In reaction to the devastating tsunami in the Indian Ocean on December 26, 2004, a project team from the faculty of Aerospace Engineering, part of the Delft University of Technology, started to investigate the feasibility of a tsunami global early warning system using reflections of a Global Navigation Satellite System (GNSS). A conceptual design of a demonstrator satellite to prove the principles of the Space-borne Tsunami Warning System (STWS) was made. This chapter provides background information about the characteristics and impact of tsunamis, about a Global Navigation Satellite System (GNSS) in general, and about the use of GNSS-Reflections (GNSS-R) in detecting disasters, as well as the actual design and a cost estimation for the Space-borne Tsunami Warning System.