  Radio interferometers, which are designed to observe astrophysical objects in
the universe, can also be used to study the Earth's ionosphere. Radio
interferometers like the Giant Metrewave Radio Telescope (GMRT) detect
variations in ionospheric total electron content (TEC) on a much wider spatial
scale at a relatively higher sensitivity than traditional ionospheric probes
like the Global Navigation Satellite System (GNSS). The hybrid configuration of
the GMRT (compact core and extended arms) and its geographical location make
this interferometer an excellent candidate to explore the sensitive regions
between the northern crest of the Equatorial Ionization Anomaly (EIA) and the
magnetic equator. For this work, a bright radio source, 3C68.2, is observed
from post-midnight to post-sunrise ($\sim$\,9 hours) to study the ionospheric
activities at solar-minima. This study presents data reduction and processing
techniques to measure differential TEC ($\delta\rm{TEC}$) between the set of
antennas with an accuracy of $1\times10^{-3}$ TECU. Furthermore, using these
$\delta\rm{TEC}$ measurements, we have demonstrated techniques to compute the
TEC gradient over the full array and micro-scale variation in two-dimensional
TEC gradient surface. These variations are well equipped to probe ionospheric
plasma, especially during the night-time. Our study, for the first time,
reports the capability of the GMRT to detect ionospheric activities. Our result
validates, compared to previous studies with VLA, LOFAR and MWA, the ionosphere
over the GMRT is more active, which is expected due to its location near the
magnetic equator.
