LIST OF ABSTRACTS

Session 3: "The role of Satellite Laser Ranging in the Global Geodetic Observing System"

The Virtual Observatory in Geodesy and Earth's Sciences: The French activities

Florent Deleflie, Sébastien Lambert, Pierre Exertier, A.-M. Gontier, C. Barache
Observatoire de la Cote d'Azur, France
E-mail: Florent.Deleflie@obs-azur.fr

Poster:
This poster presents the context of the astronomical Virtual Observatory (VO), an ambitious international proposal to provide uniform, convenient access to disparate, geographically dispersed archives of astronomical data from software which runs on the computer on the astronomer's desktop. The VO could be of interest for the geodetic community: we present some of our efforts in that direction. IVOA web site: http://www.ivoa.net OV-GAFF website: http://grg2.fr

The Contribution of Laser Ranging to the Global Geodetic Observing System

Richard Gross
Jet Propulsion Laboratory, USA
E-mail: Richard.Gross@jpl.nasa.gov

The Global Geodetic Observing System (GGOS) provides measurements of the time varying gravity, rotation, and shape of the Earth using instruments located on the ground and in space. The key components of GGOS that provide these measurements are the IAG Services, including the ILRS. Earth orientation parameters have been routinely determined from laser ranging observations to the Moon since 1970 and to artificial satellites since 1976, making them the longest available space-geodetic series of Earth orientation parameters. Such long duration homogenous series of accurate Earth orientation parameters are needed for studying long-period changes in the Earth's orientation, such as those caused by climate change. Low-degree time varying spherical harmonic coefficients of the Earth's gravitational field have also been routinely determined from laser ranging measurements to artificial satellites since shortly after the launch of Lageos; the resulting degree-2 zonal coefficient agrees better with models of the gravitational effect of surface geophysical fluids than does that determined by GRACE. Laser ranging measurements also fundamentally contribute to the determination of the terrestrial reference frame by providing its origin and contributing to its scale. Thus, satellite laser ranging is a key component of GGOS. Without satellite laser ranging GGOS would not be able to meet its goal of providing geodetic products accurate to better than a part per billion.

SLR, GNSS, VLBI, and DORIS Networks: ILRS+IGS+IVS+IDS

Carey Noll
NASA GSFC
E-mail : Carey.Noll@nasa.gov
Presenter: Michael Pearlman

Poster:
The global networks of the International Laser Ranging Service (ILRS), the International GNSS Service (IGS), the International VLBI Service for Geodesy and Astrometry (IVS), and the International DORIS Service (IDS) are the backbone of GGOS. The observations obtained from these global networks provide a continuous monitoring of the International Terrestrial Reference Frame (ITRF). Co-location of two or more techniques at sites is an important aspect for generation of the ITRF as well as providing an assessment of the observation quality, accuracy, and validation of results. As of mid-2008, these networks consisted of 42 laser ranging sites, 407 GNSS sites, 45 VLBI sites, and 58 DORIS sites. This poster will illustrate the global coverage of these networks, highlighting inter-technique co-locations.

SLR and the Next Generation Global Geodetic Networks

Erricos C. Pavlis, Magdalena Kuzmicz-Cieslak
Joint Center for Earth Systems Technology (JCET), USA
E-mail: epavlis@umbc.edu

The Global Geodetic Observing System - GGOS, places the utmost importance on the development, maintenance and wide distribution of an International Terrestrial Reference Frame (ITRF) with very stringent accuracy attributes. We present here results from simulation studies aimed at designing optimal global geodetic networks to support GGOS. At present, our goal is an origin definition at 1 mm or better at epoch and a temporal stability on the order of 0.1 mm/y, with similar numbers for the scale and orientation components. These goals are based on extensive deliberations within the Earth science community. In particular, oceanographers, a prime user group that these products are intended for, require this level of accuracy and temporal stability in order to address sea level rise issues with confidence. The stability, integrity and applicability of the ITRF are directly related to how accurately we can account for mass redistribution during the analysis and reduction process of the data used for its development. Long wavelength variations of the gravity field driven by these mass redistributions produce geometric effects that are manifested as changes in the origin and orientation between the instantaneous and the mean reference frame. This insidious coupling between the product and the reference with respect to which the product is generated makes the problem extremely complex and sensitive to systematic errors. An uneven distribution of the stations realizing the ITRF results in biases and distortions in the combined product due to the dissimilarity of the combined networks and their de facto lopsided overlap. Poor geometry results in increased correlations between the similarity transformation parameters, leading again to biased and unstable results. In this presentation, we are examining SLR's contribution in establishing the optimal network along with VLBI, since these two techniques alone are sufficient for this task. Using simulations of geodetic data that we expect to collect with the future geodetic networks, we look at various designs of several co-located networks and the resulting accuracy in the origin, scale and orientation definition of the realized ITRF.

