Longitudinal and Seasonal Variability of Equatorial Ionospheric Irregularities and Electrodynamics

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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014SW001103

Satellite‐based augmentation systems: A novel and cost‐effective tool for ionospheric and space weather studies

The impact of the St. Patrick's Day storm (17 March 2015) on the major equatorial electro-dynamical process viz., the Equatorial Ionization Anomaly (EIA) has been assessed using 2D (5 ° lat. x 5 ° long.) total electron content (TEC) maps generated from the ground based SBAS (Satellite Based Augmentation System) enabled receiver data. The various aspects of EIA specifically the i) evolution/devolution, ii) longitudinal structure, and iii) its variability during different phases of a geomagnetic storm, have been brought out. These 2D TEC maps, which have a large latitudinal (5 S-45° N) and longitudinal (55-110° E) coverage, show the complete reversal in the longitudinal structure/pattern of EIA during the recovery phase of the storm as compared to the quiet day. These results have been explained in the light of the combined effects of the storm associated processes such as i) the penetration electric fields of magnetosphere origin, ii) storm-induced thermospheric winds, and, iii) activation of the consequent disturbance dynamo effectively distorting the longitudinal wave number 4 (WN4) structure of the EIA. It has been shown unambiguously that even a separation of ~10°-15° longitude could experience significantly different forcings. The relevance and the far reaching consequences of the study in the light of the current trends and requirements for reliable satellite based navigation are highlighted.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016SW001408

The impact of the 17 March 2015 St. Patrick's Day storm on the evolutionary pattern of equatorial ionization anomaly over the Indian longitudes using high‐resolution spatiotemporal TEC maps: New insights

Abstract The present paper investigates accuracy of single and dual-frequency Global Positioning System (GPS) standard point positioning solutions employing different ionosphere error mitigation techniques. The total electron content (TEC) in the ionosphere is the prominent delay error source in GPS positioning, and its elimination is essential for obtaining a relatively precise positioning solution. The estimated delay error from different ionosphere models and maps, such as Klobuchar model, global ionosphere models, and vertical TEC maps are compared with the locally derived ionosphere error following the ion density and frequency dependence with delay error. Finally, the positional accuracy of the single and dual-frequency GPS point positioning solutions are probed through different ionospheric mitigation methods including exploitation of models, maps, and ionosphere-free linear combinations and removal of higher order ionospheric effects. The results suggest the superiority of global ionosphere maps for single-frequency solution, whereas for the dual-frequency measurement the ionosphere-free linear combination with prior removal of higher-order ionosphere effects from global ionosphere maps and geomagnetic reference fields resulted in improved positioning quality among the chosen mitigation techniques. Conspicuously, the susceptibility of height component to different ionospheric mitigation methods are demonstrated in this study which may assist the users in selecting appropriate technique for precise GPS positioning measurements.

Evaluation of GPS Standard Point Positioning with Various Ionospheric Error Mitigation Techniques

The ionosphere is a layer of the Earth’s upper atmosphere comprising high concentration of electrons and ions, mostly caused by the solar radiation producing free electrons from the existing atmospheric gases. It extends from 50 km to more than 1000 km altitude. Although solar radiation is stronger at higher altitude but there are very few gaseous atoms at this height; hence the ionization is sparse. The ionospheric properties such as electron density, ion and electron temperatures, ionospheric composition, and dynamics vary with altitude, latitude, longitude, local time, season, solar cycle, as well as magnetic activity. Ionization changes at the equatorial and polar regions are known to be high compared to relatively moderate changes in the mid-latitude region. The variability of the equatorial and low latitude ionosphere is due to the large-scale electrodynamics associated with the equatorial electrojet (EEJ), plasma fountain, equatorial ionization anomaly (EIA), equatorial wind, and temperature anomaly etc. The EEJ refers to an enhanced daytime eastward electric current in the E region due to a strong vertical polarization electric field developed in a latitude band of ±3 about the dip equator. This eastward electric field at the dip equator gets mapped onto F region through the (E × B) drift, lifting the plasma to higher altitudes. The uplifted plasma diffuses down along magnetic field lines into both hemispheres creating two crests of plasma, one in each hemisphere (EIA generation). The overall process is called as the "fountain effect" [2]. The EIA can be described by a trough (minimum) in the ionization densities around the dip equator and a crest (maximum) around ±15 magnetic latitude on each hemisphere. The EIA intensity on a day and its latitude of crest development explicitly depends on the EEJ strength at the equator. Hence, there is strong EIA on a day with strong EEJ, less prominent EIA on a day when CEJ develops, and even absence of EIA crest during certain severe geomagnetic disturbances [19]. All these unique features at the equatorial and low latitude ionosphere are due to the perfect horizontal alignment of the geomagnetic field

/pdf/ionospheric-tec-variations-at-low-latitude-indian-region-1z4ltqpma4.pdf

Ionospheric TEC Variations at low Latitude Indian Region

GAGAN is an Indian implementation of
Satellite Based Augmentation System (SBAS),
developed jointly by Airports Authority of India (AAI) and Indian Space
Research Organisation (ISRO). It is in final operational phase with all the
required ground and space segments ready. With the availability of GAGAN
signal-in-space (SIS), the improvement in position solution is investigated
using the two collocated dual frequency GPS receivers. One of the receivers was
configured as SBAS
receiver and the other was kept as GPS stand-alone receiver. It is found that accuracy in
position improved
significantly in SBAS receiver due
to GAGAN correction. The impact of scintillation on GNSS was also investigated in terms of position
degradation and loss of lock of the satellite signals. The manyfold effects of
scintillation on GPS and SBAS are discussed in detail. The results indicate
~15% reduction in number of measurements due to loss of lock during severe
scintillation.

/pdf/improvement-of-position-accuracy-with-gagan-and-the-impact-50wjubedpd.pdf

Improvement of Position Accuracy with GAGAN and the Impact of Scintillation on GNSS

Advanced space rubidium atomic frequency standard for satellite navigation

Precise calorimetric rubidium mass estimation and its application to the rubidium atomic frequency standard (RAFS)

The present paper characterizes the L-band scintillation over Indian region using the long term database from GAGAN (GPS Aided GEO Augmented Navigation) network. Around 20 scintillation monitoring receivers are operating continuously under the GAGAN network from the year 2004 onwards.

https://www.ursi.org/Proceedings/ProcGA14/papers/ursi_paper2790.pdf

K. S. Parikh

Papers

Satellite‐based augmentation systems: A novel and cost‐effective tool for ionospheric and space weather studies

Improvement of Position Accuracy with GAGAN and the Impact of Scintillation on GNSS

Advanced space rubidium atomic frequency standard for satellite navigation

Precise calorimetric rubidium mass estimation and its application to the rubidium atomic frequency standard (RAFS)

Statistically characterizing the L-band scintillation over Indian equatorial and low latitude region