Global analysis of the GGP superconducting gravimeters network for the estimation of the pole tide gravimetric amplitude factor

The tidal records of superconducting gravimeters (SG) in nine stations are analyzed, in order to determine the gravity variation due to the polar motion. In a first step the tidal constituents are estimated and subtracted from the original data, together with the estimated atmospheric pressure effects, by the computer program VAV. The data so obtained are submitted to a regression analysis by a specially developed program POLAR, whose aim is the estimation of the gravimetric amplitude factor δCH of the pole tide and its time shift with respect to the theoretical gravity signal due to the polar motion. The procedure includes an optimization of various parameters of the regression model, as well as a detection and elimination of the anomalous portions of the records. The analysis has been first applied separately on each one of the nine series. Most of the stations provided a δCH factor between 1.17 and 1.19 with mean square deviation close to 1%. Further all series have been submitted to a global analysis through which a common value of the δCH factor has been estimated. No significant global time shift has been found. The global adjustment values are δCH = 1.1816 ± 0.0047 or 1.1797 ± 0.0047, depending on the way the time shift is introduced, while the simple arithmetic mean of the stations is 1.1788 ± 0.0040. This result differs considerably from the values predicted by Earth response models, e.g. from δ = 1.158 obtained for the annual period through the non-hydrostatic anelastic model, usually called DDW99. The discrepancy is due to the indirect effects of the ocean tides. A preliminary correction scheme based on an equilibrium ocean pole tide is indeed reducing the globally adjusted δCH factor to 1.1612 or 1.1593 and the arithmetic mean to 1.1605.