Total Electron Content Fluctuations and Their Implications for GPS Signal Delay over Thailand: Insights from Low Latitudes during the Ascending Phase of Solar Cycle 25

Space Weather and the Global Positioning System

The Total Electron Content (TEC) is a vital and most dominant ionosphere parameter that can cause Global Positioning System (GPS) signal delays, signal degradation and in extreme cases loss of lock. This results into inefficient operations of ground and space based Global Navigation Satellite System (GNSS) applications. Ionosphere range delay on GPS signals is a major error source for GPS positioning and Navigation. Earlier research has shown that the ionosphere is highly variable at low latitude and equatorial regions. Since India located near to the equator, the gradient of equatorial ionization anomaly between the through and the crest is very sharp, which results in large variation of ionospheric electron content. In this thesis the seasonal variation of ionosphere TEC is compared for several stations (Bangalore(12.96°N, 77.5667°E), Hyderabad (17.36°N, 78.47°E), Lucknow (26.8000° N, 80.9000° E), Port Blair (11.6683° N, 92.7378°E)) in India from 2013 to 2014. The observed TEC from GPS is compared with those derived from the International Reference Ionosphere (IRI)-2012 model (IRI-NeQuick and IRI-2001). From the Results we observed that TEC achieves its highest value during the summer, moderate TEC values in the winter and minimum in equinox seasons. And also observed that as the longitude increases the TEC variability is more in the ionosphere. The study of TEC variability is, therefore useful for GNSS users in order to minimize errors where high level of accuracy in measurements is required.

Evaluation of long-term variability of ionospheric total electron content from IRI-2016 model over the Indian sub-continent with a latitudinal chain of dual-frequency geodetic GPS observations during 2002 to 2019

The global positioning system (GPS) signal transit time delay in ionosphere comprised of ionized plasma is a major error source in GPS range measurements. As the density of the ionized plasma varies, the velocity of the radio waves differs from the velocity of light. Due to this, the GPS signals experience group delay or phase advance. Hence, the GPS signal transit time measurement is affected, and this time delay directly propagates into pseudorange measurements when scaled by the velocity of light. The delay depends on elevation angle of the satellite since the signal takes the longer propagation path when transmitted by the satellites tracked at lower elevation angle. The delay also depends on the solar activity conditions since the ionized plasma is a result of solar radiation. To achieve the precise navigation solution, the delay in ionosphere is estimated using conventional method where the total electron content (TEC) is modeled and pseudorange measurements of Link1 (L 1) and Link2 (L 2) frequencies are used. In this method, the TEC is an additional parameter to be calculated and the accurate range measurements determine the accuracy of the TEC. To overcome this, an eigenvector algorithm is proposed in this paper. The algorithm decomposes the pseudorange and carrier phase measurement coefficient matrix. The ionospheric time delay estimates of the proposed algorithm and conventional method are presented in this paper. The delays are estimated for the typical data collected on April 7, 2015, from dual frequency (DF) GPS receiver located in a typical geographic location over Bay of Bengal (Lat: 17.73° N/Long: 83.319° E). The proposed algorithm can be implemented for military and civil aircraft navigation and also in precise surveying applications.

The ionosphere is a layer in the Earth's atmosphere where free electrons exist in sufficient numbers to affect the propagation of electromagnetic waves especially the Global Positioning System (GPS) signals. The study of the Total Electron Content (TEC) variation in the ionosphere and structures is important to ensure the reliability of radio communication systems and accuracy of space weather forecasting. This paper presents the diurnal, seasonal and geographic location effects on TEC variation over Malaysia using Precise Point Positioning (PPP) technique. Since the GPS signals are broadcasted in two widely spread L-band frequency channels namely L1 and L2 consisting of code and phase, it is possible to determine the TEC by employing differencing techniques. This study is conducted using GPS data obtained from 50 stations all over Malaysia. The results of the diurnal analysis show that the mean TEC reaches its maximum during post local noon and its minimum during early morning. The results of the seasonal analysis show that the mean TEC during the equinox months is 35 TECU higher than during the solstice which is only 25 TECU. The seasonal effects on TEC variation is due to the location of the Sun, the movement of plasma around the magnetic equator, and the location of Malaysia. The latitudinal profile of TEC during equinox shows that the location of TEC maximum during the daytime is at southern Malaysia, but changes to the north during nighttime. During solstice, the location of TEC maximum during both day and nighttime is at northern Malaysia, while TEC maximum during early morning is located at southern Malaysia. These results can be used as a reference for ionospheric characterization over Malaysia.

One aspect of the Global Positioning System (GPS) is the potential to conduct geophysical research, and worldwide networks of GPS receivers have been established to exploit this potential. Several research groups have begun using this global GPS data to study ionospheric total electron content (TEC) variations, also referred to as GPS phase fluctuations, as surrogates for ionospheric scintillations. This paper investigates the relationship between GPS amplitude scintillations and TEC variations for the same line of sight using observations from Ancón, Peru. These observations were taken under equatorial spread F conditions for three nights in April 1997. As expected, only when the spectrum of TEC fluctuations includes significant power at the Fresnel scale do scintillations appear. We also find that when the TEC fluctuation spectrum includes the Fresnel scale, the S4 scintillation index is roughly proportional to measures of TEC fluctuation for the weak scintillations observed. The proportionality constant varies from night to night, however, casting doubt on the ability to predict GPS S4 successfully from TEC fluctuation data alone. We also present a simple theoretical phase screen model and show that if a relationship between TEC fluctuation measures and S4 exists, that relationship depends on the power spectrum of phase variations at the screen. Unfortunately, the available TEC data, at 30 s per sample (with some aliasing apparently permitted), offer limited spectral information. A preliminary comparison of 1 s/sample data with the same data decimated to a 30 s/sample interval suggests, however, that the level of successful S4 prediction, based on TEC fluctuation measures alone, is comparable at either sample rate.

