Laurence Wauters

Influence of ionospheric perturbations in GPS time and frequency transfer

The stability of GPS time and frequency transfer is limited by the fact that GPS signals travel through the ionosphere. 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 and carrier phase measurements made on the two frequencies is used to remove the first-order ionospheric effect. In this paper, we investigate the impact of residual second- and third-order ionospheric effects on geodetic time transfer solutions i.e. remote atomic clock comparisons based on GPS measurements, using the ATOMIUM software developed at the Royal Observatory of Belgium (ROB). The impact of third-order ionospheric effects was shown to be negligible, while for second-order effects, the tests performed on different time links and at different epochs show a small impact of the order of some picoseconds, on a quiet day, and up to more than 10 picoseconds in case of high ionospheric activity. The geomagnetic storm of the 30th October 2003 is used to illustrate how space weather products are relevant to understand perturbations in geodetic time and frequency transfer.

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.