Statistics for Spatial Data.

The differential code bias (DCB) in global navigation satellite system (GNSS) satellites should be precisely determined using real ground GNSS data when designing certain applications, such as ionospheric remote sensing, precise positioning, and timing. Different from GPS with a full constellation, the number of BDS (BeiDou Satellite System developed by China) satellites currently in orbit is very limited, and the data from BDS are insufficient for estimating satellite DCB simultaneously with ionospheric total-electron-content modeling. In view of this, a calibration approach, namely, GPS-aided DCB determination (GPSADCB), is proposed to estimate the DCB of current BDS satellites with the aid of GPS data. The data from GPS are used to derive the ionospheric delay along the propagation path of the BDS signal. Before applying GPSADCB to estimate the DCB of current BDS satellites, the performance of this approach is first validated using two sets of GPS data in a simulated BDS case. Then, a continuous four-day data set gathered by two BDS receivers is used to determine the DCB of current BDS satellites based on GPSADCB. Validation results indicate that the difference of DCB estimates between GPSADCB and the Center for Orbit Determination in Europe is about 0.3 ns and the stability of DCB estimates in one month is about 0.2 ns. The DCB of different BDS satellites ranges from -10 ns to 13 ns, and the mean of day-to-day scatter for DCB estimates is below 0.2 ns over the period of data collection. It is demonstrated that the proposed approach can be used to estimate BDS satellite DCB at the initial stage with a few satellites in orbit.

Determination of the Differential Code Bias for Current BDS Satellites

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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016JA023119

Conjugate hemisphere ionospheric response to the St. Patrick's Day storms of 2013 and 2015 in the 100°E longitude sector

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.

Two-Shell Ionospheric Model for Indian Region: A Novel Approach

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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014JA020156

Effects of prolonged southward interplanetary magnetic field on low‐latitude ionospheric electron density

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.

http://nopr.niscair.res.in/bitstream/123456789/2899/1/IJRSP%2036%285%29%20394-404.pdf

Ionospheric studies for the implementation of GAGAN

Future Satellite Based Navigation Systems based on GPS, like US Wide Area Augmentation Sysytem (WAAS), are of global interest to the scientific community. Precise estimation of ionospheric delay is most crucial for successful implementation of the systems. Due to the complex ionospheric structure and large variation of Total Electron Content (TEC) in low latitudes, it is necessary to compare and validate the efficiency of the existing algorithms in this region. In this study, performances of the various interpolation algorithms for TEC calculation at ionospheric grid point (IGP) and user position for gridbased Single Shell Model have been tested for 72 test days of 2005. Based on the results obtained from this analysis, it has been found that, for the Indian region, it would be more suitable to use Ordinary Kriging in place of Planar Fit to estimate delay at an IGP (as used by US WAAS). It has also been found that Ordinary Kriging performs better than the Bilinear Interpolation technique at the user end.

/pdf/statistical-comparison-of-various-interpolation-algorithms-4thc3ooa5a.pdf

Neha Nagori

Papers

Ionospheric studies for the implementation of GAGAN

Two-Shell Ionospheric Model for Indian Region: A Novel Approach

Statistical Comparison of Various Interpolation Algorithms for Grid- Based Single Shell Ionospheric Model over Indian Region