  This paper presents the storm time comparative analysis of the performances
of latest versions of global ionospheric models: International Reference
Ionosphere (IRI) 2016, NeQuick 2 (NeQ) and the IRI extended to Plasmasphere
(IRI-P) 2017 with respect to Navigation with Indian Constellation (NavIC) and
Global Navigation Satellite System (GNSS) derived ionospheric Total Electron
Content (TEC). The analysis is carried out under varying geomagnetic storm
conditions during September 2017-November 2018, falling in the declining phase
of solar cycle 24. TEC data from Indore, located near the northern crest of the
Equatorial Ionization Anomaly (EIA) along with data obtained from the
International GNSS Service (IGS) stations at Lucknow, located beyond the
anomaly crest; Hyderabad, located between anomaly crest and magnetic equator
and Bangalore, located near the magnetic equator have been analysed. The models
generally overestimated during the storm periods with the exception of IRI-P,
which matched (with an offset of about 3-5 TECU) with the enhancement observed
on September 7, 2017 (during the strong storm of September 2017), from stations
around the anomaly crest. No significant match was observed by the other two
models. This match of IRI-P is attributed to the plasmaspheric contribution as
well as the capability of assimilating measured TEC values into this model. In
the present study, to the best of our knowledge, first comparisons of the
empirical model derived TEC with NavIC and GNSS measurements from an anomaly
crest location, combined with the IGS observations from the magnetic equator to
locations beyond the anomaly crest, are conducted during geomagnetically
disturbed conditions. Since NavIC satellites are at higher altitudes(~ 36000
km), the inclusion of NavIC data to the existing model could give better
ionospheric predictions over the Indian subcontinent.
