Estimation of Ionospheric Delays in Dual Frequency GNSS Positioning

The correct estimation of ionospheric delays is very important in precise satellite-based kinematic positioning, especially over long baselines. In the case of triple frequency system, the ionospheric delays can be estimated from the measurements. However, in the case of dual frequency system, the situation becomes more complicated. The precision of those ionosphere models supplied by the external information source such as JPL IONEX Data is not sufficient, as the high frequency component is neglected. The precision of the low frequency component is not sufficient for the use in long baseline kinematic positioning. On the other hand, the high frequency component can be estimated from the phase range measurements. If the low frequency components are estimated by using an external information source or pseudo-range measurements, more reasonable estimates of ionospheric delays may be possible. In this paper, the estimation using the pseudo-range measurements is discussed. The accuracy of the estimates for ionospheric delays is not sufficient at present because of the bias errors in the pseudo-range measurements. However, if the accuracy of the pseudo-range measurements is improved in future by GPS Modernization and GALILEO, the method would become very promising and convenient, since any external informations are needed and realtime estimation of ionospheric delays becomes possible.