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Journal ArticleDOI

Ionospheric Gradients Estimation and Analysis of S-Band Navigation Signals for NAVIC System

01 Jan 2018-IEEE Access (Institute of Electrical and Electronics Engineers (IEEE))-Vol. 6, pp 66954-66962
TL;DR: S-band signals of NAVIC are used for the first time to investigate ionospheric gradients over low-latitude region and it is evident that RLS model can estimate ionospherical gradients for a single NAVIC station.
Abstract: Navigation with Indian constellation (NAVIC) is the operational name given to the Indian Regional Navigation Satellite System developed by the Indian Space Research Organisation, India. The most influential factor is ionospheric gradients that can degrade the positional accuracy of the global navigation satellite system users especially in low-latitude regions. The main aim of this paper is to estimate the ionospheric gradient variations obtained from the NAVIC receiver located at Guntur, India (16.23° N, 80.44° E). Code and carrier phase measurements of S-band (2492.028 MHz) signals are used to derive ionospheric time delays and total electron content (TEC) values. In this paper, S-band signals of NAVIC are used for the first time to investigate ionospheric gradients over low-latitude region. The recursive least squares (RLS) algorithm is implemented as a single frequency ionospheric model for estimating the absolute TEC, and longitudinal (E–W) and latitudinal (N–S) ionospheric gradients. Ionospheric gradient analysis has been carried for three consecutive days during September equinox, December solstice in 2016, and for a geomagnetic disturbed event observed during May 2017. The annual statistical analysis in the periodic structure of spatial ionospheric gradients from NAVIC S-band signals during June 2016–May 2017 is also discussed. It is evident that RLS model can estimate ionospheric gradients for a single NAVIC station. The outcome of this work would be useful for understanding ionospheric irregularities climatology over low-latitude region.
Citations
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Journal ArticleDOI
TL;DR: The experimental results of SAKARMA for NavIC have revealed that the MAPE for proposed SAKarMA model is 9-17% (accuracy: 83-91%), while 34-53% (Accuracy: 47-66%) for the Klobuchar model, illustrating that the proposed SakARMAmodel is capable of predicting the ionospheric delays for single frequency GNSS/NavIC users.
Abstract: The single-frequency users of Global Navigation Satellite System (GNSS) require an effective mathematical model that mitigate the dominant errors due to ionospheric delays. Klobuchar model approximately reduces ionospheric effect up to 50% through its coefficients in the navigation message, which is not sufficient for the GNSS single frequency users at critical applications. Hence, a new model, for Single Frequency GNSS User Applications using Klobuchar model driven by Auto Regressive Moving Average Method (SAKARMA) is proposed to forecast and enhance the precision of ionospheric delay estimations for GNSS users. The hourly VTEC maps are obtained from the Assimilated Indian Regional Vertical Total Electron Content (AIRAVAT) by the process of data assimilation using the Kalman filter exclusively for the Indian region (longitude: 65°E to 100°E; latitude: 5°N to 40°N) using 26 GPS TEC stations over Indian region. The accuracy of the SAKARMA model is investigated using the AIRAVAT maps for various Indian geographic regions during both geomagnetic quiet and disturbed conditions of September month in 2016 year. Furthermore, in order to test SAKARMA model, a dual-frequency Navigation with Indian Constellation (NavIC) receiver located at the KL Education Foundation, Guntur, India (geographic: 16.37°N, 80.37°E; geomagnetic: 7.44°N, 153.75°E) is used to collect the observations during 2 - 12 September 2017. Furthermore, SAKARMA model is also validated with Klobuchar model, Klobuchar-style coefficients provided by the Center for Orbit Determination in Europe (CODE) (CODKlob) Model, BeiDou System (BDS2) Model and NeQuick 2 Model over a low latitude NavIC station in forecasting the ionospheric delays. The experimental results of SAKARMA for NavIC have revealed that the MAPE for proposed SAKARMA model is 9-17% (accuracy: 83-91%), while 34-53% (accuracy: 47-66%) for the Klobuchar model. Thus, the results illustrate that the proposed SAKARMA model is capable of predicting the ionospheric delays for single frequency GNSS/NavIC users.

20 citations


Cites background from "Ionospheric Gradients Estimation an..."

  • ...As the Indian geographical region comes under the low latitudes and prone to EIA effects on the satellite communication and navigation links, NavICGEO satellites would facilitate inmonitoring the dynamic changes in low-latitude ionosphere [23]....

