The GPS buoy system introduced in this study successfully detected the tsunami due to the 2010 Chilean Earthquake. Its amplitude and phase are generally consistent with the predicted tsunami waveform. However, there are some inconsistencies with the simulated data. First of all, the arrival time of the tsunami was about 30 minutes later than the predicted arrival time. This difference was unanimously observed at all GPS buoys that are established along the Japanese coast. Comparisons by other studies (e.g., Satake et al., 2010) show similar results.
The difference of arrival times of about 30 minutes may have to be investigated by considering various factors such as the water depth model, the spatial resolution of gridding, modeling errors, as well as the source location. The prediction of tsunami heights is fairly consistent with observed heights, suggesting that the prediction of inundation height at the coast may be made with considerable precision. Further improvements of the numerical simulation may be necessary for a better prediction of tsunami arrival time and the differential effects of arrival times due to dispersion between long period and short period sea waves.
When a comparison between a numerical simulation and observation is being carried out, one significant advantage of a GPS buoy compared with a coastal monitoring system such as tide gauges is that the GPS buoy is less affected than tide gauges by local geomorphological effects or non-linear effects due to basal friction, etc. Moreover, a GPS buoy can record not only tsunami but also wind waves. Therefore, a GPS buoy can be utilized for daily sea surface monitoring, and not just for tsunami.
Currently, the GPS buoy system uses RTK-GPS which requires a land base for the precise positioning of the buoy. This limits the distance of the buoy from the coast to, at most, 20 km. Establishment of the buoy further from the coast is truly important to achieve a longer lead-time for evacuating nearby coasts. There are two problems to be solved in this regard; one concerns accuracy and the other data transmission. Since tsunami amplitudes decrease as water depths become larger, the detectability requirement of a GPS buoy is more demanding in deeper ocean. If the distance of a GPS buoy from the coast is larger, currently used RTK-GPS may not achieve centimeter accuracy. We are trying to introduce another algorithm for solving this problem. One possibility is the so-called precise point positioning algorithm in which a baseline is not used for estimating the position, but only a single station is used (Geng et al., 2010a, b). We are now testing if such an algorithm can achieve centimeter accuracy in the current GPS buoy system.
Another problem of deployment further from the coast would be data transmission. Currently, we are using radio for data transmission. Since we use a dual radio band, data transmission is very reliable; data have been acquired without loss of lock in rough water, even close-by the passage of a typhoon. However, radio transmission would not be feasible, if the distance of the GPS buoy is far from the coast, say more than 50 km. Satellite data transmission would be more reliable in such a situation. However, such satellite data transmission is still not cost-effective. Future cost reduction of satellite data transmission is truly needed for earlier tsunami detection.