Evaluation of earth rotation parameters from modernized GNSS navigation messages

Abstract

Modernized navigation messages of global navigation satellite systems like GPS CNAV include earth rotation parameters (ERPs), namely the pole coordinates and UT1-UTC (∆UT1) as well as their rates. Broadcast ERPs are primarily needed for space-borne GNSS applications that require transformations between earth-fixed and inertial reference frames like navigation in earth orbit as well as to the moon. Based on a global tracking network of 23 stations, broadcast ERP values are obtained for the global systems GPS and BeiDou as well as the regional QZSS and IRNSS. Subsequent data sets at daily intervals show polar motion discontinuities of 0.4 to 0.7 mas for GPS, QZSS, and IRNSS, whereas BDS is worse by a factor of about two. Discontinuities in ∆UT1 range from 0.17 to 0.45 ms. External comparison with the C04 series of the International Earth Rotation and Reference Systems Service results in polar motion RMS differences of 0.3 to 1.0 mas and ∆UT1 differences of about 0.13 ms for GPS, QZSS, and IRNSS. Due to less frequent update intervals, BDS performs worse by a factor of 2 – 4. In view of the current GNSS-based positioning errors at geostationary or even lunar distances, the accuracy of GPS, QZSS, and IRNSS broadcast ERPs is sufficient to support autonomous spacecraft navigation without the need for external data.