New correction approaches for mitigating ionospheric higher order effects in GNSS applications

Abstract

By combining two or more signals of Global Navigation Satellite Systems (GNSS) more than 99% of the ionospheric propagation delay can be corrected in real time precise positioning. However, higher order propagation effects such as ray path bending errors remain uncorrected in dual- or triple-frequency ionosphere-free combination. The range computation between a satellite and a ground receiver is affected up to several centimeters due to higher order ionospheric terms. Therefore, they cannot be neglected in precise point positioning applications, especially during times of high total electron content (TEC). Neglecting the ray path bending by assuming a straight Line of Sight (LOS) propagation introduces mainly two errors in the range computation. Firstly, the TEC along a curved path is slightly larger than that along the straight LOS. This causes differential TEC between two GNSS signal paths which results in uncorrected ray path bending error in the first-order ionosphere-free combination in addition to the higher order terms of the refractive index. Secondly, the total length of a curved path is slightly longer than the LOS one. Since, the ionosphere is dispersive in nature; the path length will not be the same for two GNSS signals. This indicates that the dual-frequency range equation must have additional terms for correcting differential bending. It has been found that in the range equation the excess TEC and excess path terms practically compensate each other. In other words, if we consider one term ignoring the other term would degrade the accuracy of the range computation. To mitigate the LOS propagation assumption error, i.e., the ray path bending error, we have derived different correction formulas based on simulation studies. Our study shows that in average about 60-70% and above 80% of the excess path and excess TEC errors can be corrected by different correction approaches during high and low solar activity conditions, respectively.