Comparative studies of Ionospheric models with GNSS and NavIC over the Indian Longitudinal sector during geomagnetic activities
Sumanjit Chakraborty, Abhirup Datta, Sarbani Ray, Deepthi Ayyagari, Ashik Paul
CComparative studies of Ionospheric models with GNSSand NavIC over the Indian Longitudinal sector duringgeomagnetic activities
Sumanjit Chakraborty a, ∗ , Abhirup Datta a,b , Sarbani Ray c , DeepthiAyyagari a , Ashik Paul c a Discipline of Astronomy, Astrophysics and Space Engineering, IIT Indore, Simrol,Indore 453552, Madhya Pradesh, India b Center for Astrophysics and Space Astronomy, Department of Astrophysical andPlanetary Science, University of Colorado, Boulder, CO 80309, USA c Institute of Radio Physics and Electronics,University of Calcutta, Kolkata 700 009,West Bengal, India
Accepted for publication in Advances in Space Research
Abstract
This paper presents the storm time comparative analysis of the perfor-mances of latest versions of global ionospheric models: International Ref-erence Ionosphere (IRI) 2016, NeQuick 2 (NeQ) and the IRI extended toPlasmasphere (IRI-P) 2017 with respect to Navigation with Indian Con-stellation (NavIC) and Global Navigation Satellite System (GNSS) derivedionospheric Total Electron Content (TEC). The analysis is carried out un-der varying geomagnetic storm conditions during September 2017-November2018, falling in the declining phase of solar cycle 24. TEC data from In-dore, located near the northern crest of the Equatorial Ionization Anomaly ∗ Corresponding author.
Email addresses: [email protected] (Sumanjit Chakraborty), [email protected] (Abhirup Datta), [email protected] (Sarbani Ray), [email protected] (Deepthi Ayyagari), [email protected] (AshikPaul)
Preprint submitted to ASR May 12, 2020 a r X i v : . [ phy s i c s . s p ace - ph ] M a y EIA) along with data obtained from the International GNSS Service (IGS)stations at Lucknow, located beyond the anomaly crest; Hyderabad, locatedbetween anomaly crest and magnetic equator and Bangalore, located near themagnetic equator have been analysed. The models generally overestimatedduring the storm periods with the exception of IRI-P, which matched (withan offset of about 3-5 TECU) with the enhancement observed on September7, 2017 (during the strong storm of September 2017), from stations aroundthe anomaly crest. No significant match was observed by the other two mod-els. This match of IRI-P is attributed to the plasmaspheric contribution aswell as the capability of assimilating measured TEC values into this model.In the present study, to the best of our knowledge, first comparisons of theempirical model derived TEC with NavIC and GNSS measurements from ananomaly crest location, combined with the IGS observations from the mag-netic equator to locations beyond the anomaly crest, are conducted duringgeomagnetically disturbed conditions. Since NavIC satellites are at higheraltitudes( ∼ Keywords:
NavIC, GNSS, TEC, Ionospheric Models, Geomagnetic Storms, IndianLongitude Sector 2 . Introduction
Ionosphere, the ionized region of the atmosphere, extends from 60 kmto beyond 1000 km above the surface of the Earth, with the equatorial re-gion, confined within ± ◦ magnetic dip, accounting for nearly one-third ofthe total global ionization. There is renewed interest in the upper atmo-spheric ionized regions primarily because of its deleterious effects on HF,VHF, and UHF radio communications and navigation and, aerospace, andthe Global Navigation Satellite System (GNSS). As long-range radio wavesare significantly affected by the ionosphere, constant observation of the iono-spheric uncertainties is crucial for communication and navigational systemsto work with higher efficiency (Atıcı, 2018). A fundamental parameter tostudy the ionosphere is the ionospheric Total Electron Content (TEC) de-fined by the number of electrons integrated between two points, along atube of unit cross-sectional area, expressed in TECU, where 1 TECU = 10 electrons/m . With the abundance of ionospheric data from the multitudeof GNSS