The Global Positioning System is the satellite based positioning system over the globe. The system is primarily designed to provide precise position of ground sites. The interferometric use of the GPS system can attain centimeter accuracy, so that it is now widely used for geodynamics researches such as plate motion monitoring and crustal deformations. During the course of data reduction for coordinate estimation, we have to segregate the effect of propagation media such as ionosphere and troposphere. Total electron content along the slant path and the tropospheric path delay in the zenith direction can be obtained. The tropospheric path delay can then be related to the total content of the water vapor content over the site.

Under these circumstances, permanent array of GPS observations have been increasingly established at many areas in the world. In Thailand, we have established a nationwide GPS network under the meteorological-hydrological project called GAME-T (GEWEX Asia Monsoon

Experiment - Tropics). We have conducted GPS observations at six sites in Thailand since March 1998. The project is to clarify water and energy circulation system in the Asia Monsoon area. Thus monitoring of water vapor content over the Thailand based on GPS observations provides invaluable data for the project.

The data taken at this network is serving invaluable data for meteorology. However, it is also providing invaluable data for monitoring crustal deformation in this country. Thailand is located in the tectonic area called as Sunda block, which is presumably stable craton that moves toward east with about three centimeters a year. The area is also under the effect of the subducting Indian plate from southwest and the southeastward extrusion of the Eurasian continent due to the collision of India toward Himalaya. However, detailed deformation field in this area is yet unknown. It is thus truly necessary to keep maintaining the present permanent array for a long time for geophysical studies in Thailand.