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Journal ArticleDOI
TL;DR: In this paper, the relation between the occurrence of ionospheric irregularities and the spatial gradient of total electron content (TEC) derived from two closely located GNSS receivers located within the equatorial region, over Ethiopia, during the postsunset hours was investigated.
Abstract: . The relation between the occurrence of ionospheric irregularities and the spatial gradient of total electron content (TEC) derived from two closely located stations (ASAB: 4.34 ∘ N, 114.39 ∘ E and DEBK: 3.71 ∘ N, 109.34 ∘ E, geomagnetic), located within the equatorial region, over Ethiopia, during the postsunset hours was investigated. In this study, the Global Positioning System (GPS)-derived TEC during the year 2014 obtained from the two stations were employed to investigate the relationship between the gradient of TEC and occurrence of ionospheric irregularities. The spatial gradient of TEC ( ΔTEC∕Δlong ) and its standard deviation over 15 min, σ(ΔTEC∕Δlong ), were used in this study. The rate of change of TEC-derived indices (ROTI, ROTI ave ) were also utilized. Our results revealed that most of the maximum enhancement and reduction values in ΔTEC∕Δlong are noticeable during the time period between 19:00 and 24:00 LT. In some cases, the peak values in the spatial gradient of TEC are also observed during daytime and postmidnight hours. The intensity level of σ(ΔTEC∕Δlong) observed after postsunset show similar trends with ROTI ave , and was stronger (weaker) during equinoctial (solstice) months. The observed enhancement of σ(ΔTEC∕Δlong) in the equinoctial season shows an equinoctial asymmetry where the March equinox was greater than the September equinox. During the postsunset period, the relation between the spatial gradient of TEC obtained from two closely located Global Navigation Satellite System (GNSS) receivers and the equatorial electric field (EEF) was observed. The variation in the gradient of TEC and ROTI ave observed during the evening time period show similar trends with EEF with a delay of about 1–2 h between them. The relationship between σ(ΔTEC∕Δlong) and ROTI ave correlate linearly with correlation coefficient of C=0.7975 and C=0.7915 over ASAB and DEBK, respectively. The majority of the maximum enhancement and reduction in the spatial gradient of TEC observed during the evening time period may be associated with ionospheric irregularities or equatorial plasma bubbles. In addition to latitudinal gradients, the longitudinal gradient of TEC has contributed significantly to the TEC fluctuations.

15 citations


Cites background from "Ionospheric Gradients Estimation an..."

  • ...A variation in the spatial gradient of TEC observed after midnight may be due to the plasma bubbles (Ratnam et al., 2018)....

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Journal ArticleDOI
TL;DR: In this article , an encoder-decoder structure with a convolution long short-term memory (ED-ConvLSTM) network was proposed to forecast global total electron content (TEC) based on the International GNSS Service (IGS) TEC maps from 2005 to 2018 with 1-hr time cadence.
Abstract: In this paper, we proposed an innovative encoder-decoder structure with a convolution long short-term memory (ED-ConvLSTM) network to forecast global total electron content (TEC) based on the International GNSS Service (IGS) TEC maps from 2005 to 2018 with 1-hr time cadence. The ED-ConvLSTM model is used to forecast TEC maps 1–7 days in advance through iterations. To investigate the model's performance, we compared the model with International Reference Ionosphere (IRI2016) model in 2014 and 2018, and compared the model with 1-day Beijing University of Aeronautics and Astronautics (BUAA) model in 2018. The results show that our 7-day ED-ConvLSTM model (ED-ConvLSTM model that forecasts 7 days in advance) outperforms IRI2016 in 2014 and 2018, and our 5-day ED-ConvLSTM model (ED-ConvLSTM model that forecasts 5 days in advance) outperforms 1-day BUAA model. Furthermore, the root mean square error (RMSE) from the 1-day ED-ConvLSTM model with respect to the IGS TEC maps decreases by 51.5% and 43%, respectively, in 2014 and 2018 compared with that from IRI2016 model. The RMSE from the 1-day ED-ConvLSTM model is 20.3% lower than that from the 1-day BUAA model in 2018. In addition, our model has the highest RMSE in the Equatorial Ionospheric Anomaly (EIA) region, but can roughly predict the features and locations of EIA. However, the model fails to forecast localized TEC enhancement and the sudden ionospheric response to the geomagnetic storms. Overall, the model shows competitive performance in medium-term global TEC maps prediction during geomagnetic quiet periods.

