnitude. Within the constraints set by available hardware and required IGS product latencies, such an increase in process size requires maximum efficiency from the processing systems at the IGS Analysis Centres. Furthermore, multi-constellation solutions introduce signals and receivers from GPS, GLONASS and Galileo in one single process. This not only leads to a further increase of process size, but in addition requires a high level of flexibility to cope with constellations of varying size and composition.

Although information technology has advanced rapidly over the past decades, and has provided many new ways for coping with such increased demands to large software systems, the world of science still seems to depend mainly on large, complex FORTRAN systems. At ESOC’s Navigation Office, a prototype C++ system is being developed that explicitly minimizes the use of memory and CPU, supports multi-CPU parallelization, and has a generic interface with a high level of autonomy. The main objectives of this development are

For more than 10 years the International GNSS Service (IGS) has demonstrated a knack for innovation to maximize the benefits of GPS/GNSS signals in space. Today the IGS provides a large set of high quality products for a huge number of applications e.g. in geodynamics, surveying or atmosphere monitoring. A key objective of the IGS is to provide users anywhere in the world access to highest level GNSS data, products and resources for scientific applications, through an “open data policy”. This is naturally dependent upon the availability and performance of the various satellite systems.

Recognizing the importance of the upcoming new European satellite navigation system (GALILEO) and of the modernization programs planned for GPS and GLONASS the IGS decided to set up a GNSS-Working Group. Major goals of this WG are to prepare a consolidated feedback to GNSS system engineering based on relevant IGS experience of providing highest accuracy products for the existing systems and concerning the work of IGS Analysis Centres as well as other IGS Working Groups to reflect opportunities of the various GNSS modernization programs.

The recent launches of the first GPS IIR-M satellite as well as of additional GLONASS-M satellites offers new opportunities and signals for IGS data processing but also carries the risk of introducing new intra- and intersystem biases e.g. Satellite Antenna Offsets or Differential Code Biases. An upcoming additional challenge obviously will be the launch and data processing of the GALILEO IOV satellites in 2007. Thus special emphasis is given to calibration characterization issues such as the role of SLR for orbit determination, estimation of inter-system and inter-frequency biases, clock and orbit prediction as well as reference frame definition and realization.

This presentation will give a summary of the activities of this GNSS-WG and touch upon the strategies of the International GPS Service for optimizing the future use of multiple integrated GNSS.

The EUREF Permanent Network (EPN), consists of more than 180 permanent GPS stations from which about 25 are also tracking GLONASS satellites. The primary purpose of the EPN is to maintain the European Terrestrial Reference System (ETRS89) and EUREF does this by generating weekly coordinate estimates for all EPN stations. Up to now, all coordinate estimates have been based on only GPS data and no GLONASS data is used.

With the growing number of available GPS+GLONASS equipment, the recent revitalization of GLONASS and the fact that the IGS GLONASS orbits are now available with a similar latency as the IGS GPS orbits, it has become worthwhile to investigate the advantages and disadvantages of adding GLONASS data to the routine data analysis of the EPN network. In addition, at the last EPN Analysis Centers Workshop in March 2006, it was decided that the EPN Analysis Centers could from now on deliver GPS+GLONASS solutions for the maintenance of the ETRS89. Since the EPN is a regional network, it needs a priori orbit and clock information. We will compare the coordinates obtained used a GPS-only analysis with the ones obtained using a GPS+GLONASS analysis. In the case of the GPS+GLONASS-based coordinates, the results obtained using the IGS orbits will be compared with the results obtained using CODE orbits.

The proposed Khartoum Continuous Operating Reference Stations (CORS) system consists of seven stations and one central processing center for the processing and distribution of GNSS data. These stations are designed to form a precise geodetic network of permanent stations continuously track the visible GNSS satellites and where satellite receivers are to be installed. The GNSS receivers will telemetry the data to the Central Processing Center via a GSM media at a measurement interval of 1 second. The initial core product of the CORS data will be collected from the permanent stations using dual frequency GPS receivers to collect carrier phase and code observations, as well as precise GPS satellite ephemeris, earth rotation parameters, ionospheric and atmospheric information, coordinates and velocities of the permanent network stations. The objectives of establishing Khartoum CORS network are to increase and to improve the real time and post processing capabilities of GNSS and their geomatics and engineering applications, monitoring of recent crustal movements and atmospheric and geodynamic studies.