satellites, presently numbering more than 80 and projected to be inexcess of 120 in the near future, coupled with the need for accurate naviga-tion, various ionospheric models have been established which give predictionsof the ionospheric TEC where actual data is absent.The equatorial and low-latitude ionosphere consists of several unique fea-tures such as the Equatorial Ionization Anomaly (EIA), the equatorial elec-trojet and the equatorial plasma irregularities, as a result of the horizontalorientation of the magnetic field at the geomagnetic equator, making thisregion to be the most dynamic and geosensitive (Booker and Wells, 1938;Egedal, 1947; Moffett and Hanson, 1965; Hanson and Moffett, 1966; Wood-3an and La Hoz, 1976; Oyama et al., 1997; Balan et al., 1998, 2018). Theelectron density is expected to be maximum around the equator and decreasetowards the poles as a result of sun’s incident solar radiation over the equator((Appleton, 1946; Balan et al., 1998, 2018) and references therein). However,observations show this density to have peculiar crests around ± ◦ magneticlatitudes and trough around the magnetic equator (Appleton, 1946) and atthe time of peak daytime electron density, a crest-to-trough density ratioof ∼ • -30 nT ≤ Dst < -50 nT signifies a weak storm • -50 nT ≤ Dst < -100 nT signifies moderate storm • -100 nT ≤ Dst < -200 nT signifies strong storm • -200 nT ≤ Dst < -350 nT signifies severe storm • Dst ≤ -350 nT signifies great stormCME induced storms are generally strong and affect the Space Weather;however, even weak-to-moderate CIR induced storms have impacts on theionosphere in a way similar to what a strong CME induced storm might5ave (Buresova et al., 2014). Recently, the effects of the CME and CIR re-lated strong geomagnetic storms on the ionization over Indian low-latitudes,in terms of the neutral dynamics, has been studied by (Chakraborty et al.,2020), thus highlighting the importance of studying the ionosphere duringgeomagnetically disturbed conditions, under low solar activity over the dy-namic Indian longitude sector.Studies on the performance of ionospheric models compared to real mea-sured data observations from various satellite navigation systems have beenperformed by several researchers. (Adebiyi et al., 2016) assessed the perfor-mance of the older versions of the International Reference Ionosphere (IRI)2012 and IRI-extended to Plasmasphere (IRI-P) 2015 over the equatorialand low-latitude regions of Africa and showed that IRI-P performed bet-ter compared to IRI model that underestimated the observed TEC. Theyfurther concluded that height limitation and inaccurate predictions of theelectron densities of the IRI model created discrepancies in the observed andmodel data. Studies on the prediction capabilities of the NeQuick2 (NeQ)web model and IRI-P over the South American sector were performed by(Ezquer et al., 2018), where they infer that model mismatch could be dueto erroneous prediction of plasmaspheric contribution of TEC. (Atıcı, 2018)have compared the Ionolab derived GPS TEC over Istanbul, falling in themid-latitude region, with the IRI 2016 and the IRI-P models, and concludedthat IRI-P derived TECs are closer to the observed TEC compared to IRI.Comparison of International GNSS Service (IGS) VTEC with the IRI 2016was made by (Shi et al., 2019) where they concluded that IRI derived TECis consistent with the general trend of the ionosphere during low solar ac-6ivity but vastly underestimated in low latitudes near EIA under high solaractivity. (Reddybattula and Panda, 2019) analyzed the performance of IRI2016, IGS-GIM, and IRI-P 2017 with IGS GPS based observations duringhigh solar activity period 2012-2015 and concluded that IRI-P presents bet-ter results compared to the other models but requires reliable performanceduring the disturbed periods. (Tariku, 2019) evaluated the performance ofIRI 2016 and IRI-P 2017 over central Asian mid-latitude regions and inferredthat the models performed poorly during days of high solar irradiance. Mostrecently, (Maltseva and