Tectonic plate movements relative to each other are the main cause behind natural hazards of earthquakes. On the occasion of an earthquake part of seismic energy propagates upward into the overlying atmosphere and can disturb the partially ionized region of the atmosphere i.e. ionosphere. These seismic induced ionospheric perturbations and associated co-seismic crustal deformation are being well studied using the Global Navigation Satellite System (GNSS). This talk will highlight the merits and demerits of GNSS technique while deriving the seismic rupture characteristics based on their GNSS measured ionospheric imprints. Moreover, new insights developed towards estimating the detection altitudes of GNSS measured seismic induced ionospheric perturbations will be discussed.