The geomagnetic storms are turbulence in geomagnetic field when interplanetary magnetic field driven by solar wind move southward and continue for extended period of time. Although these occur less frequently, but may energize ionospheric electrons and particles adversely affecting ground- and space-based electronic systems. Ionosphere at higher latitude is more prone to geomagnetic storms. Over lower latitude region like Indian sub-continent, the effect is less prominent but still can exhibit many distinctive effects like scintillations, equatorial ionization anomaly, fountain effect and equatorial electrojets. The increased numbers of free electrons in ionosphere introduce delays in global positioning system (GPS) satellite signals resulting in errors during GPS positioning. In a dual frequency GPS receiver, the line integral of free electron density along the pathway of signal through the ionosphere (i.e., Total Electron Content, TEC) can be measured. In this present paper, GPS observation data of three low latitude GPS stations in India located at Bangalore, Hyderabad and Mumbai during four severe geomagnetic storms from 2003-2005, are processed to measure ionospheric TEC during the events. The measured TEC at each of the station is compared with quietest days of the months to investigate its abnormal changes in responses to severe geomagnetic storms. The consequences of TEC variation is analyzed and correlated with interplanetary magnetic field (IMF-Bz), geomagnetic Kp and Dst-indices to study its behavioral changes during the storms. Eventually the aim of the study is to estimate the influence of ionospheric condition on GPS positioning to devise suitable method for accurate position measurements in the low latitude Indian region.