Mozhaeva, 2019) have investigated the possibilitiesof TEC usage over the low-latitude region and showed that the closest matchis presented by IRI-P derived values.Given the availability of a regional navigation satellite system like theNavigation with Indian Constellation (NavIC) along with the aid of thelegacy GPS satellites, it becomes essential to look at various aspects ofthe ionosphere of the Indian longitude sector. While several studies haveshown the performance of ionospheric models with respect to real measuredGPS data, geomagnetic storm time model deviations with respect to NavICand GNSS (GPS, GLONASS and GALILEO) taken together over the In-dian longitude sector, has not been reported extensively in the literature.In order to address this problem, the storm time performance of the latestionospheric models: IRI 2016, NeQ and IRI-P 2017 have been evaluated withrespect to NavIC and GNSS measured values. The period of analysis consistsof strong, moderate, and weak storms spanning September 2017-November2018, falling in the declining phase of the 24 th solar cycle. The stationsconsidered for analysis are: Lucknow (26.91 ◦ N, 80.95 ◦ E geographic; mag-7etic dip 39.75 ◦ N, located beyond the anomaly crest), Indore (22.52 ◦ N and75.92 ◦ E geographic; magnetic dip: 32.23 ◦ N, located near the anomaly crest),Hyderabad (17.41 ◦ N, 78.55 ◦ E geographic; magnetic dip 21.69 ◦ N, located inbetween crest and magnetic equator) and Bangalore (13.02 ◦ N, 77.5 ◦ E ge-ographic; magnetic dip 11.78 ◦ N, located near the magnetic equator). Thepaper presents, for the first time, to the best of our knowledge, evaluationof ionospheric model derived TEC, covering a large spatial distribution overthe Indian longitude sector, under variable geomagnetic conditions, duringthe declining phase of solar cycle 24.
2. The Ionospheric models
The IRI 2016 is an empirical model of the ionosphere. The sources ofdata to this model are the incoherent scatter radars and the dense worldwidenetwork of ionosondes along with the Alouette topside sounder’s in-situ in-struments on board satellites. The inputs to this model are the day of theyear, the geographic/geomagnetic latitude, longitude, and altitude. To namea few, the model output provides the electron temperature and density, iontemperature, and composition and the TEC from 60-2000 km altitude range(Rawer and Bilitza, 1989). The NeQ is an upgraded version of the NeQuickmodel. This model uses a modified Di Giovanni and Radicella (DGR) profileformulation (Di Giovanni and Radicella, 1990) that consists of five semi-Epstein layers (Rawer, 1982) with modelled thickness parameters (Radicellaand Zhang, 1995), for describing the ionospheric electron density from 90 kmto the maximum height of the F2 layer. The model topside is represented by asemi-Epstein layer with a height-dependent thickness parameter that is deter-8ined empirically (Hochegger et al., 2000),(Coisson et al., 2006). The inputsto this model are the time, the position (geographic latitudes and longitudes),and either the F10.7 solar radio flux or the daily sunspot number while theoutputs are the electron concentration corresponding to the user input lo-cation and time. Specific routines are present in NeQ to evaluate electrondensity and the corresponding TEC by the method of numerical integration(Nava et al., 2008). The IRI-P 2017 (Gulyaeva, 2011) is the IRI extended tothe plasmasphere, which consists of the most developed plasmaspheric andionospheric model. The model presents a better representation of the iono-sphere due to the input of real values into the model (Gulyaeva, 2003). Themodel additionally has a scale height parameter which determines the struc-ture of the IRI topside electron density profile (Maltseva et al., 2013). Theinputs of this model are the day of year, latitude, longitude with the F10.7solar radio flux, or the daily sunspot number. The outputs of the model arethe maximum height of the F2 layer (hmF2), critical frequency of F2 layer(foF2), ionospheric TEC and, electron density, among others.
3. The NavIC
NavIC, previously known as the Indian Regional Navigation Satellite Sys-tem (IRNSS), is a regional satellite navigation system developed by ISRO.The space segment consists of three Geostationary Earth Orbit (GEO) andthree Geosynchronous Orbit (GSO) satellites. The position of these satel-lites are such that all of them have continuous radio visibility with the Indiancontrol stations. These satellites broadcast signals at 24 MHz bandwidth ofspectrum in the L5 and S band having carrier frequencies 1176.45 MHz and9492.03 MHz, respectively (Mruthyunjaya and Ramasubramanian, 2017).
4. Data and Methodology
A multi-constellation(GPS, GLONASS and GALILEO) and multi-frequency(GPS L1, L2 and L5) GNSS receiver along with a NavIC receiver (providedby the Space Applications Centre (SAC), ISRO) capable of receiving L5 andS1 signals along with GPS L1 signal, are operational in the Discipline ofAstronomy, Astrophysics and Space Engineering of Indian Institute of Tech-nology, Indore (IITI). In order to avoid the sharp latitudinal spatio-temporalgradient in the electron density that exists in and around the anomaly region,the Slant TEC (STEC) obtained at the output of the receivers are convertedto the equivalent Vertical TEC (VTEC) using:
V T EC = ST ECM F (1)where M F is the mapping or obliquity factor ((Klobuchar, 1996; Breed et al.,1997; Jakowski et al., 2011) and references therein) given by: M F = (cid:34) − (cid:16) R e cosER e + h (cid:17) (cid:35) − / (2)where R e is the radius (6371 km) of the Earth, h the altitude of the iono-sphere that is considered as a thin shell at 350 km and E the satellite’selevation angle. VTEC data have also been analysed and obtained fromthe IGS stations at Lucknow (26.91 ◦ N, 80.95 ◦ E geographic; magnetic dip39.75 ◦ N), Hyderabad (17.41 ◦ N, 78.55 ◦ E geographic; magnetic dip 21.69 ◦ N)and Bangalore (13.02 ◦ N, 77.5 ◦ E geographic; magnetic dip 11.78 ◦ N), avail-able at the Scripps Orbit and Permanent Array Center (SOPAC) website10http://sopac-csrc.ucsd.edu/index .php/sopac). The elevation cut-off chosenfor analyzing the VTEC, for the GNSS constellation, is 20 ◦ to avoid the effectof multipath on the receiving signals. The geographic location of Indore andthe three IGS stations (Lucknow, Hyderabad and Bangalore) spanning overthe Indian subcontinent is shown in Figure 1.The hourly Dst (nT) and the K p p and AE indices.11 igure 1: The map of India depicting the geographic locations of the NavIC and GNSSreceivers over Indore(IDR) and IGS-GPS receivers over Lucknow(LCK), Hyderabad(HYD)and Bangalore(BLR). The locations of the magnetic equator and northern crest of EIAare also indicated in the map. able 1: Minimum Dst values with the corresponding time and type of storms in additionto the daily SSN, F10.7 solar radio flux, the K p and AE indices. The dashed lines indicateunavailable data during that period. Date SSN F10.7(s.f.u) K p Dst(nT) AE(nT) UT(hh:mm) Type SourceSep 08, 2017 88 118.5 7- -124 791 02:00 strong CMESep 28, 2017 42 91.2 5 -55 789 07:00 moderate CIROct 14, 2017 11 68.6 5 -57 741 06:00 moderate CIRDec 04, 2017 00 66.4 4 -45 292 22:00 weak CIRJan 19, 2018 12 68.5 4 -30 220 09:00 weak CIRFeb 23, 2018 00 68.2 4 -31 500 12:00 weak CIRMay 06, 2018 15 68.4 5 -56 — 02:00 moderate CIROct 07, 2018 00 69.4 5 -53 — 22:00 moderate CIRNov 05, 2018 00 67.2 5 -53 — 06:00 moderate CIR . Results and Discussions In this section, the performance of the three models (IRI-P, NeQ and IRI)have been evaluated with respect to real measured data (NavIC and GNSS)observed from the stations: Lucknow, Indore, Hyderabad and Bangalorethat ranges from beyond the EIA to near the magnetic equator over theIndian longitude sector. The strong, moderate and weak storms during theperiod of September 2017-November 2018, falling in the declining phase ofsolar cycle 24, have been analyzed. As sample cases, storms of September 8,2017, January 19, 2018 and November 5, 2018 are discussed in the followingsubsections.
