An overview of GNSS for tsunami early warning: From lithosphere to ionosphere

Rapid and accurate earthquake source characterization plays a vital role in tsunami early warning (TEW). However, traditional seismic observations can suffer from clipping or baseline shifts, leading to magnitude saturation and underestimation of tsunami potential. Global Navigation Satellite System (GNSS) measurements provide an alternative: they record co-seismic displacements with centimeter- or even millimeter-level accuracy in real time without saturation. Moreover, GNSS techniques can detect co-seismic ionospheric disturbances (CIDs) induced by tsunami waves, offering an additional observational window to assess tsunami potential. In this review, we summarize the application of GNSS for TEW from the ground deformation (lithospheric) and ionospheric perspectives. We outline the main algorithms for precise co-seismic deformation monitoring and fast earthquake source inversion that feed tsunami forecasts, and we describe the principles and recent achievements of GNSS-based ionospheric sensing for tsunami detection. We also discuss emerging improvements toward more reliable GNSS-based TEW systems, including low Earth orbit (LEO) satellite data for improving coverage, GNSS-interferometric reflectometry for direct tsunami wave sensing, and machine learning approaches that enhance real-time decision-making.