VADASE: State of the art and new developments of a third way to GNSS seismology

In recent years, extensive work has been done to effectively exploit Global Navigation Satellite Systems (GNSS) for estimating important earthquake parameters such as the seismic moment and magnitude (i.e. GNSS Seismology). The rapid and accurate assessment of these parameters is of crucial importance to achieve reliable tsunami generation scenarios and eventually dispatch an early warning. In this framework, Geodesy and Geomatics division (AGG) of Sapienza University of Rome developed a new approach to obtain in real-time the 3D displacements of a single GNSS receiver. This solution, called VADASE (Variometric Approach for Displacement Analysis Standalone Engine), utilizes the broadcast orbits and the time differences of the high-rate (i.e. 1 Hz or more) carrier phases observations to ascertain the receiver movements over short intervals at a few centimeters accuracy level in real-time. First we summarize the state-of-art of VADASE. Then, we illustrate the most recent developments of the algorithm, which include model refinements, single frequency (L1) capability and functionality with Galileo real data. Finally, we present the first results of an automatic procedure enabled by VADASE real-time capabilities. The epoch-by-epoch displacements (i.e. velocities) of approximately 100 stations of the IGS (International GNSS Service) high-rate (i.e. 1 Hz) network are retrieved every 15 min using VADASE, and the whole network can be characterized in terms of noise level (ranging from 1 to 5 mm/s for the horizontal and from 2 to 10 mm/s for the height); on this basis, corresponding thresholds (i.e. 3-sigma) could be set up in order to highlight significant displacements caused by an earthquake and eventually raise a tsunami alarm.