Daniela Thaller

Geocenter Coordinates from GNSS and Combined GNSS-SLR Solutions Using Satellite Co-locations

Satellite Laser Ranging (SLR) data to LAGEOS, ETALON and to Global Navigation Satellite Systems (GNSS) were combined with GNSS microwave data for 5 years. Including SLR data to GNSS satellites and estimating common orbit parameters allows it to connect both space-geodetic techniques using satellite instead of station co-location. We show that only SLR data to the spherical satellites can improve the geocenter estimates, whereas SLR data to the GNSS satellites suffer from the same GNSS orbit modelling deficiencies as in the analysis of microwave data.

Sensitivity of Lageos Orbits to Global Gravity Field Models

Sensitivity of Lageos Orbits to Global Gravity Field Models Precise orbit determination is an essential task when analyzing SLR data. The quality of the satellite orbits strongly depends on the models used for dynamic orbit determination. The global gravity field model used is one of the crucial elements, which has a significant influence on the satellite orbit and its accuracy. We study the impact of different gravity field models on the determination of the LAGEOS-1 and -2 orbits for data of the year 2008. Eleven gravity field models are compared, namely JGM3 and EGM96 based mainly on SLR, terrestrial and altimetry data, AIUB-CHAMP03S based uniquely on GPS-measurements made by CHAMP, AIUB-GRACE03S, ITG-GRACE2010 based on GRACE data, and the combined gravity field models based on different measurement techniques, such as EGM2008, EIGEN-GL04C, EIGEN51C, GOCO02S, GO-CONS-2-DIR-R2, AIUB-SST. The gravity field models are validated using the RMS of the observation residuals of 7-day LAGEOS solutions. The study reveals that GRACE-based models have the smallest RMS values (i.e., about 7.15 mm), despite the fact that no SLR data were used to determine them. The coefficient C20 is not always well estimated in GRACE-only models. There is a significant improvement of the gravity field models based on CHAMP, GRACE and GOCE w.r.t. models of the pre-CHAMP era. The LAGEOS orbits are particularly sensitive to the long wavelength part of the gravity fields. Differences of the estimated orbits due to different gravity field models are noticeable up to degree and order of about 30. The RMS of residuals improves from about 40 mm for degree 8, to about 7 mm for the solutions up to degrees 14 and higher. The quality of the predicted orbits is studied, as well.

Pre-combined GNSS-SLR Solutions: What Could be the Benefit for the ITRF?

In standard combination approaches (e.g., for the International Terrestrial Reference Frame), the space-geodetic techniques are connected by the Earth rotation parameters and by the station coordinates at co-located sites, using the so-called local ties. These local ties are usually derived from terrestrial measurements together with GNSS measurements for linking to a global reference system. The local ties often differ from the coordinate differences derived from space-geodetic observations. However, the sources for the discrepancies are not always clear, and a validation within the combination is difficult as long as the local ties are needed for the combination. We provide an alternative combination method by using the co-location of GNSS and SLR observations onboard GNSS satellites. As the local ties do not need to be applied in this approach, the resulting station coordinates are consistently estimated, but independent of the local ties. This allows us to evaluate the agreement of the terrestrial local ties and the space-geodetic coordinate differences derived from a 12-years solution. The 61 co-locations investigated in a multi-year solution show an agreement better than 1 cm in the horizontal and height components for 41 and 27 co-locations, respectively. Co-locations showing big discrepancies can be explained by the shortness of the data set included, or the rare distribution over the time span. Only the co-location site San Fernando shows unexplained differences of several centimeters. When using satellite co-locations, the counterpart of local ties at stations are space ties at satellites. The offsets of the microwave satellite antenna form one component of the space tie, with the offset of the laser retro-reflector array (LRA) forming the second part. We show that corrections to the space ties can be estimated from combined GNSS-SLR solutions. The corrections to the LRA offsets are only 5–6 mm, whereas the corrections estimated for the microwave antennas can exceed 1 dm, with a mean correction of −86.1 mm and −110.4 mm for GPS and GLONASS, respectively. The corrections to the microwave satellite antennas cause a scale difference for the GNSS ground network of 0.67 ppb. We show that this scale is consistent to the SLR scale, thus, the scale of SLR is properly transferred to the GNSS network via the co-location at the GNSS satellites.

Towards a rigorous combination of VLBI and GPS using the CONT02 campaign

The International VLBI Service for Geodesy and Astrometry (IVS) set up a 15-day campaign of continuous VLBI observations, named CONT02, that took place October 16–31, 2002. The goal of CONT02 was to acquire state of the art VLBI data over a continuous two-week period to demonstrate the highest accuracy of which VLBI is capable. Using the two weeks of CONT02 data from VLBI and the global GPS data for the same time span, daily unconstrained normal equations were generated taking much care to use identical models and the same parameterization of the common parameters for both space geodetic techniques. Based on these normal equations first steps towards a fully rigorous combination of all common parameters were performed by including station coordinates and Earth orientation parameters (EOP) into the solutions. In addition, the impact of the local tie information on the combined solutions was studied. To assess the quality of the individual and combined solutions, polar motion and UTI-UTC parameters were set up with a two-hour time resolution and the results were compared with sub-daily models for ocean tide variations derived from altimetry and other space geodetic techniques.