7 citations

Journal ArticleDOI
TL;DR: In this paper , a carrier-aided dual-frequency vectorized tracking (CA-DFVT) architecture for the Navigation with Indian Constellation (NavIC) is presented, which uses the precise carrier phase measurements from the S-band signal and the unambiguous code phase measurement from the L5 signal to form a new measurement model for the EKF to estimate the position, velocity, and time (PVT) solutions.
Abstract: A new carrier-aided dual-frequency vectorized tracking (CA-DFVT) architecture for the Navigation with Indian Constellation (NavIC) is presented. CA-DFVT tracks both NavIC L5 (1176.45 MHz) and S-band (2492.028 MHz) signals concurrently. It uses the precise carrier phase measurements from the S-band signal and the unambiguous code phase measurements from the L5 signal to form a new measurement model for the extended Kalman filter (EKF) to estimate the position, velocity, and time (PVT) solutions. The new measurement model takes advantage of the benefits of the higher frequency S-band signal, i.e., less ionospheric delay and carrier phase noise, as well as the L5 signal’s inherent noise mitigation capabilities. Compared to the single-frequency approach, the dual-frequency approach in CA-DFVT eliminates the ionospheric effect and minimizes other errors, resulting in better navigation solutions. The proposed CA-DFVT enhances the reliability and robustness of NavIC signal tracking and position estimation in interference and high dynamics environments. We used static and dynamic field tests to validate the performance and robustness of the proposed CA-DFVT receiver architecture. In comparison to single-frequency (L5/S-band) vector tracking, the CA-DFVT receiver demonstrated consistent signal tracking and position estimation with higher position accuracy. In the static case, the mean horizontal position accuracy of CA-DFVT improves by approximately 2–4 and 9–14 m compared to L5-only VT and S-only VT, respectively, while, in the dynamic case, it improves by approximately 2–5 and 25–42 m, respectively.

6 citations

Journal ArticleDOI
TL;DR: In this paper, the vertical ionospheric delay of the Indian NavIC system at S1 (2492.028 MHz) and L5 (1176.45 MHz) frequencies has been investigated.
Abstract: To meet the growing requirements of Standard Positioning Services (SPS) and Precision Services (PS), more and more GNSS systems operating at conventional GPS frequencies and higher frequency bands are launched. The Indian NavIC system is one of such systems transmitting navigational signals at S1 (2492.028 MHz) and L5 (1176.45 MHz) frequencies. For GPS at L-band frequencies, comprehensive research work has been conducted to analyze the ionospheric delay to estimate precise user position, although very little research work is available in the public domain at the navigational S-band level. The NavIC program provides opportunities to explore the ionospheric delay effect on S-band navigational signals. The precise position determination demands accurate estimation of the vertical ionospheric delay which is generally obtained using Vertical Electron Content (VTEC) of the ionosphere. The VTEC can be obtained by multiplying a mapping function to the Slant Total Electron Content (STEC). Conventionally a thin shell (also known as a single shell) model is used to map STEC to VTEC, but it introduces error at low elevation angles. This error is significant for the NavIC receivers, located in the northern part of India, as they observe elevation angles below 50◦ for most of the time, and thus there is a need to investigate the suitability of the mapping function model for the NavIC system. As the ionospheric shell height modifies the mapping function and results in a change in VTEC, the height and thickness of the thick shell have been investigated based on the ionospheric data taken from IRI 2016 and were estimated as 300 km and 250 km, respectively. In the present work, the thick shell model has been compared to thin shell model mapping functions to improve the accuracy of VTEC estimation at the low elevation. The reduction in vertical delay using the thick shell mapping function at low elevation indicates its suitability for the locations like Dehradun, India, which lies in the mid-latitude region. Furthermore, the temporal variability of vertical delay at S and L band frequencies has also been investigated to understand the diurnal and seasonal characteristics of ionospheric vertical delay over a period of 12 months to cover all the seasons during the year 2017–18. The vertical delay at the S-band frequency was found to be less than that at the L-band frequency and is almost constant over a month. This finding will be beneficial for single-frequency users and could be used to develop the Grid Ionospheric Vertical Delay (GIVD) map for the NavIC system to enhance positional accuracy.

