External Evaluation of the Terrestrial Reference Frame: Report of the Task Force of the IAG Sub-commission 1.2

Ideally, the origin of the Terrestrial Reference Frame (TRF) is defined as the center of mass of the whole Earth system, the time evolution of its orientation is such that no global net rotation of the whole Earth’s surface is possible and the TRF scale is specified through the adoption of some physical constants and time-scale. These parameters need to be accurately determined since their choice has an influence on many Earth’s science applications. The aim of the task force “External evaluation of the Terrestrial Reference Frame” is to review all the applications for which the TRF accuracy is of fundamental importance. As the TRF choice has an influence on the interpretation of the results in these specific applications, we investigate if some evaluation procedures could be established. We classified the methods that allow evaluation of the TRF using ground, geodetic data or models that have not been used in the TRF construction, based on their expected contributions. Some of these methods have been applied to the latest International Terrestrial Reference System realizations and the results are presented here. