GPS TEC Research Articles

Comparison of GPS derived TEC with the TEC predicted by IRI 2012 model in the southern Equatorial Ionization Anomaly crest within the Eastern Africa region

We have compared the TEC obtained from the IRI-2012 model with the GPS derived TEC data recorded within southern crest of the EIA in the Eastern Africa region using the monthly means of the 5 international quiet days for equinoxes and solstices months for the period of 2012 – 2013. GPS-derived TEC data have been obtained from the Africa array and IGS network of ground based dual-frequency GPS receivers from four stations (Kigali (1.95°S, 30.09°E; Geom. Lat. 11.63°S), Malindi (2.99°S, 40.19°E; Geom. Lat. 12.42°S), Mbarara (0.60°S, 30.74°E; Geom. Lat. 10.22°S) and Nairobi (1.22°S, 36.89°E; Geom. Lat. 10.69°S)) located within the EIA crest in this region. All the three options for topside Ne of IRI-2012 model and ABT-2009 for bottomside thickness have been used to compute the IRI TEC. Also URSI coefficients were considered in this study. These results are compared with the TEC estimated from GPS measurements. Correlation Coefficients between the two sets of data, the Root-Mean Square Errors (RMSE) of the IRI-TEC from the GPS-TEC, and the percentage RMSE of the IRI-TEC from the GPS-TEC have been computed. Our general results show that IRI-2012 model with all three options overestimates the GPS-TEC for all seasons and at all stations, and IRI-2001 overestimates GPS-TEC more compared with other options. IRI-Neq and IRI-01-corr are closely matching in most of the time. The observation also shows that, GPS TEC are underestimated by TEC from IRI model during noon hours, especially during equinoctial months. Further, GPS-TEC values and IRI-TEC values using all the three topside Ne options show very good correlation (above 0.8). On the other hand, the TEC using IRI-Neq and IRI-01- corr had smaller deviations from the GPS-TEC compared to the IRI-2001.

PERBANDINGAN KARAKTERISTIK AKTIVITAS SINTILASI IONOSFER DI ATAS MANADO, PONTIANAK DAN BANDUNG DARI DATA PENERIMA GPS (CHARACTERISTICS COMPARISON OF IONOSPHERIC SCINTILLATION ACTIVITIES OVER MANADO, PONTIANAK AND BANDUNG BASED ON GPS RECEIVER DATA)

Ionospheric scintillation activity on certain region need to be known its characteristics since its occurrence can degrade satellite signal quality of global satellite navigation system (GNSS) and also satellite communication that works at L-band frequency. The occurrence of ionospheric scintillation varies with location. Therefore, this paper aimed to determine comparative charasteristics of ionospheric scintillation activity over Manado, Pontianak and Bandung from amplitude scintillation index S4 data derived from GPS receiver. The data obtained from the GPS Ionospheric Scintillation and TEC Monitor (GISTM) at Manado station (1.48o N; 124.85oE geomagnetic latitude 7.7oS), at Pontianak station (0.03o S;109.33oE geomagnetic latitude 9.7oS) and at Bandung (-6.90oS;107.6oE geomagnetic latitude 16.54oS) on July 2014 to June 2015. The data were classified into three categores : quiet, moderate and strong based on s4 index. Then we calculated percentage occurrence of scintillation monthly from each observation stastions and mapping of S4 index over Manado, Pontianak and Bandung. The results show that the presentage of strong scintillation (S4>0.5) above Manado is always lower than the other stastions. Strong scintillation was detected at one stations may not also detected at other stations. For very strong scintillastion event, the occurrence of strong scintillation could be detected by all observation stastions but vary in duration. Duration of strong scintillation over Bandung was the longest (up to 4 hours) compared to Pontianak (less than 2 hours) and Manado (less than 1 hour). Based on map of distribution scintillastion occurrence, strong scintillation occurs more intensively over Bandung than over Pontianak and Manado.

Study of GPS Position Error during 16 June, 2005 (Disturb Day) at Bhopal

In order to study GPS position error, the GPS Ionospheric Scintillation and TEC Monitor (GISTM) based GPS receiver was installed at an equatorial station, Bhopal (23.2° N, 77.4° E, Geomagnetic latitude 14.23˚ N), India. We analyzed the horizontal error and the level of confidence in terms of DRMS and CEP and positional error from fixed GPS point for 16 June 2005(disturb day). In order to study the effect of storm on GPS position errors, the latitudinal error and longitudinal error in meter is studied. We observed that the maximum number of error points in the latitudinal error lies between 1.95 to -1.57 meter while longitudinal error points lies between 1.09 to – 1.28 meter. It was observed that some of the error points lie out the 95% error ellipse and it is observed that the error point’s increased in N and N-E direction. The results have been compared with the earlier ones and discussed in terms of possible source mechanism responsible for the position error at anomaly crest region.

Statistical Analysis of GPS Scintillation and TEC Depletion in Temporal and Spatial Distributions over South China

利用位于赤道异常区的深圳站（22.59°N，113.97°E）2011年1月至2012年12月及2015年1月至2015年12月监测到的GPS-TEC数据，统计分析华南地区电离层闪烁与TEC耗空同时出现、电离层闪烁单独出现和TEC耗空单独出现3种现象的时间和空间分布特性.结果表明：这3种现象均主要发生在春秋季节；闪烁与TEC耗空同时出现、闪烁单独出现和TEC耗空单独出现分别主要发生在纬度为19°-23°N，21°-24°N和24°-26°N的空间区域.探测到闪烁和TEC耗空同时出现、闪烁单独出现和TEC耗空单独出现的时间分别主要分布在20：00LT-22：00LT，21：00LT-23：00LT和22：30LT-23：30LT.闪烁与TEC耗空同时出现、闪烁单独出现和TEC耗空单独出现3种现象的时间和空间分布特性对应了华南地区不规则体和赤道等离子体泡（EPBs）从产生到消失的演变过程.

GPS TEC fluctuations over Tromsø, Norway, in the solar minimum

GPS TEC fluctuations over Tromsø, Norway, in the solar minimum

Seasonal and solar cycle effects on TEC at 95°E in the ascending half (2009–2014) of the subdued solar cycle 24: Consistent underestimation by IRI 2012

TEC measured at Dibrugarh (27.5°N, 94.9°E, 17.5°N Geomag.) from 2009 to 2014 is used to study its temporal characteristics during the ascending half of solar cycle 24. The measurements provide an opportunity to assess the diurnal, seasonal and longterm predictability of the IRI 2012 (with IRI Nequick, IRI01-corr, IRI 2001topside options) during this solar cycle which is distinctively low in magnitude compared to the previous cycles. The low latitude station Dibrugarh is normally located at the poleward edge of the northern EIA. A semi-annual variation in GPS TEC is observed with the peaks occurring in the equinoxes. The peak in spring (March, April) is higher than that in autumn (September, October) irrespective of the year of observation. The spring autumn asymmetry is also observed in IRI TEC. In contrast, the winter (November, December, January, February) anomaly is evident only in high activity years. TEC bears a distinct nonlinear relationship with 10.7cm solar flux (F10.7). TEC increases linearly with F10.7 up to about 125sfu beyond which it tends to saturate. The correlation between TEC and solar flux is found to be a function of local time and peaks at 10:00LT. TEC varies nonlinearly with solar EUV flux similar to its variation with F10.7. The nonlinearity is well captured by the IRI. The saturation of TEC at high solar activity is attributed to the inability of the ionosphere to accommodate more ionization after it reaches the level of saturation ion pressure. Annual mean TEC increased from the minimum in 2009 almost linearly till 2012, remains at the same level in 2013 and then increased again in 2014. IRI TEC shows a linear increase from 2009 to 2014. IRI01-corr and IRI-NeQuick TEC are nearly equal at all local times, season and year of observation while IRI-2001 simulated TEC are always higher than that simulated by the other two versions. The IRI 2012 underestimates the TEC at about all local times except for a few hours in the midday in all season or year of observation. The discrepancy between model and measured TEC is high in spring and in the evening hours. The consistent underestimation of the TEC at this longitude by the IRI may be attributed to the inadequate ingestion of F region data from this longitude sector into the model and exclusion of the plasmaspheric content.