GPS derived ionospheric TEC variability with different solar indices over Saudi Arab region

The total electron content (TEC) is an important parameters in the Earth's ionosphere, related to various space weather and solar activities. However, understanding of the complex ionospheric environments is still a challenge due to the lack of direct observations, particularly in the polar areas, e.g., Antarctica. Now the Global Positioning System (GPS) can be used to retrieve total electron content (TEC) from dual-frequency observations. The continuous GPS observations in Antarctica provide a good opportunity to investigate ionospheric climatology. In this paper, the long-term variations and fluctuations of TEC over Antarctica are investigated from CODE global ionospheric maps (GIM) with a resolution of 2.5°×5° every two hours since 1998. The analysis shows significant seasonal and secular variations in the GPS TEC. Furthermore, the effects of TEC fluctuations are discussed.

In this study, the characteristics of the total electron content (TEC) fluctuations and their regional differences over China were analyzed by utilizing the rate of the TEC index (ROTI) based on GPS data from 21 reference stations across China during a solar cycle. The results show that there were significant regional differences at different latitudes. Strong ionospheric TEC fluctuations were usually observed at lower latitudes in southern China, where the occurrence of TEC fluctuations demonstrated typical nighttime- and season-dependent (equinox months) features. This phenomenon was consistent with the ionospheric scintillation characteristics of this region. Additionally, compared to low-latitude China, the intensity of TEC fluctuations over mid-latitude China was significantly weaker, and the occurrence of TEC fluctuations was not a nighttime-dependent phenomenon. Moreover, the intensity of TEC fluctuations was much stronger during high solar activity than during low solar activity. Furthermore, the summer-dependent characteristics of TEC fluctuations gradually emerged over lower mid-latitude areas as equinox characteristics weakened. Similar to the equinox characteristics, the summer-dependent characteristics gradually weakened or even disappeared with the increasing latitude. Relevant discussions of this phenomenon are still relatively rare, and it requires further study and analysis.

GPS TEC variations under quiet and disturbed geomagnetic conditions during the descending phase of 24th solar cycle over the Indian equatorial and low latitude regions

Characterisation of Total Electron Content over African region using Radio Occultation observations of COSMIC satellites

Development of multivariate ionospheric TEC forecasting algorithm using linear time series model and ARMA over low-latitude GNSS station

The global positioning system (GPS) signal that is transmitted from satellite to receiver will be disturbed and degraded in the atmosphere layers due to many factors affecting the signal quality, especially in the ionosphere layer which is an ionised layer in upper atmosphere. Ionospheric scintillation and Total Electron Content (TEC) depletions are the most famous factors that can cause serious effects on GPS signal. This research investigates and analyses the measurement of the ionospheric amplitude scintillation and TEC depletions at quiet and disturbed day. The other parameters involved on this study are the carrier-to-noise (C/No) ratio, and rate of change of TEC (ROT). Data from GISTM receiver at UKM station for minimum solar cycle event were analysed on 5 April 2010 (quiet day) and 7 January 2010 (disturbed day); whereas, data from GISTM for maximum solar cycle event were analysed on 7 October 2015 (quiet day) and 2 June 2015 (disturbed day). The solar cycle event affected on the GPS signal especially when this event increased, so the analysis of ionospheric scintillation during maximum solar cycle event was compared with minimum solar cycle event to investigate the differences between these solar cycle events. The ionospheric scintillation and TEC variations are affected by solar cycle. Results showed that the ionospheric scintillation and TEC variations increased during maximum solar cycle event.

Evaluation of the improvement of IRI-2016 over IRI-2012 at the India low-latitude region during the ascending phase of cycle 24

<p>The present study investigates the variation of Total Electron Content (TEC) using Global Positioning System (GPS) satellites from four equatorial to mid-latitudes stations over a period of one year. The stations are Port Blair (11.63°N, 92.70°E), Agartala (23.75°N, 91.25°E), Lhasa (29.65°N, 91.10°E) and Urumqi (43.46°N, 87.16°E). The diurnal, monthly and seasonal variations of TEC have been explored to study its latitudinal characteristics. Analysis of TEC data from these stations reveals the characteristics of latitudinal variation of Equatorial Ionospheric Anomaly (EIA). To validate the latest IRI 2012 model, the monthly and seasonal variations of GPS-TEC at all the four stations have been compared with the model for three different topside options of electron density, namely, NeQuick, IRI-01-corr and IRI-2001. TEC predictions from IRI-2001 top side electron density option using IRI 2012 model overestimates the observed TEC especially at the low latitudes. TEC from IRI- NeQuick and IRI-01-corr options shows a tendency to underestimate the observed TEC during the day time particularly in low latitude region in the high solar activity period. The agreement between the model and observed values are reasonable in mid latitude regions. However, a discrepancy between IRI 2012 derived TEC with the ground based observations at low latitude regions is found. The discrepancy appears to be higher in low-latitude regions in comparison to mid latitude regions. It is concluded that largest discrepancy in TEC occur as a result of poor estimation of the hmF2 and foF2 from the coefficients.</p>