The raw GPS data from all the stations is to be processed, modeled, and adjusted applying various methods including Area Calculations Parameters, for Ionospheric, tropospheric effects and distance dependant errors. The processed data is then available for the whole Coverage area in RTCM Format either through GSM or Radio Network. The concept of having the GNSS reference stations is to have homogeneous network all over Khartoum state area. Users can log into the main control room through GSM/radio link and send their navigated position to the system, and the system in turn will send the user RTCM corrections for that navigated position, enabling the user to have 1-2cm RTK position accuracy anywhere within the coverage area of the whole Network. Data is fully controlled and archived and only authorized users can be linked to access the data.

The paper presents the technical details of the Khartoum CORS system network design, site selection, hardware configuration, modeling, monitoring and handling of satellite and receiver biases and atmospheric propagation delays.

A review of the IGS Reference Frame realization “IGb00” is presented along with a proposal for an update. The focus is on 3 main aspects: first, the update of the selected set of primary reference frame stations; second, the realignment to ITRF2005 and third, the impact of station antenna absolute phase centers on the reference frame. The current IGb00 realization was proposed and adopted almost 3 years ago; it included 99 stations. For various reasons, about 80 stations remain useable. This situation also highlights the importance of the reference frame stations and the effort that must be made to maintain these key sites, avoid or eliminate discontinuities whenever possible to help ensure a more stable frame. The proposed new realization will take advantage of additional stations while still relying on older sites to ensure reliable link with historical data. The realization will also be realigned to the new ITRF2005, when officially available. The effect of the switch from station antenna relative calibration “IGS_01” to the absolute calibration “IGS_T05” will also be discussed. The antenna phase center shift as well as radome addition/removal also introduces discontinuities in the station coordinates time series. Each aspect of this update will introduce a small discontinuity (rotation, translation, scale and rates) between the existing “IGb00” and the proposed realization.

The IGS analysis centers and user community in general need to be assured that the data centers archive a consistent set of files. Changes to the archives can occur because of the re-publishing of data, the transmission of historic data, and the resulting re-distribution (or lack thereof) of these data from data center to data center. To ensure the quality of the archives, a defined data flow and method of archive population needs to be established. This poster will diagram and review the current IGS data flow, discuss problems that have occurred, and provide recommendations for improvement.

NASA supports the Global Navigation Satellite System (GNSS) infrastructure through a network of 75 permanent stations called the Global GPS Network (GGN). The GGN is operated cooperatively by JPL and UNAVCO. GGN data are contributed to the International GNSS Service (IGS) global network. GGN stations make up approximately 20 percent of the IGS and are some of its longest running core stations. GNN sites provide 1 to 30 second sampling and a number of stations have available real-time data streams. Data are used to produce highly accurate products that are essential for Earth science research and other multidisciplinary and educational applications. Products include GNSS precise satellite orbits, Earth rotation parameters, global tracking station coordinates and velocities, satellite and tracking station clock information, zenith tropospheric path delay estimates, and global ionospheric maps. These global data and products form the critical framework that regional GNSS networks depend upon. The GGN is currently being upgraded to accommodate additional GNSS observables as they become available including the new GPS L2C and L5 signals, Galileo, and GLONASS. Careful consideration is being made to integrate new equipment and observations without adversely affecting the time series measurements at critical stations. As part of this effort, a special new monument design is being tested at UNAVCO’s Colorado test facility. The monument can accommodate multiple antennas that can be used for collocated observations while new site equipment is phased into operation.

The Aegean and its surrounding area is one of the most active seismic region in the Mediterranean and West Eurasia. This area lies on the collision zone between the Eurasian and the African lithospheric plates.

