Study of common aperiodic displacements at ITRF co-location sites

In historical versions of the International Terrestrial Reference Frame (ITRF), the time evolution of station positions was described by piece-wise linear models. These kinematic models have been extended with exponential and logarithmic functions in ITRF2014 to account for post-seismic displacements, then with annual and semi-annual sine waves in ITRF2020 to account for the seasonal deformation of the Earth. However, part of the Earth’s surface deformation, such as inter-annual hydrological loading deformation, or high-frequency atmospheric loading deformation, is still not captured by such deterministic functions.

A reference frame in the form of a time series could allow such aperiodic displacements to be taken into account. This would require the aperiodic motions sensed by the different space geodetic techniques to be tied in a common frame by means of co-motion constraints. However, common aperiodic displacements between co-located space geodetic stations have not been evidenced at a global scale so far, and the relevance of such constraints is thus debatable. In this study, in order to investigate the possible existence of common aperiodic displacements at ITRF co-location sites, we use the solutions provided by the technique services for ITRF2014. Those solutions are first carefully aligned to a common reference frame in order to minimize differential network effects and obtain comparable position time series across techniques. The obtained position time series are then cleaned from linear, post-seismic and periodic signals (including seasonal deformation and technique systematic errors). The remaining aperiodic displacements are finally inter-compared at co-location sites.

Modest correlations are thus evidenced between the GNSS residual position time series and the other space geodetic techniques, mostly in the vertical component. The magnitude of the common aperiodic displacements evidenced in this study is finally discussed.