Modeling the Ionosphere Using Extended Spherical Harmonics Approach for Generation of IRNSS/NavIC based Precise Ionospheric Maps/Products (NPIM) over Indian Region

IRNSS (Indian Regional Navigation Satellite System)/NavIC (Navigation with Indian Constellation) is a regional satellite based navigation system over Indian region being developed by Indian Space Research Organisation (ISRO). There are many error sources affecting the navigation performance of satellite based navigation systems. Among them, the error due to ionosphere is the pre dominant source of error especially while working with radio frequencies in real time as well as in post processing mode for areas of ionospheric research. In real time mode, ionospheric error corrections are available to users as broadcast ionospheric corrections for single frequency users and frequency dependent advantage for dual frequency users to correct the ionospheric delay in their line of sight. But there are some applications where precise ionospheric corrections are required in post processed mode for their scientific research and analysis which is a global requirement over equatorial region which is more prone to ionospheric anomalies and high ionospheric variations. Precise ionospheric products in general provide precise total electron content (TEC) values for the identified grid points over the globe at a specified height above the earth considering a thin shell approximation of the ionosphere. These precise ionospheric products are usually generated by monitoring the ionosphere using ground station network ranging to navigation satellites orbiting the earth i.e., GNSS (Global Navigation Satellite System) data. A perfect example is the Global Ionosphere Maps (GIM) generated using IGS (International GNSS Service) stations network. In this paper, effort has been made to generate precise ionospheric products similar to GIM using NavIC ionospheric data for grid points identified over Indian region. A novel technique based on spherical harmonics with an extended polynomial fit is designed for generating NavIC based Precise Ionospheric Maps (NPIM). The generated NPIM over Indian region using this technique is then compared and evaluated with GIM data over Indian region. It is found that NPIM provides better accuracy of 2 to 4 TECU (TEC units) as against 5 to 6 TECU by GIM data over Indian region.