Admittedly, the level of our knowledge about the geological structure of the region and its tectonic deformation remains limited as a result of the lack of geodetic measurements of high quality.

Since 1997, the Technical University of Crete (TUC), Chania, Crete, Greece, has been operating a permanent GPS station, i.e., TUC1. The station has been the southernmost European permanent GPS site. Its data are useful for geodynamical studies of the southern fringe of the European plate and its interactions with African plate.

In 2004, a second station, named TUC2, was installed in the TUC campus next to TUC1. After a period of testing, the station has been accepted for inclusion in the EUREF. The hourly data feed and meteorological data upstream have been established. TUC2 is collocated with the specially equipped pad for satellite laser ranging measurements. The measurement campaign with the French Transportable Laser Ranging Station had been carried out in 2004, and new campaigns are planned for the future.

Another permanent GPS site, named GVD0, has been installed by TUC on the island of Gavdos, about 80 km south of Chania. The GVD0 is collocated with the DORIS beacon.

These permanent GPS sites have provided a wealth of precise data for a number of research activities, such as the Establishment of a European radar altimeter calibration and sea-level monitoring site for JASON, ENVISAT and Euro-GLOSS, the development of statistical quality control algorithms, and the monitoring of the "silent and slow earthquakes" happening in the boundary zone.

We present the main results derived from the permanent GPS measurements carried out by TUC and our plans for extending our array over other parts of Crete.

This paper examines the application of the Statistical Process Control for monitoring the quality of GPS-coordinate time series in real time. Quality control is constrained to monitoring the location (i.e., mean value) and scale (i.e., accuracy) of the available data in one-dimension.

The detection of failures or changes of small magnitude in GPS coordinate solutions is critical for applications requiring continuous and reliable results. Examples include real-time deformation monitoring for dams, high-rise buildings, bridges, earth surface tectonic movements, landslides, etc.

Control charts are implemented as modules in a software package being developed at the Crete Tech University, Greece. The software has been designed to monitor data in real time and triggers alarms whenever predefined critical values are exceeded.
The conventional cumulative sums and the adaptive Cusums have been applied to Real-Time-Kinematic GPS data, as produced in an experiment. An abrupt shift in data has been assumed to vary between 0.5 to 2 standard deviations from a target mean value.
Comparative results show that the conventional Cusums are suitable and efficient tools in monitoring quality of the RTK-GPS data. Results also show that the control charts for the detection of location shifts should also be accompanied by control charts on the scale.

The phase center and variations (PCV) of an GNSS antenna can be precisely determined using the Geo++® Absolute Field Calibration with a Robot. The PCV are determined free or significantly reduced of any mutipath effects depending on the antenna type. However, there are remaining multipath effects caused by the actual setup and the environment on the GNSS site, which can significantly modify the phase variations.

The site multipath influence itself can be separated into near-field and far-field effects, which do have different properties. Near-field effects cause a systematic bias especially in the coordinate height component. Far-field effects can be averaged out by sufficient length of observation data.

The absolute antenna calibration with the robot is an excellent instrument to investigate near-field effects on phase variations. A particular antenna setup mounted on the robot will be constantly rotated and tilted by the calibration procedure, but the geometry between received satellite signals and setup will not change. Due to the very long-periodic multipath in the close vicinity and electro-magnetic interaction of the antenna, the phase variation pattern change. Therefore, the near field effect of the antenna can be determined and investigated.

The capabilities of the robot concerning size and weight of the antenna setup as well as a representative mock-up of the GNSS antenna set-up are limited. Therefore also other means are currently investigated by Geo++ to determine the near-field effect.

Investigations on near-field effects using a robot, the separation of site dependent error components and feasible approaches to determine GNSS site near-field effects are discussed.

The problem of vertical crustal deformations occurring at seasonal timescales in response to soil moisture loading is investigated.

