The thesis consists of nine chapters. In Chapter 1 (Introduction), first, the importance of gravity observation made at high latitude is described. The remaining part of Chapter 1 explains the outline of contents of this thesis. Chapter 2 reviews the tidal phenomena and the definition of several quantities appeared in the thesis. Chapter 3 describes the method to estimate the ocean effects and the computer program used in this thesis. The characteristics of SG is described in the first part of Chapter 4 in connection with the observation results shown in the later Chapters. The locality of Syowa Station, the procedures for setting up of the SG and the data acquisition system used in the observation are also introduced in Chapter 4. We used the computer program called BAYTAP-G and -L for the tidal analysis. The method and some problems on the actual analysis used this analysis method are mentioned in Chapter 5. The observed results are discussed in Chapters 6, 7 and 8 for the short-period tides, the long-period tides and the polar motion effect especially focusing into the annual component, respectively. Finally, the concluding remarks are given in Chapter 9.

In Chapter 6, we reexamined the gravity tidal factor (δ-factor) of the diurnal and semi-diurnal tides at Syowa Station. The 2-year SG data obtained in the period from March 1993 to March 1995 were used in the analysis. The ocean tide effects (the effects of the attraction and loading due to the ocean mass) were estimated using a new global ocean tide model by Matsumoto et al. (1995). As the δ-factors corrected for the ocean tide effects, we obtained the values of 1.144, 1.127, 1.157 and 1.111 for O

In Chapter 7, we examined the long-period tides (M

In Chapter 8, the results for the polar motion effect are described. First, the previous analysis results and the problems on the analysis for this effect, which were obtained from the analysis by Sato et al. (1997) using the two years Syowa SG data, are summarized. They discussed the two problems, i.e. on an interference problem between the annual and Chandler components and on an effect of the inaccurate estimation of step-like changes including the observed data. Based on their experiences, a revised analysis model is applied here and the analysis is carried out using the Syowa SG data much longer than those used in the previous analysis. Thus, it is revised so that (1) the annual component of the polar motion data was excluded from the IERS EOP (International Earth Rotation Service Earth Orientation Parameter) data before fitting and (2) the term to estimate the step-like changes using the Heviside's function was added to the previous model. It was shown that, by using the revised model, the analysis error for the annual component is improved by about 15% in the case of Esashi, for example. The reliability of the analysis results is also affected by the stability of the period of Chandler component in time. We, therefore, examined this using the 22 years IERS EOP data, and we recognized that the period of Chandler component is stable within ±1.3 days during the observation period of the 17 years from 1983 to 1999. We obtained a value of 435.4 as the mean Chandler period averaged over the 17 years. This value was used for the fitting though this study.

In Chapter 8, we discussed also the results for the estimation of annual gravity changes by calculating the four effects of the solid tide, ocean tide, polar motion and SSH variations, In order to pull out the effect of mass changes in the SSH variations, it is needed to estimate the thermal steric changes in SSH variations, and to correct its effect. We evaluated the steric coefficient based on mainly the POCM (Parallel Ocean Climate Model, Stammer, 1996) SSH (Sea Surface Height) data and the SST (Sea Surface Temperature) data, and we obtained a value of 0.60×10