G31C-01 INVITED
Monitoring Global Sea Level Change: What do we Need From a Geodetic Observing System?
The launch of TOPEX/Poseidon (T/P) in 1992 ushered in a new era in sea level change studies, one that has been continued by the Jason-1 (2001) and OSTM/Jason-2 (2008) missions. The launch of GRACE in 2002 provided another important tool for studying sea level change via the monitoring of water mass movement on the Earth's surface. These missions require considerable geodetic infrastructure to support the orbit determination, defining and maintaining the reference frame, etc. In addition, geodetic monitoring of tide gauges on the Earth's surface is critical for calibration and validation of satellite altimeter measurements. This talk will review the major scientific questions in sea level change research, what we have learned so far from satellite geodetic measurements, and what geodetic infrastructure is needed to answer the important questions of the future.
G31C-02
Towards the International Altimeter Service (IAS), a core element of GGOS
Recognizing the multiple interdisciplinary applications of satellite altimetry and its specific importance for geodesy, the International Association of Geodesy, IAG, has decided to set up an International Altimetry Service (IAS) in order to extend or gradually improve existing services for the benefit of the broad altimetry user community. The rationale to establish this service as a sustainable core element of GGOS, the Global Geodetic Observing System, is based on the potential to provide a precise and near global mapping and monitoring of the Earth surface and to essentially improve the knowledge on the Earth gravity field. Moreover, mapping and monitoring of seasonal and secular changes of the mean sea level helps to understand fundamental processes of the System Earth: the ocean water mass redistribution with its impact to the Earth centre-of-gravity, to Earth rotation by the ocean angular momentum functions, the temporal variations of the Earth gravity field, and the sea level rise. IAS will act in a mission- and agency-independent capacity. In view of many organisations already providing data and products for satellite altimetry, IAS can only be realised by an integrating effort, e.g., as collaboration between data providers, archive and product centres, research laboratories and experienced users. One essential step towards IAS will consist in knowledge transfer: where to find what data, products and documents, to provide a mission overview and basic information on different altimetry technologies. Further initial activities consists in the definition and invitation to tender of pilot projects e.g. for compilation and comparison of precise altimeter satellite ephemeris, for supporting calibration and validation activities, for compilation and comparison of ocean tide models and other topics. The present paper provides information on status and the development of IAS.
G31C-03 INVITED
The Future Global Geodetic Networks to Support GGOS
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 attributes. 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 first step, we are focusing on establishing the optimal SLR and VLBI network, since these two techniques alone are sufficient, and they are also the most costly, necessitating a very conservative deployment of the minimum number of such systems. Using simulations of geodetic data that we expect to collect with the future geodetic networks, we provide various designs of several co-located networks and the resulting accuracy in the origin, scale and orientation definition of the realized ITRF. We present here the results of extensive simulation studies aimed at designing optimal global geodetic networks to support GGOS.
G31C-04
The Global Geodetic Observing System and the Gravity Field
The gravity field of the earth is a central pillar of GGOS. With the time-changing gravity field from GRACE providing important information on the movements of mass in the earth system, and the continued refinement of gravity field models and geoids, the role of gravity in GGOS is increasing. Gravity field quantities are special in the way that measurements at the ppb-accuracies, matching the space geodesy geometrical parameter determination, is either a function of wavelength (e.g., the long-wavelength mm-geoid from satellite gravity field missions), or based on point sampling (absolute or superconducting gravimetry). The International Gravity Field Service acts for coordination of the activities of the gravity-field related services of the IAG and GGOS. From a global perspective, one of the main challenges is to ensure the consistency of the global and regional geopotential and geoid models, with a definition of a global vertical reference system spanning the oceans and continents a natural element of this. The current EGM08 geopotential model, producing for the first time a global 10-cm geoid at very high spherical harmonic degree (2160), serves as a first example of a global static gravity field model serving GGOS needs. Other IGFS coordination actitivities include the establishment of a global network of repeated absolute gravity stations. For time changing satellite gravity fields, a first attempt at collecting all available data from different processing centers have been done at the ICGEM (International Center for Global Earth Models), one of the new components of IGFS.
G31C-05
Time-varying gravity comparison of the GSFC solutions derived from DORIS, SLR and GRACE
The GRACE gravity mission has now supplied us with solutions for the time variations in the Earth's gravity field that now span more than five years, from 2003 to the present. Solutions are available from different analysis centers: GSFC, CNES, JPL, and UT/CSR. Satellite laser ranging (SLR) and DORIS (Doppler Orbitography and Radiopositioning) Integrated by satellite have also been used to develop time series of geopotential solutions, which describe the low degree variations in the geopotential. In the case of SLR, these data span more than 25 years, and in the case of DORIS the solutions span the past 15 years (since 1993). The SLR satellites that contribute include Lageos1 and Lageos2, Starlette, Stella, Ajisai, and other spacecraft. The DORIS satellites include the full constellation of DORIS spacecraft including TOPEX, ENVISAT, and the SPOT series of satellites. We describe the time series obtained from the various solutions, and examine statistically the similarities and differences between the time series, and the quality of the solutions, and describe the annual and interannual signals that appear in the solutions. We compare the low degree harmonics for the periods where the SLR, DORIS and GRACE time series overlap, and we intercompare the geopotential rate solutions for the three sets of solutions, bearing in mind that the techniques supply solutions over different time periods.
