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Ionospheric studies for the implementation of GAGAN

TL;DR: In this article, a near real-time grid-based ionospheric delay model for correcting propagation delay at 1575.42 MHz and 1227.6 MHz was proposed to meet the requirement of correction with 0.5 m maximum residue over Indian region.
Abstract: Satellite Based Augmentation System (SBAS), being developed by Indian Space Research Organization (ISRO) in collaboration with Airports Authority of India (AAI) is known as “GPS Aided GEO Augmented Navigation” (GAGAN). It is expected to offer better accuracy and integrity of navigation service than with GPS alone by providing correction terms to the GPS signals. This is achieved by modelling a Near Real Time Grid Based Ionospheric Delay Model for correcting propagation delay at 1575.42 MHz (L1) using measurements at 1575.42 and 1227.6 MHz (L2). Existing algorithms are replaced by Kriging based model to meet the requirement of correction with 0.5 m maximum residue over Indian region. Details of the data collection and pre-processing, including estimation of the Total Electron Content (TEC), which is a measure of ionospheric delay, has been described. Kriging algorithm and some preliminary results of studies are also presented in this paper. This includes the spatial decorrelation of the stochastic random field over the deterministic variation of ionospheric TEC. Its variation with time and locations are investigated and a temporal dependence found to exist. Large scale ionospheric irregularities and depletions that cause severe amplitude and phase scintillations are also studied. Their impacts on GAGAN are also shown. Some major scientific studies required to be carried out over Indian region to improve the GAGAN performance is discussed.
Citations
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Journal ArticleDOI
TL;DR: In this paper, the effects of the St. Patrick's Day geomagnetic storms of 2013 and 2015 in the equatorial and low-latitude regions of both hemispheres in the 100°E longitude sector is investigated and compared with the response in the Indian sector at 77°E.
Abstract: The effects of the St. Patrick's Day geomagnetic storms of 2013 and 2015 in the equatorial and low-latitude regions of both hemispheres in the 100°E longitude sector is investigated and compared with the response in the Indian sector at 77°E. The data from a chain of ionosondes and GPS/Global Navigation Satellite Systems receivers at magnetic conjugate locations in the 100°E sector have been used. The perturbation in the equatorial zonal electric field due to the prompt penetration of the magnetospheric convective under shielded electric field and the over shielding electric field gives rise to rapid fluctuations in the F2 layer parameters. The direction of IMF Bz and disturbance electric field perturbations in the sunset/sunrise period is found to play a crucial role in deciding the extent of prereversal enhancement which in turn affect the irregularity formation (equatorial spread F) in the equatorial region. The northward (southward) IMF Bz in the sunset period inhibited (supported) the irregularity formation in 2015 (2013) in the 100°E sector. Large height increase (hmF2) during sunrise produced short-duration irregularities during both the storms. The westward disturbance electric field on 18 March inhibited the equatorial ionization anomaly causing negative (positive) storm effect in low latitude (equatorial) region. The negative effect was amplified in low midlatitude by disturbed thermospheric composition which produced severe density/total electron content depletion. The longitudinal and hemispheric asymmetry of storm response is observed and attributed to electrodynamic and thermospheric differences.

31 citations

Journal ArticleDOI
TL;DR: A two-shell model which incorporates two different shells, at 300- and 500-km altitudes, having different weights at different time domains has been implemented and it is observed that there is at least 60% improvement in the performance of the two- shell model in comparison to the single-shell models for the Indian region.
Abstract: In the U.S. Wide Area Augmentation System, European Geostationary Navigation Overlay Service, and Indian Global Positioning System Aided Geo Augmented Navigation, a near real-time grid-based single-shell model is proposed to correct the ionospheric delay at the user aircrafts. The single-shell model is based on the assumption that the whole ionosphere is compressed at a fixed altitude at 350 km. This assumption may not be appropriate for the Indian region, which falls in the Equatorial Ionospheric Anomaly belt. In this paper, a two-shell model which incorporates two different shells, at 300- and 500-km altitudes, having different weights at different time domains has been implemented. A statistical comparison between single- and two-shell models has been done for all quiet days of year 2005. Based on the results, it is observed that there is at least 60% improvement in the performance of the two-shell model in comparison to the single-shell model for the Indian region.

31 citations

Journal ArticleDOI
TL;DR: The role of electrodynamical/neutral-dynamical and compositional disturbances are discussed in view of these observations to understand low-latitude ionospheric response when geomagnetic disturbance persists for longer duration.
Abstract: The present work describes the low-latitude ionospheric variability during an unusually prolonged (~33 h) geomagnetically disturbed condition that prevailed during 15–16 July 2012. The low-latitude electron density in summer hemisphere, investigated using ground- and satellite-based observations, responded to this by generating strong negative ionospheric storm on 16 July. The maximum electron density on 16 July over Indian low latitudes was reduced by more than 50% compared to that on a geomagnetically quiet day (14 July 2012). In contrast to the extreme reduction in total electron content (TEC) in the Northern Hemisphere, TEC from a winter hemispheric station revealed substantial (~23 total electron content unit, 1 TECU = 1016 el m−2) enhancements on the same day. This contrasting hemispherical response in TEC is suggested to be due to the combined effects of strong interhemispheric and solar-driven day-night winds. Further, very weak equatorial electrojet (EEJ) strength on 16 July indicated that the westward electric field perturbations in the low-latitude ionosphere were possibly due to the disturbance dynamo effect associated with meridional circulation from polar to equatorial latitudes. Interestingly, despite reduction in the integrated EEJ strength on 15 July, the low-latitude electron density showed substantial enhancement, highlighting the significant effect of the positive ionospheric storm on the low-latitude ionosphere. The roles of electrodynamical/neutral-dynamical and compositional disturbances are discussed in view of these observations to understand low-latitude ionospheric response when geomagnetic disturbance persists for longer duration.

