Chapter 2 Ocean Tidal Loading Effects to Displacements at GNSS Sites

摘要

Ocean tidal loading (OTL) effects to displacements should be considered during GNSS precise positioning. Especially, along with the shift of geodesy from mainland to ocean, the OTL corrections to displacements play more and more important roles in precise positioning. The approaches to resolve OTL effects in precise GNSS data processing software GAMIT are that, either directly reading out the amplitudes and phases of several main tidal constituents (also called tidal coefficients) at site from file station.oct, or interpolating the site's tidal coefficients from global grid file grid.oct. It's not ideal to modify the OTL effects in China mainland (especially in coast areas, islands and reefs) using GAMIT directly, because of tidal coefficients' errors or the limitations of tracking stations' distribution. This paper detailed describes the OTL effects theories to displacements based on convolution integration approach about OTL and Green's functions. Numerical integration of OTL is performed using the Gauss quadrature method and the integration areas are separated to inner zone and outer zone. A newest 2' × 2' resolutions local ocean tidal model of the East China Sea and South China Sea was adopted for inner zone, and a global model TPXO7.2 for the outer zone. Some OTL corrections to displacements at coast GNSS sites were computed, and which were applied to the GNSS data processing. The estimation of amplitudes and phases for the main tidal constituent M2 were acquired at some sites, variation functions changed with time for displacement were constructed. Numerical tests show that, the displacements at coast sites are bigger than those at inland sites, amplitudes of local loading on displacements reach the order of centimetre. If the amplitudes and phases of tidal constituents calculated by this method are appended into the station.oct file, the baselines' accuracies will be improved greatly for GAMIT software. Not only the OTL corrections but also suitable ocean tidal models and tide constituents should be taken into account in GNSS data analyses.