Combination of the seasonal displacement parameters estimated in the ITRF2014 processing

Coordinates of ITRF2014 have been computed using data from the four geodetic techniques, namely Global Navigation Satellite Systems, Very Long Baseline Interferometry, Satellite Laser Ranging and Doppler Orbitography and Radiopositioning Integrated by Satellites. In the ITRF2014 computation process, annual and semi-annual periodic coordinate variations have been simultaneously estimated with reference positions and velocities at each geodetic station. Those parameters were not published since they were not provided in the same reference frame and required to be assessed. At a co-location site, nearby instruments are supposed to measure the same physical ground motion. Based on this assumption, we have combined estimated annual and semi-annual displacement parameters of co-located stations in order to determine coordinate seasonal variations at every ITRF co-location site in a well-defined reference frame. The level of agreement of seasonal parameters estimated by different techniques can thus be rigorously evaluated in both horizontal and vertical components. In addition, we model and supplement the elastic deformation induced by surface loading derived from the Gravity and Recovery Climate Experiment (GRACE) and the thermoelastic deformation of the ground to assess the quality of the combined seasonal parameters.