GNSS Positioning of Ocean Buoys in Japan for Disaster Prevention

Japan is an island nationhit by frequent earthquakes,tsunamis, and severe storms. Several government agencieshave thus deployed an array of ocean buoysin the seas surrounding Japan. We have succeeded to monitor tsunamisinduced by several earthquakes withGPS buoysdeployed within 20 km from thecoast using RTK. The short distance limitation of RTK only gives us a very short warning period for residents near the shore. Because tsunami wave heights are smaller in the deeper ocean, the accuracy requirements for GPS tsunami buoys is about 5 cm rms in the open ocean. Therefore we developed a real-time PPP-ambiguity resolution (PPP AR) correction system for tsunami height monitoring in Japan. Because we need to avoid coordinate errors in the reference network due to earthquake induced deformation it is desirable to base the PPP AR corrections on small networks at great distances from the buoys. For testing, a small network in northern Japan was used as server network. We use both buoy and static stations to validate the accuracy of PPP-AR positioning, where 13 km RTK solutions and official coordinate solution for static station are considered as reference for the PPP-AR solution, respectively. The PPP-AR demonstration showed that 1-2 cm horizontal and 2-4 cm vertical kinematic PPP-AR positioning is feasible up to 1000 km from the reference network. The fact suggests that a reliable tsunami height monitoring system is feasible with a setup of several small networks in the Japanese Islands for PPP-AR servers and by establishing the infrastructure to communicate with the GPS-buoy receivers.