Measurements of periodical elastic compensation of solid Earth on tide and surface load, determined from GRACE data are used to this purpose. We propose to use a spectral method to convert the 10-day GRACE gravity field CNES-solutions (interpreted as surface load) in terms of vertical displacements comparable to GPS and DORIS measurements. We apply this strategy to regions with high amplitudes of seasonal variations of continental water storage (tropical basins of Africa and South America, in the South East Asia during monsoon events and several basins of the Northern hemisphere) as observed by the GRACE space mission.

The today's geodynamical and navigation applications of GNSS are being processed by a large variety of softwares where each one of them implements its own analysis strategy. The estimated unknown parameters of a LSQ (recursive or iterative) procedure can depend on theses strategies (reference frame definition, tropospheric parameterisation, ambiguity resolution, receiver’s antenna effect, stochastic parameterisation etc.) which can induce artefacts in the estimated positions and velocities of geodetic points.

The aim of this study is to analyse the geodynamical results coming from the methodology used by GINS GPS software. We are currently examining a DD network solution together with the Precise Point Positioning (PPP) strategy implemented in our software. To compare the strategies analysis, we use a set of 10 days from the 6 months GPS data acquired in the north-western France, Brittany in 2004 in order to study ocean loading. The ocean tides of this region can reach up to 10 m and produce loading effects up to 12 cm peak-to-peak on the vertical component and some cm-level displacements on the horizontal components of geodetic stations. In this specific case we need high time resolution GPS solutions to study short-periodic signals (diurnal, semi-diurnal, tier-diurnal, quart-diurnal, fifth-diurnal, sixth-diurnal, seventh-diurnal, and eighth-diurnal period signals) instead of classical 24h or hebdo-average solutions. Moreover, the equivalence in some cases between the loading effect and the processing artefacts sets up a sensitivity condition for the processing strategy (ambiguity resolution problem, constraints, tropospheric delay, ad-hoc models etc.). For example in GRGS we are currently producing our own GPS orbits and a comparison of the solutions with the ones from IGS orbits is examined. So it is essential to quantify the software’s strategies impact on the GPS positioning.

The different solutions are compared to the predicted positioning time series based on FES2004 (LEGOS) model in a local geodetic system NEU, which is considered as our reference in this study.

Co-located very long baseline interferometry (VLBI) and Global Positioning System (GPS) reference stations were installed near Fortaleza, Brazil in 1993. Both have been important in the realization and maintenance of the International Terrestrial Reference Frame (ITRF). A new generation GPS system was installed in 2005 to replace the original station. Experience gained in the prior 12 years has been used to improve the design of the GPS antenna mount. Preliminary indications are greatly improved data quality from the new station. Simultaneous observations from the nearly half-year of overlapping operation have been used to determine the local tie between the new and old GPS reference points to about 1 mm accuracy. This can be used to update the 1993 survey tie between the original GPS and the VLBI points, although there are questions about the accuracy of that measurement based on a comparison with space geodetic data. A test of removing the conical radome over the old GPS antenna indicates that it has biased the station height by about 16 mm downward, which probably accounts for most of the previous survey discrepancy.

A new set of anechoic chamber calibrations of geodetic GPS antennae was ob-served in September 2005. The measurements aimed at the validation and com-parison of these laboratory results to the absolute values obtained from the IGS accepted robot calibration by GEO++. Remaining systematic discrepancies and errors sources of the anechoic chamber set-up leading to different levels of agreement for different antenna-types between these two methods were studied. These are effects of cables, remaining multipath, calibration mechanisation, among others. Results will be shown for Trimble Zephyr, Leica AX1202 and Dorne Margolin type antennae. In addition experiences of the use of absolute antenna calibration parameter in the German SAPOS network and antenna change related effects will be discussed.

