Abstract
Since Kepler, Newton and Huygens in the seventeenth century, geodesy has been concerned with determining the figure, orientation and gravitational field of the Earth. With the beginning of the space age in 1957, a new branch of geodesy was created, satellite geodesy. Only with satellites did geodesy become truly global. Oceans were no longer obstacles and the Earth as a whole could be observed and measured in consistent series of measurements. Of particular interest is the determination of the spatial structures and finally the temporal changes of the Earth's gravitational field. The knowledge of the gravitational field represents the natural bridge to the study of the physics of the Earth's interior, the circulation of our oceans and, more recently, the climate. Today, key findings on climate change are derived from the temporal changes in the gravitational field: on ice mass loss in Greenland and Antarctica, sea level rise and generally on changes in the global water cycle. This has only become possible with dedicated gravity satellite missions opening a method known as satellite gravimetry. In the first forty years of space age, satellite gravimetry was based on the analysis of the orbital motion of satellites. Due to the uneven distribution of observatories over the globe, the initially inaccurate measuring methods and the inadequacies of the evaluation models, the reconstruction of global models of the Earth's gravitational field was a great challenge. The transition from passive satellites for gravity field determination to satellites equipped with special sensor technology, which was initiated in the last decade of the twentieth century, brought decisive progress. In the chronological sequence of the launch of such new satellites, the history, mission objectives and measuring principles of the missions CHAMP, GRACE and GOCE flown since 2000 are outlined and essential scientific results of the individual missions are highlighted. The special features of the GRACE Follow-On Mission, which was launched in 2018, and the plans for a next generation of gravity field missions are also discussed.
Highlights
Today, Helmert’s classical definition of geodesy (Helmert 1880) could perhaps be reformulated as follows: "Geodesy is the science of determining the geometric and gravimetric figure of the Earth and its orientation and how these properties change over time"
For example, a model of the sea topography was available as an independent quantity, one could claim that satellite altimetry includes both the determination of the geometrical and the gravimetric shape of the oceans
Already from these first monthly data, a gravity field based on data from a single satellite could be calculated for the first time, which showed an improvement in the long wavelength proportions by a factor of 10 compared to pre-CHAMP models (Reigber et al 2002)
Summary
Helmert’s classical definition of geodesy (Helmert 1880) could perhaps be reformulated as follows: "Geodesy is the science of determining the geometric and gravimetric figure of the Earth and its orientation and how these properties change over time". In view of the achievable accuracies of geodetic measuring methods, the determination and analysis of the temporal changes of the geometric and gravimetric figure of the Earth and the Earth orientation are moving into the centre of geodetic research, in addition to the spatial variations. A fascinating special role is played by ocean altimetry, i.e. the centimetre-accurate scanning of the sea surface with satellites This method will be briefly characterized already here. The scanning of the ocean surface corresponds almost exactly to measuring the most important equipotential surface of the Earth’s gravity field, the geoid, and to the determination of the gravimetric figure of the Earth. The textbook (Seeber 1993) belongs to this series of general overviews including its comprehensive literature review
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