27 citations

Journal ArticleDOI
TL;DR: This paper attempts to predict the Total Electron Content using adaptive recurrent Neural Network at three different locations of India using in-situ Learning Algorithm and finds that the mean and root mean square values of prediction errors remain small enough for all practical applications.

25 citations

Journal ArticleDOI
TL;DR: This paper proposes and demonstrates SBAS applicability to ionospheric and space weather research in a novel and cost‐effective way and vindicate the potential of SBAS over extended areas.
Abstract: Satellite-Based Augmentation Systems (SBASes) are designed to provide additional accuracy and robustness to existing satellite-based radio navigation systems for all phases of a flight. However, similar to navigation systems such as GPS which has proven its worth for the investigation of the ionosphere, the SBASes do have certain advantages. In the present paper, we propose and demonstrate SBAS applicability to ionospheric and space weather research in a novel and cost-effective way. The recent commissioning of the Indian SBAS, named GPS Aided Geo Augmented Navigation (GAGAN), covering the equatorial and low-latitude regions centered around the Indian longitudes provides the motivation for this approach. Two case studies involving different ionospheric behavior over low-latitude regions vindicate the potential of SBAS over extended areas.

24 citations


Cites methods from "Ionospheric studies for the impleme..."

  • ...It is worthy to mention that a lot of data processing was carried out in GPS measurements to remove the interfrequency biases [Acharya et al., 2007] and to estimate the vertical TEC by averaging the measurements above an elevation cutoff angle of 40° [Rama Rao et al....

    [...]

  • ...GAGAN project known as GAGAN-TEC network [Acharya et al., 2007] are used for comparison....

    [...]

  • ...It is worthy to mention that a lot of data processing was carried out in GPS measurements to remove the interfrequency biases [Acharya et al., 2007] and to estimate the vertical TEC by averaging the measurements above an elevation cutoff angle of 40° [Rama Rao et al., 2006a]....

    [...]

References
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Journal ArticleDOI
TL;DR: In this paper, a method based on a Kalman filtering approach was proposed to estimate the biases in the GPS satellites and receivers and the total electron content at each GPS station using dual GPS data.
Abstract: In the estimation of the ionospheric total electron content from the Global Positioning System (GPS) observables, various instrumental systematic effects such as the biases in the GPS satellites and receivers must be modeled. This paper describes a procedure, based on a Kalman filtering approach, for estimating these instrumental biases as well as the total electron content at each GPS station, using dual GPS data. The method is applied to six data sets, of 48 hours each, spanning one year, from the Deep Space Network with GPS stations in Australia, Spain, and the United States. The formal errors for the estimated satellite biases and for the total electron content at each station are about 0.07 ns and 0.2×1016 el/m2, respectively. The variation in time of the satellite biases (relative to the mean of all of them) estimated in different epochs during 1-year period, is below 1 ns.

437 citations

Journal ArticleDOI
TL;DR: In this article, the authors used the Wideband ionospheric scintillation model (WBMOD) to calculate the S 4 on a two-way channel, where the data from three additional stations (Ascension Island, in the South Atlantic Ocean, Huancayo, Peru, and Manila, Phillipines; data collected under the auspices of the USAF Phillips Laboratory Geophysics Directorate) were used to provide a greater diversity in both latitude and longitude.
Abstract: One of the main limitations of the modeling work that went into the equatorial section of the Wideband ionospheric scintillation model (WBMOD) was that the data set used in the modeling was limited to two stations near the dip equator (Ancon, Peru, and Kwajalein Island, in the North Pacific Ocean) at two fixed local times (nominally 1000 and 2200). Over the past year this section of the WBMOD model has been replaced by a model developed using data from three additional stations (Ascension Island, in the South Atlantic Ocean, Huancayo, Peru, and Manila, Phillipines; data collected under the auspices of the USAF Phillips Laboratory Geophysics Directorate) which provide a greater diversity in both latitude and longitude, as well as cover the entire day. The new model includes variations with latitude, local time, longitude, season, solar epoch, and geomagnetic activity levels. The way in which the irregularity strength parameter C k L is modeled has also been changed. The new model provides the variation of the full probability distribution function (PDF) of log(C k L) rather than simply the average of log(C k L). This permits the user to specify a threshold on scintillation level, and the model will calculate the percent of the time that scintillation will exceed that level in the user-specified scenario. It will also permit calculation of scintillation levels at a user-specified percentile. A final improvement to the WBMOD model is the implementation of a new theory for calculating S 4 on a two-way channel

158 citations

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, a new approach to model the ionosphere based on GPS data is presented, which includes a second shell to account for the distribution of the electrons in the outer part of the Ionosphere.
Abstract: We present a new approach to model the Ionosphere based on GPS data. Previous authors have used models with an unique shell. In this case we have included a second shell to account for the distribution of the electrons in the outer part of the Ionosphere. We have analyzed the ionospheric electron content of a region above 30 degrees in declination in different conditions of ionospheric activity using the Kalman filter. The data used has been obtained from the International GPS Service for Geodynamics (IGS) network. Simultaneously we have studied the receiver and transmitter differential biases showing the effects of neglecting the outer part of the Ionosphere in the model. It appears a systematic variations for the receivers—depending on its latitude—not for the satellites.

76 citations