As a result of the arrival of a CME on September 6, 2017, a G4 level(K p igure 2: Dst, AE and interplanetary parameters for September 6-10, 2017. September 8(48-72 UT(h) in the plot) signifies the day of Dst minimum. to highly perturbed electric fields at high latitudes. This higher magnitudeof the second peak of AE is in accordance with the Dst which also showed asecond dip at 18:00 UT. Figure 2c and 2d show the IMF, Bz and IEF, Ey,having variations in the opposite direction, with a minimum value of -31.21nT and a maximum value of 21.68 mV/m respectively at 23:32 UT.Diurnal variations of VTEC obtained from IGS GPS observables from thestations Lucknow, Hyderabad, Bangalore along with NavIC, GPS, GLONASS15nd GALILEO observables from Indore at a resolution of 1 minute is shownin Figure 3. Surprisingly, a higher value of VTEC is observed on Septem-ber 7 while a lower value is observed on September 8, i.e the day when Dstdropped to a minimum, by IGS GPS at Lucknow and the GNSS observablesfrom Indore. This signifies a decrease in TEC on September 8 over Luc-know and Indore which are beyond and near the anomaly crest respectively,whereas no significant changes in TEC due to the storm is observed over thestations Hyderabad and Bangalore located closer to the magnetic equator.This shows that during storm time conditions, ionosphere in and around theanomaly crest become perturbed in comparison to that near the magneticequator or the anomaly trough as because the equatorial fountain intensifiesduring geomagnetic storms resulting in strengthening the EIA crest.Figure 4 shows the comparative analyses of IRI, NeQ and IRI-P mod-els with observations along with a one sigma error-bar from NavIC, GPS,GLONASS and GALILEO over Indore, depicting a multi constellation pic-ture of the ionosphere during this period. In this figure, the available GEOand GSO satellites of NavIC, namely PRNs 2-7, are depicted in panels A-Frespectively. The GPS, GLONASS and GALILEO satellites which are withinthe 2 ◦ × ◦ IPP reception zone of the NavIC satellites, are taken for analysis.It can be observed from Figure 4A that the model derived TEC are overes-timating the measured values as well as they are unable to capture the truevariation during disturbed conditions. From Figure 4B a shift in the diurnalvalue is observed by NavIC (panel a) which is not captured by the models,however there is a close match ( ∼ igure 3: Diurnal variation of VTEC during September 06-10, 2017 observed from adistributed chain of four stations in the Indian longitude sector. ∼ igure 4: Diurnal variations of model derived VTEC from IRI-P(green),NeQ(red) and IRI(blue) are compared with PRNs 2-7(panels A-F respectively) ofNavIC(subplots:(a)) and all PRNs of GPS(subplots:(b)), GLONASS(subplots:(c)) andGALILEO(subplots:(d)), over Indore during September 5-9, 2017. One sigma error-barof the measured values(black) are also shown for the period. igure 5: Diurnal variations of VTEC from observed values of GPS are compared withmodel derived values of IRI(blue), NeQ(red) and IRI-P(green) over (a) Lucknow, (b)Indore, (c) Hyderabad and (d) Bangalore during the period of September 6-10, 2017. .2. Weak storm of January 19, 2018 A CIR that originated from a positive polarity coronal hole, accompaniedby HSSWS, hit the Earth’s magnetic field on January 14, 2018. This eventsparked a G1 level (K p =5, minor) geomagnetic storm as reported by NOAA.Figure 6, in a way similar to Figure 2, shows the Dst and interplanetaryparameters’ variation over the selected period. Figure 6a shows the Dstreaching a minimum with a value of -30 nT at 09:00 UT on January 19thus signifying the storm to be weak in nature. Figure 6b shows the AEvalues which was 499 nT at 10:38 UT on January 19 around the time of Dstminimum. AE had a second peak with a higher value of 739 nT at 13:44UT and a third peak having a value of 621 nT at 20:01 UT on January 21.Figures 6c and 6d show the IMF, Bz and IEF, Ey with a minimum