STUDY OF ASTROMETRY AND GEODYNAMICS BY MEANS OF INTERNATIONAL VLBI OBSERVATIONS

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1. Results obtained by VLBI observations(1) Detection of rapid variations of the earth's rotationThe International Earth Rotation Service (IERS) and the International GPS Geodynamics Service (IGS) jointly organized the international observational campaign to investigate feasibility of GPS in detecting rapid variations in the earth's rotation. The campaign entitled EPOCH92 was conducted from July 27 through August 10 with the participation of the already established space techniques, namely, VLBI and SLR. We participated in EPOCH92 as the network center of the VLBI Pacific network and organized five 24-hour session observations during the 2-week campaign. Correlation processing made at the correlation center at the US Naval Observatory was recently completed and the database has been delivered to the worldwide analysis centers. One of the scientific objectives of the research project supported by the scientific grant-in-aid is to clarify the characteristics of rapid earth rotation by an … More alyzing the VLBI data obtained during the EPOCH92 campaign. The research group has just started analysis of the VLBI database. The results of analysis will be presented at the coming international meetings.(2) Southern extension of the radio reference frameWe conducted VLBI observations using the network involving an Australian station, Tasmania. Due to the participation of the southern station, we could observe radio sources of declinations down to -30ﾟ and thereby contribute to extending the radio reference frame to the southem hemisphere with high accuracy. We also investigated position changes of all the observed radio sources and found that several of them have shown position changes probably due to the structure changes.2. Station displacements due to atmospheric loading effectsIn consideration of the current accuracy in three-dimensional positioning by VLBI is approaching to sub-centimeter level, it is indispensable to model all possible geophysical factors yielding this amount of displacements. In particular, modeling vertical displacement is essential in monitoring the sea level change in the context of global change. Atmospheric loading effect is one of them causing a few mm to a few cm vertical displacements.Based on the global pressure data calculated by the Japan Meteorological Agency, we estimated the vertical Displacements of all the stations taking account of the characteristics of ocean responses. The results parameterized by the sitesurface pressure data have been incorporated in the VLBI analysis software. Less