A journey of the improvement in analysis model of the GPS Earth Observation Network System (GEONET) is reviewed along a history of GPS observation technique in the 1990s as a background. Results of the studies were applied to the analysis strategy of GEONET and the routine solutions are obtained with dramatically improved quality; high precision of coordinates, uniform quality and consistency in biases of vertical coordinates . The accomplished quality is the essential contribution to many geodetic/geophysical applications of the GEONET solutions. Insufficiency of calibration and correlation among unknown parameters are major difficulties to understand error factors of GPS. To resolve the complicated situations of problems and to get insight into "error factors", I suggest to analyze two aspects of errors: "error sources" and "compliance/sensitivity of models". Decorrelation of parameters by introduction of independent observation or tuning of analysis model is as essential as elimination/correction of error sources by calibration of observable to reduce errors in solutions. With widening and deepening of geodetic/geophysical applications, requirements for precise GPS observation are still becoming higher for more precision, higher temporal resolution, more consistency with other techniques and long-term stability . Comprehension of error factors should be a base of efforts to meet with those requirements and further exploitation of ability of GPS observation.

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The Satellite Laser Ranging (SLR) observation by Japan Coast Guard (JCG) has been continued since 1982 at the Simosato Hydrographic Observatory in use of a fixed-type SLR system. Simosato is the base station of the Marine Geodetic Control Network connecting the mainland and isolated islands of Japan to a world geodetic system in order to delineate the limits of Japanese jurisdictional area at sea defined in the United Nation Convention of the Law of the Sea. Observation performance at Simosato has been improved, i.e., the single-shot precision of SLR observation has been improved: from 9.5 cm for LAGEOS-1 yearly RMS in 1986 to 1.5 cm in 2005, the total number of passes obtained at Simosato has increased from, e.g., 541 passes of 4 satellites in 1986 to 2, 331 passes of 24 satellites in 2005. The SLR data acquisition at Simosato for twenty-four years has been significantly contributing to establishing the International Terrestrial Reference Frame as well as to various research fields in global geodesy. A transportable SLR station, also developed by JCG, has been used to determine the positions of nine major isolated islands and other six sites at the edge of the mainland of Japan by observing geodetic satellites such as AJISAI launched in August 1986 by Japan. The processing of SLR data have been carried out using orbital processors developed by JCG and NASA. Geocentric coordinates and velocities of the SLR stations as well as geodynamical and earth-rotation parameters have been estimated by using SLR data. SLR observation and data processing by JCG have contributed to the establishment of the world geodetic system in 2002 as the national geodetic coordinate system of Japan .

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Microgravity survey at Ogasawara Iwo-jima has been repeatedly performed by the National Research Institute for Earth Science and Disaster Prevention in every two years since 1998 by using two Scintrex CG-3M gravimeters (No.212 and No.284). Iwo-jima is an active volcanic island belonging to the Izu-Ogasawara islands, and the purpose of this study is to detect the gravity changes associated with the large scale crustal deformation of volcanic origin in the island. Our gravity survey line consists of one return trip from Tsukuba (Bosai BS) to Iwojima through Iruma air base (Iruma BS). As the reference of standard gravity value, we tied Bosai BS to Tsukuba gravity station (Tsukuba GS) of Geographical Survey Institute, Japan, before and after the Iwo-jima measurements. Because of the large gravity difference between Bosai BS and Iwo-jima, about 870 mGal (1mGal=1×10^-5ms^-2), we calibrated scale factors of the gravimeters, which were determined twice by calibration surveys along the line connecting the 1st-order gravity stations of JGSN96, Haneda GS, Sapporo GS, Chitose GS and Naha GS, in 1999 and in 2003. The obtained two coefficients for scale factor correction for each gravimeter exhibit temporal change amounted to about 1×10^-4 . In the present study we assumed temporally linear change of the scale factor for each instrument, and then interpolated or extrapolated the calibrated values to apply to the Iwo-jima survey . We present here the gravity values of the base benchmark (Iwo101) in Iwo-jima estimated for the four surveys in 1998, 2000, 2002 and 2004. The differences between the estimated gravities of Iwo101 by the two in struments range within 0.01 mGal for the 2000 and 2002 surveys, for which the scale factors were interpolated from the calibrated values. Larger differences ranging between 0 .02 and 0.06 mGal appeared in the 1998 and 2004 surveys, for which they were extrapolated, indicating that the errors in the absolute gravity values at Iwo-jima depend strongly on the accuracy of scale factors. The average gravity values of the two instruments at Iwo101 were 979, 081.670 meal (1998), 979, 081.719 mGal (2000), 979, 081.489 mGal (2002) and 979, 081.473 mGal (2004). The large gravity decrease of 0.230 mGal at Iwo101 during the period from December 2000 to December 2002 is well correlated to the large uplift of 0.955 m with the ratio of gravity change to the uplift, -0.241 mGal/m. This ratio corresponds to the intrusion of materials with the density of 1.6×10³ kg/m³ for the case of an infinite flat intrusion, indicating a significant contribution of magma in the 2001 large crustal deformation.

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Taking part in the Asia-Pacific Space Geodynamics (APSG) Project cooperation campaigns in JAG and the work plan in the Permanent Committee on GIS Infrastructure for Asia & the Pacific (PCGIAP), we have collaborated to establish the Absolute Gravity Standard Station Network in East Asia and South-East Asia (AGSSN-ESEA) with FG-5 absolute gravimeters. During the period from 2002 to 2005, we have already determined absolute gravity values in the order of μGal at Wuhan, Nanning, Shanghai, Beijing, Kunming, Lhasa, Hong-Kong Wulumuqi, Xi'an and Xining in China, Hsinchu in Taiwan, Bandung, Yogyakarta Cibinong and Pontianak in Indonesia, Kuala Lumpur and Kota Kinabalu in Malaysia, Bangkok and Cheng-Mai in Thailand, and Manila in Philippines as well as Kyoto, Esashi, Matsushiro, Kamioka, Muroto, Aso, Mizunami and Naha in Japan. As an extension of this campaign in the southern hemisphere, we have conducted absolute gravity measurements in Perth and Canberra in Australia, and the Syowa Station in Antarctica in 2003-2004.

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