value of-6.87 nT and a maximum value of 2.94 mV/m respectively at 11:54 UT onJanuary 21.Figure 7 depicts diurnal VTEC variations obtained from IGS-GPS ob-servables over Lucknow, Hyderabad, Bangalore and NavIC along with GPS,GLONASS and GALILEO observables over Indore. No significant TEC en-hancements are observed on the day of Dst minimum, i.e. January 19. Figure8 shows the comparison of the three models with real time observations fromNavIC, GPS, GLONASS and GALILEO VTEC over Indore in a mannersimilar to that explained in Figure 4. It is clearly observed from Figures8A-F that the models are insensitive to weak storm conditions and hencelargely overestimate the real value measured during this period. The closestmatch is found by PRN 7 of NavIC of 8F. This points out to the fact thatthe models are unable to capture quieter ionospheric variations around the22 igure 6: Dst, AE and interplanetary parameters for January 17-21, 2018. January 19(48-72 UT(h) in the plot) signifies the day of Dst minimum. igure 7: Diurnal variation of VTEC during January 17-21, 2018 observed from a dis-tributed chain of four stations in the Indian longitude sector. igure 8: Diurnal variations of model derived VTEC from IRI-P(green),NeQ(red) and IRI(blue) are compared with PRNs 2-7(panels A-F respectively) ofNavIC(subplots:(a)) and all PRNs of GPS(subplots:(b)), GLONASS(subplots:(c)) andGALILEO(subplots:(d)), over Indore during January 17-21, 2018. One sigma error-bar ofthe measured values(black) are also shown for the period. igure 9: Diurnal variations of VTEC from observed values of GPS are compared withmodel derived values of IRI(blue), NeQ(red) and IRI-P(green) over (a) Lucknow, (b)Indore, (c) Hyderabad and (d) Bangalore during the period of January 17-21, 2018. .3. Moderate storm of November 5, 2018 Due to a high speed stream of solar wind that made contact with theEarth’s magnetic field, a G2 level (K p =6, moderate) geomagnetic storm con-ditions were observed on November 5, 2018. The variation in the Dst and theinterplanetary parameters during this period are plotted in Figure 10. It is tobe noted that the AE data were unavailable for this particular period. Figure10a shows the development of a moderate storm as the minimum value ofDst reached to -53 nT at 06:00 UT on November 5. Figure 10b shows theIMF, Bz with a minimum value of -11.07 nT at 20:35 UT on November 4while Figure 10c shows the IEF, Ey with a maximum value of 05.45 mV/mat 04:15 UT on November 5.In Figure 11a, due to unavailability of IGS data from Lucknow during thisperiod, it is shown as blank. No significant TEC enhancements are observedduring this period from all the satellite constellations over the Indore. Fig-ure 12 depicts model comparison with real time data analysed from differentconstellations over Indore in a manner similar to as stated in the previoustwo storms. Close match with an offset of about 5-6 TECU are observed byNavIC PRNs 3, 6 and 7(panels B, E and F respectively) and GPS valueswith the NeQ model. While the other PRNs’ values are not at all replicatedby these models. Thus the model predictions fail to emulate the storm timevariations over this period. Figure 13 compares GPS TEC over Indore andIGS TEC over Hyderabad and Bangalore with the three models. Since therewere no available data for Lucknow during the period, only the model valuesare plotted in Figure 13a. From the other panels, it can be observed thatall the models are overestimating the measured values except over Banga-28 igure 10: Dst, AE and interplanetary parameters for November 3-7, 2018. November9(48-72 UT(h) in the plot) signifies the day of Dst minimum. igure 11: Diurnal variation of VTEC during November 3-7, 2018 over the Indian sector. igure 12: Diurnal variations of model derived VTEC from IRI-P(green),NeQ(red) and IRI(blue) are compared with PRNs 2-7(panels A-F respectively) ofNavIC(subplots:(a)) and all PRNs of GPS(subplots:(b)), GLONASS(subplots:(c)) andGALILEO(subplots:(d)), over Indore during November 3-7, 2018. One sigma error-barof the measured values(black) are also shown for the period . 