Comparison of ionospheric characteristic parameters obtained by GPS and ionosonde with IRI model over China

This paper presents a comparison of ionospheric characteristic parameters obtained by a GPS network and three ionosondes at Mohe (122.4∘E, 53.5∘N, dip angle 70.983∘N), Zuolingzhen (114.6∘E, 30.5∘N, dip angle 46.350∘N), and Fuke (109.1∘E, 19.5∘N, dip angle 27.083∘N) located in China with an IRI model in the year 2011. Observed NmF2 and hmF2 values at the three stations are compared with IRI2007 and IRI2012 predictions, respectively. The results show that there are clear linear correlations between the observed values and the IRI model predicted values. The IRI model gives overestimations at the three stations mostly in 2011. For the NmF2 and hmF2 values, most of the results show that the IRI2012 predicted values are closer to the observed values compared with those of IRI2007. Additionally, the GPS TEC values derived from the Crustal Movement Observation Network of China (CMONOC) are compared with the IRI2012 predictions. From the results, it is evident that the IRI2012 model predictions follow the normal trend of diurnal variation of GPS measured TEC monthly means but do not reproduce the measured data well. The IRI2012 model overestimates electron density in the latter part of spring as well as the first half of autumn and winter and underestimates electron density in early spring and the latter part of autumn. Referring to GPS TEC, the precision of the IRI2012 model predicted TEC values is ∼5 TECU over China. It may also be noted that there are two discontinuities of IRI-TEC monthly means appearing in November and December of the year 2011. This brings a bias of ∼3 TECU of TEC values between two adjacent months.

PREDICTION OF TOTAL ELECTRON CONTENT OF THE IONOSPHERE USING NEURAL NETWORK

This paper presents the prediction of hourly Vertical Total Electron Content (VTEC) using a neural network by utilizing the data from a GPS Ionospheric Scintillation and TEC Monitor (GISTM) receiver for six years (from 2005 to 2010) during low to medium solar activity (Sunspot number (SSN) between 0.0 and 42.6). Several network configurations were investigated to observe the effect of the number of neurons, and hidden layers. Overall testing process for several network set-up yielded Root Mean Square Error (RMSE) value of 3 to 7 TECU, absolute error of 2 to 6 TECU and relative error of 8% to 28%. Testing using April 2010 to November 2010 data (SSN from 8.0 to 25.2) produced RMSE value of 2.95 to 3.88 TECU,absolute error of 2.39 to 3.09 TECU and relative error of 8.11% to 16.18%, which are within the acceptable range.

Application of Modified Periodogram Algorithm on GPS TEC for Earthquakes

Background/Objectives: Ionospheric total electron content perturbations generated by earthquake occurred in Indonesia are analyzed. Methods/Statistical Analysis: The vertical total electron content data on the earthquake day is detrended such that it can be analyzed using statistical signal processing algorithms. Modified periodogram algorithm is implemented on both the disturbed and undisturbed vertical total electron content data. Findings: From the power spectral densities of modified periodogram, the time of occurrence of earthquake is observed and it is noted that it is matching with the actual time of occurrence of event considered. Application/Improvement: This analysis aids in developing an early warning system for earthquakes.

Application of Multiple Signal Classification Algorithm on GPS TEC for Earthquakes

Background/Objectives: Earthquakes are the more vulnerable disasters for mankind and are inevitable. The major objective of our work is to identify the precursors of earthquakes and analysed the precursors in ionosphere prior to the occurrence of earthquake using Global Positioning System Total Electronic Content (GPSTEC). Methods/Statistical Analysis: In this work Multiple Signal Classification algorithm (MUSIC) is explored. MUSIC estimates the frequency content of a signal or autocorrelation matrix using a Eigen space method. Findings: It is clearly observed that there is a significant raise in energy of the ionosphere on the day of earthquake occurrence. Application/Improvement: If the quality data is available before and after the occurrence of earthquake also, then it may be possible to develop the early warning system for earthquakes.

