Higher-order ionospheric effects in GPS time and frequency transfer

In high-precision geodetic time and frequency transfer, which requires precise modeling of code and carrier phase GPS data, the ionosphere-free combinations P ₃ and L ₃ of the codes and carrier phases, made on the two GPS frequencies, are used to remove the first-order ionospheric effect. We quantify the impact of the residual second- and third-order ionospheric effects on geodetic time and frequency transfer solutions for continental and intercontinental baselines. All time transfer computations are 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 ionospheric effects in the satellite clock products, only single-difference, common-view processing is used, based on code and carrier phase measurements. The results are shown for weak and strong solar activity, as well as for particular epochs of ionospheric storms. Second-order ionospheric delays can lead to corrections up to about 10 ps in the common-view clock solution of intercontinental baselines with very different longitudes. However, realistic values of the geomagnetic field in the ionosphere are required to assess the amplitude of second-order ionospheric effects in time and frequency transfer during an ionospheric storm.