G31C-06
GGP - An International Project Dedicated to Studies of Geodynamics Based on Superconducting Gravimeter Observations
The Global Geodynamics Project (GGP) first established in 1997 and further extended in 2003 comprises a worldwide network of superconducting gravimeters (SG). Recent new observation sites include Pecný (Czech Republic), Ghuttu (India), and Austin (Texas). Several new stations are about to be established resp. panned extending the network to about 30 stations in 2009. During the last IUGG assembly in 2007 it was decided to move to a permanent network hosted by IAG and being part of GGOS. Specific GGP recommendations have been given for observation conditions, data processing, and reductions. Raw SG data are made available to the scientific community via the GGP database. A standard processing of the data sets is carried out by the International Center for Earth Tides (ICET). Of ongoing interest within GGP is the issue of consistently combining observations from absolute gravimeters and permanent GPS at the SG stations for studies related to long-term phenomena such as tectonic uplift, subduction zone slip, post-glacial rebound, and present-day ice melting, but also for research related to changes in continental hydrology. One of the most attractive innovative ideas within GGOS is the determination of the geocenter using a combination of satellite and terrestrial gravimetry. GGP can contribute in a unique way in this respect through continuous high-resolution gravity data sets (better than 1 nm/s2) in addition to measurements of absolute gravity. The continuous monitoring of temporal variations of the Earth's gravity field provides a tool to investigate many aspects of dynamics in the Earth's system and to contribute to other disciplines like seismology, volcanology, tectonics, earth rotation, oceanography, and hydrology. Another promising application is the deployment of SG sub-networks to assess temporal gravity field variations derived from satellite observations i.e. from the GRACE mission and large-scale hydrological modeling.
G31C-07
Realization of the Terrestrial Reference System by global GPS observations
The Global Geodetic Observing System (GGOS) aims the coordination and integration of geodetic observations on the ground and in space in order to generate a uniform set of geodetic parameters for the monitoring of processes and phenomena in the system Earth. For the compilation of the measurements of the time varying gravity, rotation and shape of the Earth within GGOS and their interpretation the definition and realization of the terrestrial reference system (TRS) with highest accuracy is essential. We present a concept of a terrestrial reference system realization by global GPS-observations only. Our terrestrial reference frame PDR05 is based on the results of a homogeneous reprocessing of a global GPS network over the time span 1994.0-2006.0. The reference frame is realized in the center of mass system, which implies a self-consistent model of the reference frame and the loading dynamics. The determined set of coordinates at epoch 2000.0 and their linear rates of change are evaluated in terms of self-consistency. Additionally, we compare the results to other TRS realizations, such as ITRF2000 and ITRF2005 as well as the GPS-only realizations IGb00 and IGS05. We clearly show, that GPS is able to realize the origin in the center of mass and the scale on the same level of self-consistency, stability and precision as other space techniques. (Paper is invited by Richard Gross (AGU membership number 10190565))
G31C-08 INVITED
Global deformation of the Earth, surface mass anomalies, and the geodetic infrastructure required to study these processes
Global deformation of the Earth can be linked to loading caused by mass changes in the atmosphere, the ocean and the terrestrial hydrosphere. World-wide geodetic observation systems like GPS, e.g., the global IGS network, can be used to study the global deformation of the Earth directly and, when other effects are properly modeled, provide information regarding the surface loading mass (e.g., to derive geo-center motion estimates). Vice versa, other observing systems that monitor mass change, either through gravitational changes (GRACE) or through a combination of in-situ and modeled quantities (e.g., the atmosphere, ocean or hydrosphere), can provide indirect information on global deformation. In the framework of the German 'Mass transport and mass distribution' program, we estimate surface mass anomalies at spherical harmonic resolution up to degree and order 30 by linking three complementary data sets in a least squares approach. Our estimates include geo-center motion and the thickness of a spatially uniform layer on top of the ocean surface (that is otherwise estimated from surface fluxes, evaporation and precipitation, and river run-off) as a time-series. As with all current Earth observing systems, each dataset has its own limitations and do not realize homogeneous coverage over the globe. To assess the impact that these limitations might have on current and future deformation and loading mass solutions, a sensitivity study was conducted. Simulated real-case and idealized solutions were explored in which the spatial distribution and quality of GPS, GRACE and OBP data sets were varied. The results show that significant improvements, e.g., over the current GRACE monthly gravity fields, in particular at the low degrees, can be achieved when these solutions are combined with present day GPS and OBP products. Our idealized scenarios also provide quantitative implications on how much surface mass change estimates may improve in the future when improved observing systems become available.