6 citations


Cites methods from "Ionospheric Gradients Estimation an..."

  • ...In a similar work [5], the ionospheric delay gradient has been estimated using S-band NavIC data over the low-latitude region to detect an equatorial plasma bubble (PBB) structure....

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References
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Book
01 Jan 1986
TL;DR: In this paper, the authors propose a recursive least square adaptive filter (RLF) based on the Kalman filter, which is used as the unifying base for RLS Filters.
Abstract: Background and Overview. 1. Stochastic Processes and Models. 2. Wiener Filters. 3. Linear Prediction. 4. Method of Steepest Descent. 5. Least-Mean-Square Adaptive Filters. 6. Normalized Least-Mean-Square Adaptive Filters. 7. Transform-Domain and Sub-Band Adaptive Filters. 8. Method of Least Squares. 9. Recursive Least-Square Adaptive Filters. 10. Kalman Filters as the Unifying Bases for RLS Filters. 11. Square-Root Adaptive Filters. 12. Order-Recursive Adaptive Filters. 13. Finite-Precision Effects. 14. Tracking of Time-Varying Systems. 15. Adaptive Filters Using Infinite-Duration Impulse Response Structures. 16. Blind Deconvolution. 17. Back-Propagation Learning. Epilogue. Appendix A. Complex Variables. Appendix B. Differentiation with Respect to a Vector. Appendix C. Method of Lagrange Multipliers. Appendix D. Estimation Theory. Appendix E. Eigenanalysis. Appendix F. Rotations and Reflections. Appendix G. Complex Wishart Distribution. Glossary. Abbreviations. Principal Symbols. Bibliography. Index.

16,062 citations

Journal ArticleDOI
TL;DR: The algorithm designed for this purpose, and implemented in the GPS satellites, requires only eight coefficients sent as part of the satellite message, and contains numerous approximations designed to reduce user computational requirements, yet preserves the essential elements required to obtaingroup delay values along multiple satellite viewing directions.
Abstract: The goal in designing an ionospheric time-delay correctionalgorithm for the single-frequency global positioning system userwas to include the main features of the complex behavior of theionosphere, yet require a minimum of coefficients and usercomputational time, while still yielding an rms correction of at least50 percent. The algorithm designed for this purpose, andimplemented in the GPS satellites, requires only eight coefficientssent as part of the satellite message, contains numerousapproximations designed to reduce user computationalrequirements, yet preserves the essential elements required to obtaingroup delay values along multiple satellite viewing directions.

1,181 citations


"Ionospheric Gradients Estimation an..." refers background or methods in this paper

  • ...diverges with geographic and geomagnetic effects [14] and it claims a direct impact on ionospheric delay with a good first order approximation by...

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  • ...The conventional models such as Klobuchar and NeQuick compensated the ionospheric propagation delays up to reasonable scale only [14], [5]....

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Journal ArticleDOI
TL;DR: In this paper, the authors present observations of a strong positive storm at middle latitudes and suggest that electric fields may play an important role in the generation of the observed positive storm phase.
Abstract: [1] During geomagnetic storms, the ionospheric F region electron density may be greatly increased or decreased, which are termed positive storms or negative storms. It is generally accepted that negative storm phases are caused by neutral composition changes. In contrast, different mechanisms have been proposed to explain the generation of positive storm phases. In this paper, we present observations of a strong positive storm at middle latitudes. The Millstone Hill incoherent scatter radar detected significant increases of the midlatitude ionospheric F region electron density during a magnetic storm on 3 April 2004. The positive phase of the ionospheric storm started to occur in the morning sector (0912 LT at Millstone Hill) and lasted for more than 10 hours. Compared with the quiet-time ionosphere, the daytime F peak altitude over Millstone Hill during the storm was ∼80 km higher, the F region electron density was increased by a factor of 2–4, and the F region electron temperature was decreased by ∼1000 K (or ∼40%). The radar also measured an enhanced eastward electric field and a slightly more equatorward neutral wind. Global GPS measurements show that total electron content (TEC) was increased at middle latitudes and decreased at lower latitudes, and the large TEC enhancements occurred primarily in the Atlantic sector. We suggest that electric fields may play an important role in the generation of the observed positive storm phase. The eastward electric field will cause increases in the midlatitude ionospheric electron density by moving the plasma particles upward and decreases in the equatorial ionospheric electron density by strengthening the fountain effect. The daytime poleward wind was reduced or even reversed during the storm, which may also contribute to the occurrence of the positive storm.