Modeling of ionosphere time series using wavelet neural networks (case study: N-W of Iran)

Wavelet neural networks (WNNs) are important tools for analyzing time series especially when it is non-linear and non-stationary. It takes advantage of high resolution of wavelets and feed forward nature of neural networks (NNs). Therefore, in this paper, WNNs is used for modeling of ionosphere time series in Iran. To apply the method, observations collected at 22 GPS stations in 12 successive days of 2012 (DOY# 219–230) from Azerbaijan local GPS network are used. For training of WNN, back-propagation (BP) algorithm is used. The results of WNN compared with results of international reference ionosphere 2012 (IRI-2012) and international GNSS service (IGS) products. To assess the error of WNN, statistical indicators, relative and absolute errors are used. Minimum relative error for WNN compared with GPS TEC is 6.37% and maximum relative error is 12.94%. Also the maximum and minimum absolute error computed 6.32 and 0.13 TECU, respectively. Comparison of diurnal predicted TEC values from the WNN model and the IRI-2012 with GPS TEC revealed that the WNN provides more accurate predictions than the IRI-2012 model and IGS products in the test area.

High-resolution ionospheric observations and modeling over Belgium during the solar eclipse of 20 March 2015 including first results of ionospheric tilt and plasma drift measurements

The ionospheric behavior over Belgium during the partial solar eclipse of 20 March 2015 is analyzed based on high-resolution solar radio flux, vertical incidence sounding, and GPS TEC measurements. First results of ionosonde-based ionospheric plasma drift and tilt observations are presented and analyzed, including some traveling ionospheric disturbances caused by the eclipse. Also, collocated ionosonde and GPS measurements are used to reconstruct the time evolution of the vertical electron density distribution using the Royal Meteorological Institute (RMI) ionospheric specification system, called Local Ionospheric Electron Density profile Reconstruction (Liedr).

Anomalous Variation in GPS TEC, Land and Ocean Parameters Prior to 3 Earthquakes

The present study reports the analysis of GPS TEC prior to 3 earthquakes (M > 6.0). The earthquakes are: (1) Loyalty Island (22°36′S, 170°54′E) on 19 January 2009 (M = 6.6), (2) Samoa Island (15°29′S, 172°5′W) on 30 August 2009 (M = 6.6), and (3) Tohoku (38°19′N, 142°22′E) on 11 March 2011 (M = 9.0). In an effort to search for a precursory signature we analysed the land and ocean parameters prior to the earthquakes, namely SLHF (Land) and SST (Ocean). The GPS TEC data indicate an anomalous behaviour from 1–13 days prior to earthquakes. The main purpose of this study was to explore and demonstrate the possibility of any changes in TEC, SST, and SLHF before, during and after the earthquakes which occurred near or beneath an ocean. This study may lead to better understanding of response of land, ocean, and ionosphere parameters prior to seismic activities.

MONITORING IONOSPHERIC VARIATION FOR A DEFINITE PERIOD TIME IN TURKEY

Abstract. Ionosphere has been studied by a number of scientists in recent years. Since GPS observations cannot provide TEC value directly, it can be estimated from combination of observations. In this study TEC values derived from GPS observations were produced variations with two hours increments from the eighth day to fifteenth day in months just after beginning of each season, namely January, April, July and October in 2014 for ZONG TUSAGA-Aktif station and 41 other stations (TUSAGA-Aktif, EUREF, IGS). TEC values computed by the Bernese 5.0 software were compared with the Global Ionosphere Model (GIM) TEC values regularly published by Center for Orbit Determination in Europe (CODE) and International Reference Ionosphere (IRI-2012) TEC values. As a result of this study, with the comparison of GPS TEC values to GIM TEC values, it has become obvious that GPS TEC values are quite similar to the GIM TEC values. The differences of TEC values derived from GPS TEC and GIM TEC change from 0.91 TECU (January) to 1.88 TECU (October). On the other hand, GPS TEC values are also compared with IRI TEC values, and found out that there is a considerable difference between the two TEC values ranging from 6.30 TECU (January) to 15.15 TECU (April). Moreover, TEC values derived from GPS measurements are attained similar to GIM TEC, but found to stray from IRI-2012 TEC values remarkably.

