Simultaneous observations of crustal tilt with collocated two characteristically different types of water-tube tiltmeters have been carried out at the Esashi Earth Tides Station since April 1980, one of which was designed at the International Latitude Observatory of Mizusawa (namely WTT(M)) and the other at Nagoya University (WTT(N)). Major differences between them are in supporting systems of the floats, sensors and the diameters of the water-tubes. Main purpose of this investigation is to verify the reliability of tilt observation with water-tube tiltmeter, establishing a method of precise calibration. In this paper, a method to derive sensitivity coefficient purely experimentally is proposed. This enables us to calibrate the instruments within an error of about 1.5%, which can be reduced to less than 1 % by making more careful measurements of cross-sectional areas of the floats and the vessels at the level of free surface of water. The results of tidal analyses for observations during about 21 months are also given. Both amplitudes and phases of six major tidal constituents (Q₁, O₁, K₁, N₂, M₂ and S₂) obtained by WTT(M) and WTT(N) are found to be very close to each other. This suggests that these watertube tiltmeters yield no systematic differences and serve for reliable tidal observations.

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Measurements by a Geodimeter 6BL have been carried out annually since 1977, in the evening within a few hours around sunset, at Tokatomi, Inuyama, Hamanako, Sangane and Mukaiyama base-line network in the southern part of central Japan . Strain rates along base-lines are given by assuming a linear variation in length. During the period between 1977 and 1982, the maximum shear strain rates observed at these five base-line networks amount to 0.1-0.7?Estrain/y, which are consistent with the triangulation results during the late 60-100 years by Geographical Survey Institute . On the other hand, axes of principal strain observed at these networks show various direction and only a part of them is consistent with the results by Geographical Survey Institute. Accumuration of horizontal strains in the southern part of cental Japan, seems to be irregular in time and space.

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Plate deformations in the Southeast Asian plate are studied with the use of optical observations of time and latitude at Mizusawa, Tokyo and Irkutsk. Japan, mean values of Mizusawa and Tokyo, and Irkutsk show large mutual variations of about 1 ms in longitude. The longitude variations suggest couplings with variations of the Earth's rotation. A viscous coupling model with a rigid mantle and plates may be possible to explain these couplings.

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From the analysis of tidal records and leveling data, it is clarified the principal seismic events in the east coast of the Izu peninsula since 1975 were preceded, by a few months, with anomalous uplifts of the crust. Those uplifts, however, were not detected by continuous measurements of crustal movement of high sensitivity installed in the area. This means that present monitoring system of continuous crustal movement might be insufficient to detect a precursory movemen of the crust of relatively long period, for a month or more. It is needed to develop a reliable monitoring system of crustal movement for such longer period.

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The differences between Precise and Broadcast ephemerides are studied by compar ing directly satellite positions to clarify characteristics of Broadcast ephemeris. It is found that typical magnitudes of errors in the X, Y and Z components are 100, 50 and 25m, respectively. Their period is about 2 to 3 days. There are other errors, periods of which are 23 hours 56 minutes for the X and Y components and 23 hours 58 minutes for the Z component. These errors cause larger bias in determination of longitude on the NWL-10D reference ellipsoid than in that of latitude. One of the reasons for these errors is unbalanced distribution of satellite tracking stations of OPNET. Another reason may be large errors in the broadcast ephemeris because it is extrapolated for over 30 hours using only 12 to 24 hours data and ignores some parameters in its calculation.

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Geoid for the areas of the northern part of North Hemisphere has been evaluated and mapped by means of polar azimuthal equidistant projection using SEASAT altimeter data covering north of 45°N. The relative accuracy of the geoid obtained expressed by 1σ value of the crossover errors varies from 35 to 131 cm, depending on local topographic situations. The geoid presented here has been compared with the geoid published in 1982 by R. H. Rapp of the Ohio State University. This geoid map is useful parti cularly when geodetic and geophysical features of the northernmost regions such as the Okhotsk Sea, the Bering Sea, the northern Atlantic or the Arctic Sea are discussed.

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The neutral atmosphere is one of the most influential noise sources in Very Long Baseline Interferometry. To improve the accuracy of the atmospheric correction, it is important to figure out where exactly the problem lies. For this purpose, a series of numerical experiments was carried out. Main results are as follows. ? The accuracy of the atmospheric correction depends on the accuracy of the available information on water vapor distribution. ? Marini-Murray model has an unnegligible bias in the low elevation angle under hot and humid circumstances. ? Solving for zenith path delay as one of the estimated parameters reduces the residual significantly.

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We have examined optical characteristics of cat's eyes, which are used as falling objects in absolute gravimetry, by ray tracing calculations in order to estimate the errors associated with the interferometric length measurements. Three types of cat's eyes, i.e. those with spherical, spheroidal, and paraboloidal incident surfaces, are taken into ac count in this paper. Ray tracing programs deal with the effects of non-verticality and divergence of incident rays, those of inclination and horizontal movement of the cat's eye during the measurements etc. Calculations show that the cat's eye with a spheroidal incident surface has the best optical characteristics and is capable of absolute gravimetry of 10^-9 accuracy. The cat's eye with a spherical incident surface, on the other hand, is not capable of 10^-9 but fully capable of 10-8 accuracy.

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With the purpose of clarifying the effects of the meteorological environment on astronomical observations at Mizusawa, we investigated relations between the residuals in the PZT time and latitude observations and the meteorological elements obtained at the ILOM and its vicinity. The results suggest that the group-mean values suffer in a systematic way from small influences of scintillation and/or short-period disturbances through internal gravity waves, but even more from the air stream in the atmospheric boundary layer and the diurnal variation of temperature. The geographical features around the ILOM will produce the topographically induced circulation, which may give rise to astronomical refraction through the gradient of the equi-density surface, witha considerable systematic error, especially in the time observation.

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