Space geodetic activities at the Onsala Space Observatory: 25 years in the service of plate tectonics

Abstract We review the development of the space geodetic techniques of Very-Long Baseline Interferometry (VLBI) and the Global Positioning System (GPS) at the Onsala Space Observatory. This facility has been participating already in the first transatlantic VLBI measurement. Throughout the period from 1980 to date Onsala has been involved in more than 1000 individual baseline determinations. If we include the first technical developent phase we can look back at 25 years of history during which the observatory has contributed to refining our understanding of plate tectonics. As a main theme of research Onsala Space Observatory has been striving for improved models for the propagation of radio waves through the atmosphere. Related perturbations imply critical limitations for the accuracy that can be achieved on a wide range of distance and time scales. We report on developments for the measurement of water vapour content in the atmosphere, in its importance to derive improved strategies and models for space geodetic observations and data analysis. While VLBI surveys for geodynamics are continuing, shifting their emphasis more to large scale control of reference systems, GPS provides a precise technique suitable to monitor crustal deformation on the regional scale. We relate recent developments, particularly the establishment of a continuous network of permanent sites in Sweden (SWEPOS) and experience from the first three years of operation.

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