Modulation of total electron content by global Pc5 waves at low latitudes

Earlier studies have successfully demonstrated that the GPS–TEC technique is a powerful method to study the propagation pattern of transient disturbances in the ionosphere. This technique has turned out to be sensitive enough to detect ionospheric signatures of ULF waves as well, particularly at high latitudes. It has already been reported earlier that during the recovery phase of the strong magnetic storm on Oct. 31, 2003, intense Pc5 geomagnetic activity was accompanied with distinct pulsations of the same periodicity in the TEC data from high-latitude GPS receiving stations. The present study reveals the identical features in geomagnetic and TEC data at low-latitude stations in the Indian sector as well. However, the presented observational results on TEC modulation by global Pc5 waves at low latitudes cannot be interpreted on the basis of the Alfven mode concept. The most promising mechanism that can explain the present observations is the plasma compression by fast magnetosonic mode. Theoretical order-of-magnitude estimates of the ratio between pulsation amplitudes in TEC and geomagnetic field based on the proposed mechanism is found to be about the same as the observed values.

A case study of ionospheric storm effects in the Chinese sector during the October 2013 geomagnetic storm

In this study, we investigate the ionospheric storm effects in the Chinese sector during 2 October 2013 geomagnetic storm. The TEC map over China sector (1°×1°) and eight ionosondes data along the longitude of 110°E are used to show significant positive ionospheric phases (enhancements in TEC and ionospheric peak electron density NmF2) in the high-middle latitude region and the negative effects at the low latitude and equatorial region during the storm. A wave structure with periods about 1–2h and horizontal speed about 680m/s, propagating from the high latitudes to the low latitudes is observed in electron densities within the height region from 200 to 400km, which is caused by the combined effects of neutral wind and the large-scale traveling disturbances (LSTIDs). In the low latitude regions, compared with those in the quiet day, the ionospheric peak heights of the F2 layer (hmF2) in the storm day obviously increase accompanying a notably decrease in TEC and NmF2, which might be as a result of the eastward prompt penetration electric field (PPEF) evidenced by the two magnetometers and the subsequent westward disturbance dynamo electric fields (DDEF). The storm-time TEC enhancement mainly occurs in the topside ionosphere, as revealed from the topside TEC, bottomside TEC and GPS TEC.

GPS phase scintillation at high latitudes during geomagnetic storms of 7–17 March 2012 – Part 2: Interhemispheric comparison

Abstract. During the ascending phase of solar cycle 24, a series of interplanetary coronal mass ejections (ICMEs) in the period 7–17 March 2012 caused geomagnetic storms that strongly affected high-latitude ionosphere in the Northern and Southern Hemisphere. GPS phase scintillation was observed at northern and southern high latitudes by arrays of GPS ionospheric scintillation and TEC monitors (GISTMs) and geodetic-quality GPS receivers sampling at 1 Hz. Mapped as a function of magnetic latitude and magnetic local time (MLT), the scintillation was observed in the ionospheric cusp, the tongue of ionization fragmented into patches, sun-aligned arcs in the polar cap, and nightside auroral oval and subauroral latitudes. Complementing a companion paper (Prikryl et al., 2015a) that focuses on the high-latitude ionospheric response to variable solar wind in the North American sector, interhemispheric comparison reveals commonalities as well as differences and asymmetries between the northern and southern high latitudes, as a consequence of the coupling between the solar wind and magnetosphere. The interhemispheric asymmetries are caused by the dawn–dusk component of the interplanetary magnetic field controlling the MLT of the cusp entry of the storm-enhanced density plasma into the polar cap and the orientation relative to the noon–midnight meridian of the tongue of ionization.