113 citations


"Ionospheric Gradients Estimation an..." refers background in this paper

  • ...Besides, in the case of extended storm time traveling atmospheric disturbances have been reported [10]....

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22 Sep 2000
TL;DR: The Wide Area Augmentation System (WAAS) as mentioned in this paper provides real-time differential GPS corrections and integrity information for aircraft navigation use, where the system guides the aircraft to within a few hundred feet of the ground.
Abstract: The Wide Area Augmentation System (WAAS) will provide real-time differential GPS corrections and integrity information for aircraft navigation use. The most stringent application of this system will be precision approach, where the system guides the aircraft to within a few hundred feet of the ground. Precision approach operations require the use of differential ionospheric corrections. WAAS must incorporate information from reference stations to create a correction map of the ionosphere. More importantly, this map must contain confidence bounds describing the integrity of the corrections. The confidence bounds must be large enough to describe the error in the correction, but tight enough to allow the operation to proceed. The difficulty in generating these corrections is that the reference station measurements are not co-located with the aviation user measurements. For an undisturbed ionosphere over the Conterminous United States (CONUS), this is not a problem as the ionosphere is nominally well behaved. However, a concern is that irregularities in the ionosphere will decrease the correlation between the ionosphere observed by the reference stations and that seen by the user. Therefore, it is essential to detect when such irregularities may be present and adjust the confidence bounds accordingly. The approach outlined in this paper conservatively bounds the ionospheric errors even for the worst observed ionospheric conditions to date, using data sets taken from the operational receivers in the WAAS reference station network. As we progress through the current solar cycle and gather more data on the behavior of the ionosphere, many of our pessimistic assumptions will be relaxed. This will result in higher availability while maintaining full integrity.

88 citations

Journal ArticleDOI
TL;DR: In this article, the authors used the Millstone Hill incoherent scatter radar database to investigate the spatial extent and temporal evolution of TEC and density altitude/latitude structure at middle and subauroral latitudes as a function of solar cycle, local time, and level of geomagnetic activity.
Abstract: The subauroral ionosphere, at the magnetic latitudes which characterize the northeastern United States, is subject to severe F region ionospheric density structuring due to the space weather effects of magnetospheric disturbance electric fields. Communications and navigation systems relying on transionospheric propagation must be able to compensate for the effects of the sharp changes (>10X) in total electron content (TEC) associated with the ionospheric trough and storm time disturbance effects at midlatitudes. The Millstone Hill incoherent scatter radar database has been used to investigate the spatial extent and temporal evolution of TEC and density altitude/latitude structure at middle and subauroral latitudes as a function of solar cycle, local time, and level of geomagnetic activity. More than 11,000 radar elevation scans covering >20° of latitude and altitudes between 150 and 750 km have been used to identify the characteristics of the density gradient near the equatorward edge of the ionospheric trough in a variety of circumstances spanning 20 years and two solar cycles. Pronounced density gradients can be identified in ∼35% of the Millstone Hill scans, and we present a statistical characterization of average magnitude and location for these steepest TEC gradients. In some cases (especially near noon) the equatorward edge of the trough lies poleward of our observational field of view, and gradients associated with phenomena other than the trough contribute to our statistics. On most days the trough appears in the radar scans between 1600 and 2000 magnetic local time (MLT). Larger TEC gradients occur at solar maximum and when the background TEC is higher. The steepest gradients occur in an environment of high TEC (at solar maximum and adjacent to regions of storm-enhanced density (SED)), when the processes which generate the trough are strongest (high Kp). High gradient values occur in the sunlit sector, with maximum values of TEC gradient (∼10 TEC/deg latitude, with 1 TEC unit = 10 16 el m -2 ) found in the postnoon ionosphere. Mean solar maximum TEC gradient at 1600 MLT is 3-4 TEC/deg for Kp 100 over New England and TEC gradients of ∼50 TEC/deg.

69 citations


"Ionospheric Gradients Estimation an..." refers background in this paper

  • ...A detailed study is established for the mid-latitudes reporting the steepest ionospheric density gradient under geomagnetic storms for solar maximum-minimum years [12]....

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