Higher-order ionosphere perturbations in GPS time and frequency transfer

In high precision geodetic time transfer (i.e. based on precise modeling of code and carrier phase GPS data), the so-called ionosphere-free combination of the code (P₃) and carrier phase (L₃) measurements, made on the two GPS frequencies, is used to remove the first-order ionosphere effect. In this paper, we quantify the impact of residual second- and third-order ionosphere perturbations on geodetic time and frequency transfer solutions, for both continental baselines and intercontinental baselines. All time transfer computations have been done using the ATOMIUM software, developed at the Royal Observatory of Belgium. In order to avoid contamination by some imperfect modeling of the second- and third-order ionosphere effects in the satellite clock products, only single-difference (Common-View) processing is used, based on both code and carrier phase measurements. The results are shown for weak and for strong solar activity, as well as for particular epochs of ionosphere storms.