Ionospheric delay corrections in multi-frequency receivers: Are three frequencies better than two?

The amount of mobile applications that utilize positioning information is ever increasing. Together with the demand for more location-aware applications, there is a growing demand for a higher positioning accuracy. In order to increase the accuracy level and to improve the perceived quality of positioning, the different error sources have to be mitigated. Among those, ionosphere delay accounts for the largest delay of the satellite signal and therefore it has to be effectively corrected. Dual-frequency receivers can effectively remove the first-order ionospheric delay, provided that there are no other errors present in the pseudorange measurements. However, recent work in this field showed that, when the measurements are contaminated also by multipath tracking errors, the performance of dual-frequency receivers degrades significantly. In this paper, we compare the range estimation performance of three algorithms and test the best dual-frequency pair, El-E5a, together with all possible triple-frequency combinations in terms of ionospheric correction. The results indicate that the availability of a third measurement brings little benefit if any, and only when there is a very high positive correlation between the pseudorange measurements on the three frequencies.