Impact of reference EOP time series on EOP prediction evaluation

Abstract This study evaluates the impact of different reference time series on the prediction of various Earth orientation parameters (EOP), namely, x and y pole coordinates, and the difference between Universal Time and Coordinated Universal Time (UT1–UTC). The former were the most frequently-predicted parameters during the Second Earth Orientation Parameters Prediction Comparison Campaign (2nd EOP PCC). We analyzed EOP series derived from both single measurement techniques—Global Navigation Satellite Systems (GNSS), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), Satellite Laser Ranging (SLR), and Very Long Baseline Interferometry (VLBI)—and combined solutions using multiple techniques. EOP series were compared across two periods: the maximum available period (2014–2022) and the operational period of the 2nd EOP PCC (2021–2022). The results of our spectral analysis highlight that, in general, differences in oscillation amplitudes are smaller for combined EOP series than those based on a single technique, with the exception of the GNSS-based series from International GNSS Service (IGS). Differences in amplitude were highest for the VLBI-based series relative to the C04 14 solution. A root mean square deviation (RMSD) analysis for both 2014–2022 and 2021–2022 revealed high consistency for combined and GNSS-based EOP series. We also carried out a mean absolute error (MAE) analysis. VLBI-based series from International VLBI Service for Geodesy and Astrometry (IVS) and U.S. Naval Observatory (USNO) were excluded due to pronounced differences, likely to be due to a lack of observations. Our MAE analysis identified that the highest consistency between predictions and reference series was achieved when using the IGS series for the x pole, the C04 20 and IGS series for the y pole, and the IGS series for UT1–UTC. Conversely, the lowest consistency was observed when using International Laser Ranging Service (ILRS) series for the x pole, European Space Agency (ESA) series for the y pole, and a combination of independent space-geodetic measurements of the Earth's orientation (SPACE) series for UT1–UTC. This study underscores the prevailing role of GNSS in combined solutions, and suggests that other space geodetic techniques make a minimal contribution to polar motion estimations. VLBI is essential for determining UT1–UTC, but its consistency with other EOP series may be limited due to the sparse distribution of observations. In conclusion, the choice of EOP reference series significantly affects MAE values in prediction evaluations. Overall, the taking GNSS-based IGS series as a reference in forecast evaluation minimizes errors of predictions, making it the preferred reference for EOP forecast accuracy assessments. However, caution is advised when using VLBI or SLR-based series, due to potential inaccuracy. Graphical Abstract