Addressing the Disaster Management National Priority Area, we will exploit the increasingly available global and regional real-time GPS data from NASA's operational Global Differential GPS (GDGPS) System to enable more accurate and timely assessment of the magnitude and mechanism of large earthquakes, as well as the magnitude and direction of resulting tsunamis. The additional GPS-based information will be used to enhance the USGS operational system for post-earthquake damage assessment and emergency response, and, improve tsunami warnings by NOAA's Pacific Tsunami Warning Center (PTWC). Analysis herein shows that had this system been in place in 2004, it would have enabled alerts for the Sumatra tsunami within 15 minutes or less of its genesis, enough to save many lives. This system is also extremely cost-effective to implement operationally as indicated by the proposed ~$1M prototype. Both USGS and PTWC have expressed strong interest in this project and will support the development, benchmarking, and transition to operations of the enhanced system, investing their own resources. Significant national and global societal benefits will follow from enhancing the existing disaster alert and management systems. The multi-national benefits from improved tsunami warning systems in the Pacific and elsewhere using GPS data satisfy the stated priority of this NRA to ""enhance decision support systems for disaster management applications in an international context"", as well as U.S. and NASA's commitments in support of GEO and USGEO. All the underlying earth science models and GPS data types have been validated with actual data from past events. Using a rapid prototyping approach we will build a prototype GREAT Alert System within the first year, enhancing the current USGS and PTWC damage assessment and alert systems. We will refine it and perform quantitative benchmarking during the second year with input from our partner agencies, and pursue transition to operations during the third year.