2009 Volume 55 Issue 1 Pages 17-38
The Global Positioning System was developed during 1970-80 in the United States. In 1994, The Geographical Survey Institute (GSI) of Japan began partial services of the nationwide GPS network named GEONET (GPS Earth Observation NETwork). Prior to the start of the GEONET service, Japanese geodesists and meteorologists held the first workshop on “GPS Tropospheric Delay” at the National Astronomical Observatory Mizusawa and recognized the importance of GPS meteorology with a suggestion of N. Mannoji from the Japan Meteorological Agency (JMA). After one year feasibility study based on a concept proposed by H. Tsuji of GSI, Japanese GPS meteorology project “GPS/MET JAPAN” was launched in 1997. This five year project from April 1997 to March 2002 was aiming at two birds with one stone; an application of precipitable water vapor (PWV) from the GEONET to data assimilation system in numerical weather predictions (NWP), and an improvement of geodetic accuracy of GPS based on NWP data. At the dawn of the project, accuracy of PWV from GEONET was statistically tested, and GEONET vividly monitored various atmospheric phenomena. All these analyses, however, attested critical necessity of knowing a few km-scale water vapor variations to improve the accuracy of GPS positioning and PWV retrieval. This led us to conduct a dense GPS network campaign with 75 receivers in a 400 km2area. A tomography analysis of slant path delays of GPS signals revealed a 3-D distribution of water vapors. Improvement of geodetic accuracy of GEONET was also tried using NWP data. GPS radio occultation (RO) method applied to GPS data observed by space-borne GPS receivers yielded a global-scale temperature distribution, which shed light on a vertical propagation process of atmospheric gravity waves in the tropics. GPS RO technique was also applied to mountain-based GPS downward-looking observation and succeeded in obtaining fine vertical temperature structure in the lower troposphere. In this paper, we summarize results of the “GPS/MET Japan” project, review subsequent relevant researches, and look at the future. We hope that this paper will encourage new studies aiming for further progresses not only on numerical weather prediction but also on the interdisciplinary sciences for understanding the global changes.
