GPS-technologies have many applications in the field of early warning. All of them have high standards in terms of accuracy and fast availability of GPS measurement results. As a basis of high precision for all GPS-based applications of the Indonesian early warning system, a network of newly developed Real-Time Reference stations was installed in Indonesia. The fast availability of GPS-based measurements is achieved through the automatic, near real-time processing of GPS-data. The GPS-based “Ground Tracking System” is a system for an automatic near real-time determination of co-seismic displacements, an important factor for the estimation of a tsunami threat after strong earthquakes. Furthermore, studies are conducted on new technologies, such as GPS Reflectometry, which could provide a valuable contribution to tsunami early warning in the future.