Modelling of ionospheric high-order errors for new generation GNSS

In dual frequency Global Positioning System receivers, the ionospheric propagation delays are commonly estimated and corrected using the “ionosphere free” combinations of pseudo-range observables. With this method, the ionospheric delay is estimated at the first-order proportional to the inverse of the frequency square. However, higher-order terms exist that may be taken into account when precise corrections are required. The second-order term results from the influence of the geomagnetic field on the ionospheric propagation of the signals. The third-order term includes the refractive effects due to the bending of the ray. Contributions of the second- and third-order terms typically amount to values in the centimetre and millimetre ranges, respectively. In a near future, triple-frequency Global Navigation Satellite Systems transmitting high-resolution codes will become available. These systems are expected to provide a noticeable improvement in accuracy. This paper focuses on the Galileo signals as Galileo In-Orbit Validation Element (GIOVE) signals are now available for acquisition. A solution relying on the triple-frequency characteristics of a signal-in-space is investigated with results based on first measurements of GIOVE-A signals. Eventually, a method is described that relies on precise dual-frequency phase measurements to evaluate the second-order ionospheric contribution for Galileo data.