Chapter 10 Ionospheric Grid Modeling of Regional Satellite Navigation System with Spherical Harmonics

摘要

As an important component of differential navigation service, ionospheric grid can greatly improve the accuracy of single frequency users. Wide-Area Augmentation System (WAAS) broadcasts a set of ionospheric grids data for authorized single frequency GPS users every 300 s. WAAS uses inverse distance weighting (IDW) algorithm to calculate the vertical total electron content (VTEC) at those ionosphere grid points (IGP). IDW algorithm has good correction accuracy, but has low availability in the area with few ionosphere pierce points (IPP). The availability and accuracy of ionospheric grid by IDW algorithm depend on the distribution of monitoring stations and the constellation of navigation system. As the constellation of regional satellite navigation system consists mainly of geostationary earth orbits (GEO) satellites and inclined geosynchronous orbits (IGSO) satellites, the IPPs have a concentrating distribution. Considering that the distribution of monitoring stations are all in China, using IDW algorithm to calculate ionospheric grid will not have good availability. This paper proposes a practical algorithm to calculate the regional ionospheric grid based on spherical harmonics (SH) model and International Reference Ionosphere (IRI-2012) model. Compared with IDW algorithm, the simulation data analysis indicates that the SH algorithm can greatly improve the availability from 82.62 to 99.83 % for mainland China and from 34.66 to 94.25 % for all grids, but the average correction RMS of ionospheric grid increases from 2.60TECU to 3.33TECU.