  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.
