Gravity Tide Observations at VERA Stations

Abstract

Gravity tide observations were carried out at VERA four stations. LaCoste-Romberg gravimeters D73 and G457 were used in the observations of three stations, and a superconducting gravimeter was used at Mizusawa station. The major purpose of the observations is to verify the tidal displacement model used in VERA geodetic VLBI and astrometry VLBI observations. The tidal analysis results were compared with a model calculation by GOTIC2. The results of three stations except Ogasawara show good agreement between the observations and the model estimations. The differences are estimated less than 0.2% in tidal factors. If those differences come from the ambiguity of Love number h, it is estimated to be less than 1%. This ambiguity of h is corresponding to the maximum error of 3 mm in the vertical displacement model. The difference of M₂ factor at Ogasawara is nearly twice as large as that of other stations, but it is only 0.2%. A slightly large difference between observation and model at Ogasawara station might be caused by local geology and/or ocean tide model error. Still, the error for the displacement model used in VERA is negligible small enough to affect analyses results.