The EUREF Permanent Network (EPN) consists of more than 180 stations and is still growing. It serves as a densification of the IGS network in Europe and also plays a major role in the realisation of the European Terrestrial Reference System 89 (ETRS89). This reference system is used as the standard for precise positioning, surveying and geodynamic studies throughout Europe. It is supported by EuroGeographics, which represents nearly all European national mapping and cadastral agencies (NMCA), and is therefore the dominating reference system in Europe. Many NMCA have established networks for real-time positioning services like SAPOS in Germany. The conversion to absolute phase center models has already been achieved in several of these networks due to the improved performance of the real time services. The Local Analysis Centres of the EPN still lack the conversion to absolute PCV. But this step is mandatory in order to be consistent with the networks of the NMCA. The poster will focus on the effects caused by the introduction of absolute antenna phase center models for a subnetwork of the EPN. Due to its large variety in installed GPS antennas – partly not even choke ring antennas - changes in the position of the GPS sites can be expected. Especially the impact of antenna domes, which are still not corrected in the EPN network, on the position and the use of a absolute antenna calibration models on the site coordinates for the subnetwork processed by the BEK local analysis centre will be shown.

For several years, swisstopo has being involved in data analysis of GPS permanent networks. One main activity concentrates on the computation of troposphere parameters in near real-time. Zenith total delay estimates are delivered every hour with a time delay of about 45 minutes to various partners, such as the European project E-GVAP (EUMETNET GPS Water Vapour Programme) and the Federal Office of Meteorology and Climatology (MeteoSwiss), with the goal to use the data for numerical weather prediction.
At present, a network of 75 permanent sites is processed on a routine basis. In line with the switch from Bernese GPS Software Version 4.2 to 5.0, a number of new features related to troposphere parameter determination became relevant. The modeling of the troposphere delay is now done using a continuous, piece-wise linear parameter representation, and the dry-Niell in conjunction with the wet-Niell mapping function are applied. Furthermore the impact of the network size, the usage of relative constraints and the application of ambiguity resolution strategies is studied. The influence of these effects on the resulting troposphere products, and comparisons with solutions coming from a real real-time positioning software (GPSNET) are presented in this paper.

ESA/ESOC is one of the main contributors to the IGS participating in practically all facets of the IGS activities. This presentation will focus on the analysis aspects of our contributions. As analysis centre ESOC contributes to all the IGS products: Ultra-Rapid, Rapid, Final, as well as the IGLOS, combined GPS and Glonass, processing. Over the course of the last two years several improvements have been made in our procedures, which have lead to a significant improvement in the consistency and quality of our products. The changes we made can be divided in four areas, namely: data retrieval, orbit modeling, data cleaning, and ionosphere. This presentation will highlight the most important changes made and show the positive effect these have had on our contributions to the IGS.

At the same time ESOC is in the process of replacing the IGS analysis software. The new software, called NAPEOS, should be operational for all IGS activities before the end of this year (2006). NAPEOS will by fully compliant with the IERS2003 conventions and will follow all the IGS recommendations, e.g., ANTEX and will ensure full internal consistency of our final products. With NAPEOS we expect a significant further improvement of our contributions to the IGS, which should bring us to the same quality level as the best IGS analysis centers. Furthermore, with NAPEOS we will be able to contribute significantly to the IGS reprocessing efforts. For testing purposes NAPEOS is being used routinely since November 2005 for processing a 100 station GNSS network. This routine process is similar to our IGLOS processing activities. Initial results from this routine processing will be presented. One unique feature in this process is that we estimate one bias per day for each receiver-Glonass satellite pair. The interesting results from these estimates will be presented showing the significant biases between the receivers and between the Glonass satellites.

GPS-based time and frequency transfer is presently performed either with a code-only analysis (as done for TAI, using C/A or P codes), or with a combined analysis of code and carrier phase measurements using geodetic analysis techniques (as used for the generation of the IGS time scale). When neglecting calibration issues, the accuracy of both solutions highly depends on the noise of the GPS codes. An important part of this code noise is caused by multipath. Using a linear combination of GPS codes and carrier phases, the behavior of code multipath in a specific station can be monitored and mitigated. The impact of code multipath on the time transfer results can then be estimated. On one hand, we investigate a possible reduction of the rms of the code-only CGGTTS results. On the other hand we evaluate the influence of the multipath mitigation on the results obtained from the combined code and carrier phase time transfer. In that case, a particular attention is drawn to the day boundary jumps appearing in the solution. These day boundary jumps show large dispersion between stations, which reflects stations code performances, but of which the origin is not yet fully understood. The hypothesis of a multipath origin is therefore tested.

