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The Global Geodetic Observing System (GGOS):
A Key Component in the Global Earth Observation System of Systems

Hans-Peter Plag, Nevada Bureau of Mines and Geology and Seismological Laboratory, University of Nevada, Reno, Nevada, USA, hpplag@unr.edu.

The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) is coordinating the geodetic support for Earth sciences and Earth observation. Building upon the work of the IAG Services, GGOS provides the geodetic infrastructure necessary to support the monitoring of the Earth system and global change research. With the global geodetic reference frames, that is, the International Terrestrial Reference Frame (ITRF) and the International Celestial Reference Frame (ICRF), which are linked to each other through the Earth Rotation Parameters (ERP), GGOS provides a fundamental utility for all Earth observations, and this contribution is widely recognized. Although already very high, the current accuracy of the ITRF is a major limitation for the detection and interpretation of global change processes such as sea level and ice sheet changes. The Group on Earth Observations (GEO) has acknowledged the importance of the geodetic reference frames as part of the metrological basis for the Global Earth Observation System of Systems (GEOSS), and has devoted a Task in its Work Plan to improving the framework conditions for the global infrastructure required for the determination and monitoring of the frames. However, geodesy and GGOS provide more to Earth observation: observations of the variations in the Earth's shape, gravity field, and rotation. Geodetic observations relate to changes in the shape of the Earth, including the surface of the oceans and ice sheets: Sea level and ice sheet changes are observed with satellite altimetry from space and supported by in situ observations of associated deformations and gravity changes. Geohazards all involve ground deformation: a common observational requirement for these hazards is the ability to measure surface displacements with respect to a well defined reference frame. Geodetic techniques are therefore crucial in the assessment, monitoring and early detection of geohazards, and they will play a pivotal role in early warning systems for such hazards and in mitigating the disasters. Information on surface displacements provides a basis for scientific studies of geohazards, hazard assessment, early warning, and disaster assessment. In addition to the classical, point-oriented geodetic techniques, two-dimensional imaging techniques such as InSAR are increasingly available, allowing the monitoring of relevant areas with high spatial resolution, although currently not with the low latency and temporal resolution required for some geohazards applications. Their integration with the geodetic point-techniques provides new means for the monitoring of geohazards. Geodetic observations relate to mass redistribution in the fluid envelop of the solid Earth: GGOS therefore has the potential to be developed into a monitoring system for mass transport in the global water cycle particularly on sub-continental to global scales. Thus GGOS can complement other in situ and remote sensing observations of changes in land water storage and other changes in the water cycle and aid water resource management, particularly on regional scales. Geodetic observations related to the dynamics of the Earth system: GGOS therefore can provide observational constraints on changes in the Earth's system dynamics and, for example, support validation of climate and ocean models. The prospect of a fully implemented GGOS is increased security, a better use of resources, and progress towards sustainable development. The presentation will summarize the current status of the implementation of GGOS and consider its relation to GEOSS. GEOSS provides a valuable overview of observational requirements in many societal areas and these requirements provide a tool to assess the system performance of GGOS and to identify necessary improvements.