Precise relative kinematic positioning of moving platforms using GPS carrier phase
observables has numerous applications. One prominent application is utilization of highly
stabilized GPS technology mounted on the buoy, which is specially designed for detecting
tsunami wave at open sea. The essential point of this research is to investigate a potential use of a
GPS tsunami buoy for the purpose of tsunami early warning system with long-baseline kinematic
GPS processing method.
The rule of thumb GPS positioning concept, GPS position results are affected by. baseline
length mostly due to de-correlation of atmospheric errors. As baseline lengths increase, position
results degrade due to the difficulty to correctly fix the cariier phase ambiguity to its integer
value. carrier phase fixed ambiguity solutions are more accurate that float arnbiguify solutions. It
is generally accepted that carrier phase can be successfUlly fixed for baselines of up to 10 km.
After that, fixing ambiguities becomes more difficult and risky. It would be certainty more
advantageous to have a reliable float solution rather than an unreliable fixed solution.
In this study, we have developed a new quasi-real time long-baseline kinematic analysis
method using dual-frequency carrier phase with floated ambiguities, implemented in the Bernese
GPS Software Version 5.0. We demonstrate that early detection of a damaging tsunami can be
achieved by tracking the anomalous changes in sea surface height. The movements of a GPS
buoy relative to a base station with baseline length of 500 km have been monitored in quasi-real
time mode, and the tsunami waves caused by the 5th September 2004 Off Kii Peninsula
earthquake, Japan, have been successful detected as they went by, even though these were only 15 cm high. The filtered record of the solution closely resembles that of short baseline, with
RMS of 3.4 cm over 2.5 hours.
To test the robustness of our Iong-baseline kinematic GPS method under various
meteorological, we conducted the GPS tsunami buoy data analysis continuously for 8 days to
monitor the motion of the buoy. The average scatterings of GPS buoy heights by the low-pass
filtered 1 -Hz positioning result after tidal correction are about 3.4 cm and 1.2 cm under both
typhoon and calm weather conditions. This accuracy is precise enough to be applicable to a
tsunami early warning system. Since our long-baseline kinematic GPS analysis is effective to a
long baseline up to 500 km, we can place a GPS buoy far offshore, which ensures an adequate
evacuation time even, for people living on the coast
