Refining the Klobuchar Ionospheric Model with Spherical Cap Harmonic Model for GNSS Positioning

Ionospheric delay is one of the major error sources in GNSS (Global Navigation Satellite System) positioning. For single frequency system, it is very important to effectively correct the ionospheric delay and so-called Klobuchar's ionospheric model has been widely used due to its well known properties such as computational simplicity. The Klobuchar model is one of the global ionospheric models, and it can correct about 50% RMS ionospheric errors[1]. On the other hand, there are some models that can achieve better accuracy than the Klobuchar model, the SCHA (Spherical Cap Harmonics Analysis) model is one of such models. By the SCHA model, it has been reported that the ionospheric delays can be well modeled in the regional area such as the sky over Japan[2]. However, comparing with the Klobuchar model, the SCHA model needs more number of parameters and computational burden. Therefore, in this paper, a method to generate the parameters of the Klobuchar model from the SCHA model is proposed. By this method, the regional ionospheric model that is not only simple and easy operational like the Klobuchar model but also regionally accurate like the SCHA model is obtained.