Modeling of Ionospheric Time Delays Based on a Multishell Spherical Harmonics Function Approach

Ionospheric delay is the primary source of error in the functional performance of global navigation satellite system (GNSS) receiver position and time determination. An accurate assessment and modeling of the ionospheric time delay error can improve the overall positioning accuracy. Concerning the equatorial and low-latitude region, the ionosphere is triggered by the equatorial electrodynamics introducing large-scale spatiotemporal as well as vertical gradients in the distribution of electron density over the region. The traditional employment of ionospheric thin-shell models with single-shell approximations, which assumes the electron density of ionosphere to be compressed to a thin layer at a fixed altitude above the earth, may not be appropriate for the low and equatorial latitude region. In this paper, a multishell approximation is proposed based on the spherical harmonics function and a dense network of GNSS experimental total electron content (TEC) data over in India. Further, the performance of the proposed model is evaluated; the ionospheric vertical delay estimation is improved by 24.13% as compared to the single-shell model. The results may be useful and show significant improvement in the context of ionospheric delay modeling of satellite navigation systems in the low-latitude region.