Loading-Induced Deformation Due to Atmosphere, Ocean and Hydrology: Model Comparisons and the Impact on Global SLR, VLBI and GNSS Solutions

Precise measurements of the Earth’s shape, gravity field, and rotation provide critical data for many geoscientific disciplines. In order to obtain reliable data, an accurate, stable, and global reference frame is required. The International Terrestrial Reference Frame, where station positions are modeled linearly, is commonly used throughout the geoscientific community for this purpose. Mass redistribution in the geophysical fluids, namely atmosphere, oceanic, and continental hydrology, cause time dependent variations in station coordinates, and other parameters such as the geocenter coordinates. Tidally-induced loading is described in the IERS Conventions. Models for non-tidal loading are available through the Global Geophysical Fluid Center. An overview of these models is given and comparisons were carried out. Within these comparisons, the best agreement was observed between the two atmospheric models, whereas the biggest discrepancies were found between the hydrology models. The processing setup for the Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Global Navigation Satellite Systems (GNSS) solutions is described, with loading displacement values introduced at the observation level. By using of non-tidal loading models the RMS of the height is reduced for 93% of the GNSS stations, with a max. reduction of 50%. The model impact on the station height in Wettzell derived by GNSS, SLR and VLBI shows a good agreement. In SLR results the Blue-Sky effect is visible. Applying the loading models reduced the seasonal variations visible in the geocenter time series derived by SLR almost completely.

GGOS-D Consistent, High-Accuracy Technique-Specific Solutions

Consistent and homogeneous long-time series of the space geodetic techniquesGlobal Positioning System (GPS), Satellite Laser Ranging (SLR), andVery Long Baseline Interferometry (VLBI) provide the basis for thecombination efforts of GGOS-D. For a consistent combination, thedefinition of common standards for parameterization and modeling isessential. These standards and the technique-specific processingoptions of all individual GPS, SLR, and VLBI solutions as well asthe combined SLR and VLBI solutions are discussed.

IERS Analysis Coordination

Until today the products of the International Earth Rotation and Reference Systems Service (IERS), like International Terrestrial Reference Frame (ITRF), International Celestial Reference Frame (ICRF) and Earth Orientation Parameters (EOP), are combined independently, neither intra-technique nor inter-technique combinations, including the full variance-covariance information are performed. To overcome this deficiencies in the present IERS product generation the IERS implemented a new structure in January 2001. This includes the new IERS Combination Research Centres (CRC) and the IERS Analysis Coordinator (AC). He is responsible for the long-term and internal consistency of the IERS products. To achieve the highest accuracy and consistency, it is crucial to proceed towards a fully rigorous combination of all the parameters common to more than one space geodetic technique. Since 2001 the IERS AC initiated and coordinated many different projects and campaigns towards this overall goal. Now the results of the last three years build the theoretical and practical base for the latest project, the Combination Pilot Project (CPP). This project will prepare the generation of a combined IERS product on a routine basis.

Recent Activities of the GGOS Standing Committee on Performance Simulations and Architectural Trade-Offs (PLATO)

The Standing Committee on Performance Simulations and Architectural Trade-Offs (PLATO) was established by the Bureau of Networks and Observations of the Global Geodetic Observing System (GGOS) in order to support – by prior performance analysis – activities to reach the GGOS requirements for the accuracy and stability of the terrestrial reference frame. Based on available data sets and simulated observations for further stations and satellite missions the committee studies the impact of technique-specific improvements, new stations, and additional co-locations in space on reference frame products. Simulation studies carried out so far show the importance of the individual station performance and additional stations for satellite laser ranging, the perspectives for lunar laser ranging assuming additional stations and reflectors, and the significant impact of the new VGOS antennas. Significant progress is achieved in processing VLBI satellite tracking data. New insights in technique-specific error sources were derived based on real data from short baselines. Regarding co-location in space PLATO members confirmed that E-GRASP could fulfill the GGOS requirements with reaching a geocenter and scale accuracy and stability of 1 mm and 0.1 mm/year, respectively.

Rapid response quality control service for the laser ranging tracking network

Quality control systems for satellite laser ranging (SLR) observations have been developed at a number of analysis institutes worldwide, using various software packages of precise orbit determination and data analysis. Satellite laser range observations, primarily from the two LAGEOS satellites but also from other satellites in low-Earth orbits and up to GNSS altitude, are being processed on a sub-daily to weekly basis. The generated quality control reports are widely used to detect various kinds of problems and quickly provide anomalous information to laser ranging stations. They have been effective in shortening the time to return to normal operations when anomalous data are detected and in quantifying the performance of laser ranging stations. Consequently, a rapid feedback loop has now been incorporated in the modern SLR operation.

GGOS-D Global Terrestrial Reference Frame

The GGOS-D global terrestrial reference frame has been computed from a combination of homogeneously processed VLBI, SLR and GPS observation time series. A major focus was on the analysis of station position time series, investigations regarding the handling of non-linear station motions, and the development of refined combination methods. The terrestrial reference frame (station positions and velocities) has been estimated simultaneously with the Earth orientation parameters and the celestial reference frame (quasar coordinates).