The comparison of the station coordinates between SLR and GPS

S. Schillak, M. Lehmann
Space Research Centre, Polish Academy of Sciences, Astrogeodynamic Observatory,Borowiec, Poland
E-mail : sch@cbk.poznan.pl

The paper presents results of the comparison of the station positions and velocities between two satellite techniques: Satellite Laser Ranging (SLR) and Global Positioning System (GPS). The coordinates were calculated for common epochs in the International Terrestrial Reference System 2005. The comparisons were performed for 19 stations which had long time series for both techniques. The analysis included estimation of the station positions stability, station velocities comparison and differences of geocentric and topocentric station coordinates between both techniques. The tectonic plates model NNR-NUVEL1A was used for verification station velocities. The agreement of SLR calculated coordinates with ITRF2005 in positions and velocities was also presented. Generally with some exceptions is a good agreement of the station positions and velocities between results from SLR and GPS, GPS had higher accuracy of the coordinates determination especially in horizontal components. For several stations was detected 1-2 cm difference between vertical components of the SLR and GPS results.

QUALITY ASSESSMENT OF THE ILRS EOP 'DAILY' PRODUCT

C. Sciarretta(1), V. Luceri(2), G. Bianco(3)
(1) Telespazio SpA, Roma, Italy
(2) e-GEOS SpA, CGS-Matera, Italy
(3) Agenzia Spaziale Italiana, CGS-Matera, Italy

The ILRS Analysis Community has been developing, since several years, global, long term, very accurate geodetic products. In the last few years, even short term (weekly data arcs) and fast updated (every week) geodetic solutions have been produced, conveyed into official ILRS combined 'weekly' product, thus allowing the users to benefit of SSC and EOP estimates which are either accurate and fast. In the case of EOP, the users are provided with daily estimates over a week with a minimum latency, at the generation day, of 4 days and a maximum latency of 11 days. The 'weekly' product performance triggered the generation of a faster, lower latency combined product, now still under testing. This product is labelled 'daily' due to its updating frequency: a sliding window of 7 days worth of data is analyzed daily, reducing the overall product latency to 2 days. In this presentation, the performance of the presently available 'daily' EOP combined product is reported. In particular, the stability and reliability of the product with respect to its latency is analysed, as well as the effect of the combination process.

ESOC IGS, IDS, and ILRS (Re-) Processing

Tim Springer, Michiel Otten, Nacho Romero, John Dow
ESA/ESOC, Germany
E-mail: Tim.Springer@esa.int

Early in 2008 ESOC has replaced its old GNSS analysis software with its new software, called Napeos. One of the key design criteria of this new ESOC GNSS analysis software has been short processing times! The now fully operational software allows to generate a 1-day IGS final solution using 150 stations within 60 minutes on a single core of a Linux PC with the Intel Fortran compiler. With such a performance rapid reprocessing decades of IGS data becomes feasible.

Besides efficient GNSS analysis the Napeos software is also capable of processing DORIS and SLR observations. ESOC therefore recently joined the reprocessing efforts of the International Doris Service, and is currently working on joining the ILRS reprocessing efforts. Our aim is to contribute fully reprocessed time series to all three space geodesy technique services: IGS, IDS, and ILRS, before the end of 2008. In our opinion a contribution from one software to all three techniques is unique and of significant value for the next ITRF combination! Our IGS and IDS reprocessing is well under way and several years have been completed and submitted whereas our ILRS efforts are just starting. However, since the ILRS processing is much less demanding computationally, compared to the IGS and IDS efforts, we are confident that we will be able to complete the ILRS reprocessing before the end of 2008.

At ESOC the reprocessing of the three individual techniques is considered as a "first step". Our "ultimate" goal is to do a fully combined analysis of the data of all three techniques, and in the future even 4 techniques when adding VLBI. In this combination of techniques SLR plays a crucial role as it is the only technique that provides unbiased range measurements. Thus SLR is the only technique that provides direct access to the scale of the terrestrial reference frame.

Our presentation will show the status and some selected results from our different reprocessing solutions and provide an overview of our future plans regarding a fully combined reprocessing.