A new tsunami observation system using GPS buoys has been developed, which employs the RTK-GPS technique to detect and monitor tsunamis in real-time before they reach the coast. A series of experimental GPS buoys succeeded in detecting three tsunamis with amplitudes of about 10 cm. Following this success, since 2007, the Japanese government has established GPS buoy systems for monitoring sea waves at 19 sites around the Japanese coast. These systems succeeded to detect 11thMarch 2011 Tohoku-Oki earthquake tsunami. Through these experiences, we recognized two problems that need to be solved in order to deploy buoys at farther distances from the coast: one is that positioning accuracy decreases as the distance increases and the data transmission by radio becomes difficult for a long distance. In order to overcome these difficulties, first, a new algorithm of PPP-AR for realtime application was employed. The test analysis showed that the positioning accuracy may attain a few centimeters even if the reference GPS network that generates precise orbits and clocks is farther than 1,000 km. Then, a satellite communication system was experimentally used to send data in both directions between the land base and buoy. The data that was obtained on the buoy was transmitted to the land base and was shown on a webpage in realtime, successfully. This kind of system may have further applications of earth science; for example, we are trying to implement GPS/acoustic apparatus for continuous monitoring of ocean floor crustal deformations.