Triple-Frequency Method for High-Order Ionospheric Refractive Error Modelling in GPS Modernization

New opportunities for the refinement of ionospheric modelling and reduction of the ionospheric error in GPS measurements arise since a third-frequency will be introduced for the modernised GPS system. This paper investigates theoretical models of the ionospheric refractive error. A triple-frequency method of correcting the 1 and 2 order ionospheric refraction is presented and a triple-frequency ionosphere-free combination method is proposed for GPS modernization. These new methods can be equally applied to the European GALILEO system. In addition, typical effects of the 2 and 3 order ionospheric effects are also investigated, and a correct formula for the 3 order ionospheric error is derived in a simple way for easy implementation. It is anticipated that the proposed refraction correction methods will play an important role in both the modernized GPS and GALILEO systems. Results show that the proposed triple-frequency methods can correct the ionospheric refraction effects to a millimetre level. Since the models are given in simple forms, these corrections can be easily implemented in many real-time applications and the triple-frequency methods for correcting high-order ionospheric can significantly eliminate the error remained in the current ionospheric models. The method developed will potentially contribute to a better long-range baseline ambiguity resolution and an accuracy improvement in precise point positioning.