On the impact of Group Delay Variations on GNSS time and frequency transfer

Group Delay Variations (GDVs) are azimuth and elevation dependent code delays that can limit the accuracy of the GNSS code observables. This contribution focuses on the GDV determination and discusses several solutions with respect to repeatability and separability. On-site tests at a laboratory network as well as simulations of several time links are discussed analyzing the stochastic processes apparently introduced by GDV. The stability of the P3 links are not effected. However, offsets up to 0.6 ns can occur. In a detailed study, GDVs are applied to an inter-continental Precise Point Positioning (PPP) time transfer link. This analysis shows that GDV are not an issue for the stability of the PPP links, since small weights reduce the impact of the P3 GDV. It can be shown that the stochastic process, induced by GDV in a PPP analysis, is similar to a random walk noise, well below the L3 carrier phase observation noise of σ_φ = 6 mm. Offsets for the receiver clock estimates of up to 0.4 ns are reported for the link WTZS (Wettzell) and Boulder (NIST).