The Italian Space Agency located in Matera in involved in several GPS activities. ASI manages the Italian GPS fiducial network and delivers GPS data to the users through the EUREF local data center Geodaf. GPS data are analyzed in the EUREF framework for the definition of the European reference frame and under the umbrella of the EC TOUGH and CERGOP2 Projects for meteorological applications and hazard monitoring. An overview of ASI GPS activities will be given.

The Earth Orientation Department at the U.S. Naval Observatory (USNO) produces precise Earth orientation parameters on a daily basis. As part of this activity, we contribute to the international GNSS community as an IGS Associate Analysis Center, producing Rapid and Ultra-Rapid products on a daily and sub-daily basis. The accuracies of these products have improved significantly by the implementation of modified processing strategies and the addition of modernized software. This paper describes our method of orbit determination and prediction, the improvements to our products, and relevant research currently being pursued at USNO.

We present the first results for investigation the dynamics of global electron content (GEC) that is equal to the total number of electrons in the near space bounded by the GPS orbital altitude (about 20000 km). We propose a method of GEC estimation based on the Global Ionospheric Maps technique (GIM). GEC is calculated by summation of total electron content values over all GIM cells, and multiplied by the area of a GIM cell. We have suggested the measurement unit GECU, which is equal to 10**32 electrons. We analyzed data for the period 1998-2005 and we found that during the 23-rd cycle of solar activity the average level of GEC varied significantly: from 0.5 to 3.5 GECU. We estimated dependence between variations of the 10.7-cm solar radio emission and of GEC as: GEC=0.013*(F10.7-60)+0.5.

27-day variations of GEC are very similar to the ones of the index F10.7, but GEC undergoes a lagging of about of 30-60 hours as compared to value of the F10.7 index. 27-day variations of GEC decrease from 8% to 3.5% as solar activity varies from its minimum to its maximum. These changes of relative amplitude of 27-day GEC variations agree well with the characteristics of active areas on the Sun.

It was also found out that GEC has seasonal variations, their maximum is related to equinoctial months. Relative amplitude of these variations reaches 10% during low solar activity and it changes up to 30 % during high solar activity. Deep seasonal variations are also typical for a ratio of GEC of the lighted and darken sides of the Earth. Maximal values of this ratio were observed in the periods of summer and winter solstices. It has different value depending on solar cycle: it changes from 3.1 to 3.7 during low solar activity and it changes from 2.8 to 3.5 during high solar activity. We developed a method and program for GEC estimation using IRI-2001 (GEC-IRI) and compared its results with experimental GEC values during the period from 1998 to 2005. We found a good agreement between observational and model data for GEC in general, but there are some significant distinctions. Significant overestimation of GEC-IRI values is perceptible for upper integration heights higher than 2000 km (up to 5-6 times for GPS satellites altitude, 20000 km). It was found that GEC-IRI seasonal variations are out-of-phase with experimental GEC values. A ratio of GEC-IRI of the lighted and darken sides of the Earth is lower (from 2.8 to 3.2) than the one for experimental GEC. Relative difference and RMS between GEC-IRI and experimental GEC series increase as smoothing time window decreases. Mainly this reflects the fact that IRI is a median ionosphere model and do not take into account day-to-day variations of the ionosphere parameters (e.g. 27-day variations). The minimal relative difference and RMS values are founded for smoothing by one-year time window (about 1.5% and 5% accordingly). As smoothing time window decreases to 10 days the relative difference and RMS increases nearly to 10-15%. Besides the relative difference for 10-day time window increases with year from about 1.5-2% for 1999 to 8-9% for 2005, while RMS values are maximal during the period of high solar activity.