Concepts for Spaceborne and Groundbased Radar Systems for Tsunami Detection

Kurzfassung

After the 2004 Boxing Day tsunami, the Bundesministerium für Bildung und Forschung (BMBF) sponsored the German Indonesian Tsunami Early Warning System (GITEWS) project, aiming at implementing an operational and effective life-saving system for Indonesia, India and possibly all countries surrounding the Indian Ocean. The German Aerospace Center (DLR) is committed to the development of novel concepts relying on ground-based and spaceborne radar systems. As far as ground-based tsunami detection is concerned, the interaction of HF radiation with ocean waves is of particular interest. According to the Sommerfeld identity, the radiation of a vertically aligned dipole can be described as the superposition of elementary harmonic plane waves, propagating along the surface, with exponential attenuation away from the boundary. The main contribution to the scattered field is given by resonant specular reflection (Bragg scattering). The related Doppler spectrum is given by first order scattering where two dominant Bragg peaks are on hand. If an additional coherent displacement like an ocean current is present, the two Bragg peaks are shifted in frequency, and the ocean current field can be derived. An incoming tsunami determines a relevant departure from standard current patterns. By continuously monitoring the current field, (correlation time approximately 15 min) a potentially dangerous tsunami could be detected when the first wavefront is still sufficiently far from the shore. Coastal HF radars already exist and are operationally used for surface current monitoring (e.g. WERA). Spaceborne radars are a cost-efficient solution for wide area surveillance and a robust monitoring method independent of sunlight and weather conditions. Hence, they are envisioned as a candidate to provide continuous observations of large areas as required by any early warning system. Such a radar should be designed to have a resolution high enough to resolve typical tsunami wavelengths and the received signal could undergo a Doppler processing to retrieve the line of sight displacement of the ocean surface. Achieving the necessary Doppler and angular resolution are challenging tasks and beam forming techniques as well as subspace based methods are envisioned as array signal processing procedures. In this study the basic geophysical Tsunami parameters which are accessible with remotes sensing systems are summarised and a couple of radar system concepts with potential for Tsunami monitoring are presented.