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

Abstract

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).

Group delay variations of GPS transmitting and receiving antennas

“…The GDV corrections for GPS satellite and receiving antennas obtained in this paper refer to a set of reference antennas and are, thus, relative by their nature. One way to achieve absolute corrections of receiving antennas are robot calibrations as demonstrated by Wübbena et al (2008) and Kersten et al (2012). It is desirable to combine both techniques to achieve sets of absolute corrections for GPS receiving and satellite antennas.…”

“…Previous GDV studies revealed significant azimuth-dependent variations for GPS satellite antennas (Haines et al 2004) and azimuth-dependent GDV of receiving antennas could be demonstrated by Kersten et al (2012). Nevertheless, in this first attempt to determine GDV we preferred to work with a simpler model which only takes nadir-or elevationdependent GDV into account.…”

“…Absolute GDV showed patterns with maximum variations of 0.5 m between horizon and zenith. Differences of the six antenna types with respect to each other were much smaller and reached up to 0.2 m. With a similar robot-based field calibration procedure, Kersten et al (2012) determined GDV of some individual receiving antennas, with an emphasis on rover antennas. They found azimuth-and elevation-dependent GDV of up to several decimeters.…”

Impact of antenna group delay variations on protection levels

“…Furthermore, both airborne multipath and antenna group delay variations were found to be primarily biaslike and should be treated as such in the aircraft position protection level equations. These conclusions apply to the current Ground Based Augmentation System (GBAS), the Space Based Augmentation System (SBAS), and future Advanced Receiver Autonomous Integrity Monitoring (ARAIM) concepts [8], as well as precise time transfer applications [9].…”