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Sunday, 14 September 2008

Using GPS in Tsunami Alarm Systems


The hazards that originate from tsunamis have been recognised for some time. Newspaper reports about undersea earthquakes and movies about meteor-inflicted tsunamis have contributed to public awareness of the threat. Early warning systems were constructed and deployed for instance in the Pacific Rim but many areas in the world are not covered by such traditional warning systems. At the latest in December 2004, when a tsunami devastated wide areas bordering to the Indian Ocean, the extensive media coverage has elevated the sheer possibility, the effects and the dangers of a tsunami into global public consciousness. In the memory and perception of tourists and holidaymakers seashore sites may forever bear tsunami-related dangers, resulting in the desire for effective, reliable and easy-to-use tsunami alarm systems. A tsunami early warning system is a system that should be able to detect tsunamis and issue warnings to prevent loss of life and property. It consists of two equally important components: a network of sensors to detect tsunamis and a communications infrastructure to issue timely alarms to permit evacuation of coastal areas. The main beneficiaries of the tsunami alarm system are of course the people who live and travel near the seaside.



There are two distinct types: international tsunami warning systems, and regional warning systems. Both depend on the fact that, while tsunamis travel at between 500 and 1000 km/h (around 0.14 and 0.28 km/s) in open water, earthquakes can be detected almost at once as seismic waves travel with a typical speed of 4 km/s (approximately 15000 km/h). This gives time for a possible tsunami forecast to be made and warnings to be issued to threatened areas. The main problem to overcome is finding a model that is able to predict with a high reliability and certainty which earthquakes will produce significant tsunamis. Otherwise this Earthquake based approach will produce many more false alarms than verified warnings. In the currect operational scenarios, the seismic alerts are used to send out the watches and warnings. Then, data from observed sea level height (either shore based via tide gauges or deep ocean DART buoys) are used to verify the existence of a tsunami. The first rudimentary system to alert communities of an impending tsunami was attempted in Hawaii in the 1920s and more advanced systems were developed in the wake of the April 1, 1946 and May 23, 1960 tsunamis which caused massive devastation in Hilo, Hawaii.

Recently new modern systems have been proposed to augment the existing warning systems. In these new systems GPS, or more generally GNSS, plays a central role. To detect the tsunami wave front and wave height offshore instruments are essential. Offshore instruments are determining the sea level height and the deviation from normality (anomalies) with high accuracy. For the new modern systems high-tech buoys were developed to make several different observations like air pressure, water temperature, its own movement based on movement sensors, and tracking the GPS signals. Furthermore, these high-tech buoys are used as a relay station for ocean bottom pressure sensors, which are anchored on the ocean floor. Thus these high-tech buoys are equipped with meteorological sensors, movement sensors, a computer for data processing, a power supply unit as well as with GPS and a satellite antenna for the communication between the buoys and a warning centre. The integration of these different sensors ensures the security of the system.

The role of GPS and GNSS.
The key role in these systems is played by GPS.The GPS observations allow to determine the change of the sea level height at the few centimeter level provided accurate GPS orbits and clock are available in (near-) real time. Thanks to the developments made in GNSS data processing, especially in the framework of the International GNSS Service (http://igs.org), the computation of accurate GPS orbits and clocks is a reality today. Thus a relatively cheap infrastructure of high-tech high-sea buoys can provide a very accurate and efficient tsunami early warning system.

Here it is worthwhile to point you to the web site of the so-called "GITEWS" (http://www.gitews.org) project which plans the deployment of 10 buoys along the Indonesian coast line to demonstrate the usability of GPS for tsunami alarm systems.

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