Ocean tide loading, body tide and polar motion effects on very long baseline interferometry
Response functions for radial and horizontal displacements, gravity perturbations and tilt due to ocean tide loading were derived, using Green’s functions computed by Farrell for the Gutenberg-Bullen A earth model. The general procedure of convolving response functions with an ocean tide model was used in this study. The ocean tide model used was that developed by Schwiderski, using the six leading constituents of the tidal spectrum. The derived response functions for gravity perturbations were tested against existing determinations of the effect from actual data, at eight stations covering the whole earth. This comparison yielded a mean of the absolute differences between the observed gravity perturbations and those obtained in this study of 0.32 µGal with a root-mean-square scatter of 0.19 µGal. A body tide model was also developed. The ephemerides of the moon and the sun were based on approximate formulae. Tests performed on the above approximate formulae indicated that the right ascension and declination of the moon and the sun can be obtained with an accuracy of the order of one minute of arc with respect to the values obtained in the astronomical ephemerides. This uncertainty affects the evaluation of the radial displacement of the terrain by less than 1 cm. The mathematical models for terrain deformations due to body tide and ocean loading ad well as an interpolation procedure for polar motion were added to the Canadian VLBI software package. VLBI data pertaining to the observing period of May 1977, obtained by a 3-station VLBI array were used to test the above models. Due to the large standard errors of these data, there is little that can be concluded from the analysis at this time. There is an improvement in the distribution of the delay-rate residuals. Their root-mean-square scatter decreases by 9.2x10[squared by -5] picoseconds/second while the root-mean-square scatter of the group-delay residuals increases by 3 picoseconds. C-range and co-tidal charts for north America were also produced, showing the radial and the horizontal displacements of the terrain due to load for the six leading tidal constituents.