The Global Positioning System/Meteorology Project (GPS/MET), a 5-year project begun in April 1997, will closely monitor atmospheric water vapor over Japan. Using this information, scientists can improve forecasting of catastrophic weather in Japan and also correct transient drifts of a few centimeters per week that occur in estimates of crustal deformation derived from GPS data. Though satellite-based GPS has become popular for monitoring crustal deformation with mm to cm accuracy, transient drifts, which change seasonally, are probably caused by changing amounts of water vapor in the atmosphere that delay GPS microwave signal propagation. Determining the amount of water vapor will enable scientists to improve the orecasting of catastrophic weather events and also correct GPS data. For example, water vapor data with high space-time resolution is urgently needed to predict local torrential rain events that cause serious damage in Japan during the rainy season.