Latitudinal variation in the occurrence of GPS L-band scintillations associated with the day-to-day changes in TEC, h′F and the E×B drift velocity and their impact on GPS satellite signals

The present study describes the day-to-day variations in the occurrence of GPS L-band scintillations from equator to the anomaly crest location associated with the changes in TEC, h\(^{\prime }\)F and E ×B drift velocities. The GPS–TEC and S4 index data from an equatorial station, Trivandrum (8.47∘N, 76.91∘E), a low latitude station, Waltair (17.7∘N, 83.3∘E) and an anomaly crest location Kolkata (22.6∘N, 88.4∘E) during the low solar activity years of 2004 and 2005 are used. It is observed that the day-time ambient TEC is higher during scintillation days compared to that during the days on which there are no scintillations at the three different locations mentioned above. Further, the diurnal variation of TEC shows a rapid decay during 1700–2000 hr LT over the three different locations during scintillation days which is observed to be comparatively much less during no scintillation days. The average height of the F-layer in the post-sunset hours over Trivandrum is found to be higher, around 350 km during scintillation days while it is around 260 km during the days on which there is no scintillation activity. The average pre-reversal E ×B drift velocity observed around 19:00 hr LT is higher (20 m/s) during scintillation days, whereas during no scintillation days, it is found to be much less (7 m/s). Further, it is observed that the GPS receivers lose their locks whenever the S4 index exceeds 0.5 (>10 dB power level) and these loss of lock events are observed to be more around the anomaly crest location (Kolkata). It may be inferred from the present observations that the level of ambient ionization around noon-time, and a fast decay (collapse) of the ionization during afternoon hours followed by rapid increase in the height of the F-layer contributes significantly to the occurrence of scintillations. The present study further indicates that the S4 index at L-band frequencies increases with an increase in latitude maximizing around the crest of the equatorial ionization anomaly during the post-sunset hours resulting in more loss of lock events in the GPS receiver signals around the EIA crest region.

Suspected seismo‐ionospheric coupling observed by satellite measurements and GPS TEC related to the M7.9 Wenchuan earthquake of 12 May 2008

AbstractAnomalous changes in the ionospheric conditions related to the Wenchuan earthquake of 12 May 2008 are investigated using electron density (Ne) from Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) and CHAMP satellites, electric field from DEMETER, and GPS‐total electron content (TEC) maps. The normalized Ne from the DEMETER satellite reveal that the previously reported TEC increments before the earthquake can be considered as fragments of the gradual equatorial ionization anomaly (EIA) enhancements near the epicenter longitude that began approximately 1 month before the earthquake and reached its maximum with an exceptionally large strength index 8 days prior to the main shock. This feature is indirectly confirmed through the CHAMP Ne and GPS TEC data. Following the EIA intensity peak, disturbances in the Ne and O+ density were observed in the nightside. Based on the concurrent electric field and Ne changes, it is suggested that EIA intensification could be triggered by the E field disturbances over the epicenter.

Observation of ionospheric irregularities around midnight and post-midnight near the northern crest of the Equatorial Ionization Anomaly in the Indian longitude sector: Case studies

Multistation and multi-technique observation of equatorial ionospheric irregularities from locations over the magnetic equator through the northern crest of the Equatorial Ionization Anomaly (EIA) and beyond is one of the important objectives of the Indian CAWSES program. For this purpose, multistation observational campaign was conducted during September 2011, April 2012 and September 2012 involving GPS TEC and S4 from Calcutta situated virtually underneath the northern crest of the EIA, and GPS S4 measurements from Siliguri, located beyond the northern crest of the EIA. Intense amplitude scintillation observations have been noted around midnight and post-midnight hours on certain GPS links located north of Siliguri with no scintillation patches during pre-midnight period. Satellite links experiencing scintillations from Siliguri were unaffected at Calcutta. Day-to-day variability has been noted in the maximum northern observation of GPS scintillations at different S4 levels from both Calcutta and Siliguri. Simultaneous observations from Siliguri and Calcutta indicate that post-midnight GPS amplitude scintillations are possibly associated with interaction of traveling ionospheric disturbances from mid-latitudes towards the equator, with transionospheric GPS links contrary to early evening hours when such movement are usually from over the magnetic equator towards the northern and southern crests of EIA. The present paper reports three cases of ionospheric irregularity tracking using the above stations on September 25, 2012, April 12, 2012 and September 11, 2011, all magnetically quiet days. Information related to the variabilities of latitudinal extent of scintillation observations along 88°E meridian at different times during 13–19UT and different levels of intensities of scintillation could provide useful information for transionospheric satellite link design and Satellite-based Augmentation System (SBAS).