31ore on the storm day where almost close correspondence is shown by NeQ.Similar observations are seen from Indore on November 4-6. In all the casessignificant overestimation is observed for IRI-P and IRI. It is expected thatif the models are reliable enough around the dynamic Indian subcontinent,deviations will generally be minimum when the storms are less severe butthe present study show that the models are yet to predict with precisionin the low-latitude region. Therefore, due to the lack of consistency in theprediction of the variable storm time model deviations, additional data fromNavIC is necessary for incorporation in the IRI-P, such that there is an im-provement in the model derived predictions during geomagnetic storm time,especially over the dynamic Indian longitude sector.32 igure 13: Diurnal variations of VTEC from observed values of GPS are compared withmodel derived values of IRI(blue), NeQ(red) and IRI-P(green) over (a) Lucknow, (b)Indore, (c) Hyderabad and (d) Bangalore during the period of November 3-7, 2018. . Conclusions There has been renewed interest in the ionosphere mainly attributed todegradation in satellite navigation performance. Ionospheric variations andcomplexities, along with its effects on high frequency communications, hasbeen an important field of study for decades. In order to eliminate theseeffects on the operation of satellite-based navigational and positioning sys-tems (GNSS and NavIC), the existence of global ionospheric models thatwould provide reliable specifications of the ionospheric parameters, espe-cially during geomagnetic storm time conditions, is essential. In this presentstudy, for the first time, to the best of our knowledge, the performances ofthe empirical models: IRI-P, NeQ, and IRI predicted TEC were comparedwith the NavIC and GNSS measured TEC over Indore, highlighting on themulti-constellation study over a single location near the anomaly crest. Si-multaneous study of the IGS stations at Lucknow, Hyderabad, and Banga-lore highlighted a single constellation study in terms of multiple locations,thereby maintaining a careful spatial distribution over the Indian longitudesector. The comparative analysis was performed under strong, moderate, andweak geomagnetic storm conditions spanning the period September 2017-November 2018 in the declining phase of solar cycle 24. Some correspon-dences, as well as inconsistencies, were observed between the measured andthe model derived TEC values. During the strong storm of September 8,2017, IRI-P showed the best performance in terms of matching with NavICand GPS TEC, with an offset of about 3-5 TECU, and being able to observethe enhancement on September 7, 2017. Poor predictions were observed fromall the models during the weak storm of January 2018 thereby stressing on34he inaccuracy of these models during the weak storm time conditions. Themismatch between the observed and model-derived values is attributed to themodels’ inherent height limitation and the difference in the topside profileestimation. The topside estimation in these models does not incorporate theeffect of EIA, which leads to a poor prediction over such a dynamic region.NavIC derived values account for TEC up to geostationary, and plasmapausealtitude, whereas GNSS measured values account up to ∼ ∼ cknowledgments SC acknowledges Space Applications Centre, ISRO for providing researchfellowship under the project number: NGP-17 of the GAGAN/NavIC Uti-lization Program. DA acknowledges Department of Science and Technology,India for providing INSPIRE Fellowship. The authors also acknowledge Prof.Gopi Krishna Seemala of the Indian Institute of Geomagnetism, Navi Mum-bai, India for providing the software to analyze the IGS data (available athttp://sopac.ucsd.edu/dataBrowser.shtml). 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