Chapter 1 Introduction

摘要

The surface of the Earth is constantly moving and changing its appearance, periodically as well as permanently. The periodic deformations are predominantly products of tidal forces, exerted by the Moon and Sun, and the rotation of the Earth. Tides of the solid Earth have a magnitude of several tens of centimeters, whereas ocean tides can reach heights of several meters (mostly due to the presence of the continents and resonance effects in ocean basins). Tidal phenomena continuously affect directly or indirectly all kinds of global and also local geodetic observations, geometric as well as gravimetric ones. Mass displacements due to tidal height variations and relative particle motion due to tidal current flows cause significant changes in the Earth rotation speed and can alter the direction of the rotation axis. The variability of both the Earth rotation speed and the direction of the rotation axis again has considerable consequences for the observation of the positions of objects in space and stations on the Earth surface. Accordingly, the explicit knowledge of Earth rotation irregularities, whatever source of excitation they have, is substantial for all tasks of precise positioning and navigation relying on space observations. As to geodesy that applies in particular for the space techniques, VLBI (Very Long Baseline Interferometry), GNSS (Global Navigation Satellite System), SLR, and LLR (Satellite and Lunar Laser Ranging) and DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite). On the one hand these space geodetic techniques partially depend on information about the spatio-temporal relation between positions on Earth and in space and on the other hand they as well observe and furnish such information. The space geodetic observations thus play a key role in Earth rotation research and vice versa independent and realistic models of Earth rotation variability can considerably improve the results of space geodesy.