Study on discrepancies in DORIS solutions provided by different analysis centers: comparison to the official IDS ITRF2020 solution

We study the position time series of permanent stations of the DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) technique processed by the four IDS (International DORIS Service) analysis centers which were the input for the DORIS contribution to the latest International Terrestrial Reference Frame (e.g. ITRF2020): ESA (European Space Agency), GOP (Geodetic Observatory of Pecny), GRG (CNES/CLS) and GSC (NASA/GSFC). These IDS ACs time series are compared with the IDS weekly combined solution "ids21wd02". The coordinate time series "ids21wd01" are obtained from the IDS 16 series (IDS contribution to the ITRF2020) aligned to the ITRF2014.We compare the time series of north, east and vertical coordinates recorded at 115 stations located at 87 DORIS positions from 1993.0 to 2020.0. We describe the position time series using a time series model consisting of nonlinear signals, with a period of several years, a linear trend, seasonal oscillations and a stochastic part. Seasonal oscillations include the annual period, the semi-annual period, and the draconic oscillation, along with its overtones. The results show that both deterministic and stochastic parameters differ between the time series provided by each analysis center. They also differ from the "ids21wd02" solution, mainly because of the changes introduced by the combination, which causes suppression of stochastic information in particular (a shift toward white noise). A comparison of the series acquired at the same locations, but at different stations, shows that the noise parameters have been continuously improving over the years, due to continuous improvements in background models, antenna types, etc. This leads to a slight change in the velocities of the stations, but mainly to a reduction in their errors. This is highly important, especially in the context of kinematic reference frames realizations or in geodynamic interpretations DORIS-derived velocities.