A surface buoy with a Global Positioning System (GPS) antenna moored about 20 km from coast to monitor sea level changes using real time kinematic (RTK) GPS technique with a land-based station. The GPS buoy is used as a wave gauge to detect tsunami before its arrival to the coast. In Japan, the system has been in operation since 2008, and in 2012, 15 GPS buoys were in operation by Ports and Harbours Bureau, Ministry of Land, Infrastructure, Transport and Tourism (ML IT). GPS data are transmitted to land, processed in real time by PARI (Port and Airport Research Institute), and then sent to JMA who is responsible for tsunami monitoring and warnings. During the 2011 Tohoku tsunami, the JMA detected the tsunami offshore and upgraded its tsunami warning for Japan.gps_wave_gauge