Abstract

Research on total electron content (TEC) empirical models is one of the important topics in the field of space weather services. Global TEC empirical models based on Global Ionospheric Maps (GIMs) TEC data released by the International GNSS Service (IGS) have developed rapidly in recent years. However, the accuracy of such global empirical models has a crucial restriction arising from the non-uniform accuracy of IGS TEC data in the global scope. Specifically, IGS TEC data accuracy is higher on land and lower over the ocean due to the lack of stations in the latter. Using uneven precision GIMs TEC data as a whole for model fitting is unreasonable. Aiming at the limitation of global ionospheric TEC modelling, this paper proposes a new global ionospheric TEC empirical model named the TECM-GRID model. The model consists of 5183 sections, corresponding to 5183 grid points (longitude 5°, latitude 2.5°) of GIM. Two kinds of single point empirical TEC models, SSM-T1 and SSM-T2, are used for TECM-GRID. According to the locations of grid points, the SSM-T2 model is selected as the sub-model in the Mid-Latitude Summer Night Anomaly (MSNA) region, and SSM-T1 is selected as the sub-model in other regions. The fitting ability of the TECM-GRID model for modelling data was tested in accordance with root mean square (RMS) and relative RMS values. Then, the TECM-GRID model was validated and compared with the NTCM-GL model and Center for Orbit Determination in Europe (CODE) GIMs at time points other than modelling time. Results show that TECM-GRID can effectively describe the Equatorial Ionization Anomaly (EIA) and the MSNA phenomena of the ionosphere, which puts it in good agreement with CODE GIMs and means that it has better prediction ability than the NTCM-GL model.

Highlights

  • The ionospheric model is widely used in GNSS systems to provide real-time ionospheric delay correction for single frequency users

  • As the TECM-GRID is a discrete model based on 5183 grid points, it can effectively avoid the influence of ionospheric anomalies that are only related to locations, such as Equatorial Ionization Anomaly (EIA)

  • The Center for Orbit Determination in Europe (CODE) Global Ionospheric Maps (GIMs) total electron content (TEC) sequences of each grid point from 1 January 1999 to 30 June 2015 were taken as the modelling data set

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Summary

Introduction

The ionospheric model is widely used in GNSS systems to provide real-time ionospheric delay correction for single frequency users. Different GNSS systems use different ionospheric models. The Klobuchar model is used in GPS systems to correct ionospheric delay for single frequency users. The model regards the night ionospheric delay as a constant with a value of 5 ns and the daytime delay as a positive part of the cosine function. The Beidou navigation satellite system (BDS) introduces a modified Klobuchar model [1,2]. Single-frequency users of the European Galileo Satellite Navigation System (Galileo) use the broadcast NeQuick-Gal model to correct ionospheric delay. The ionospheric coefficients of the NeQuick-Gal model are updated and transmitted daily by broadcast ephemeris. The International Reference Ionosphere (IRI) [3] and NeQuick/NeQuick2 [4,5] are the well-known empirical ionospheric models, which are available for estimating ionospheric total electron content (TEC) and three-dimensional electron profiles

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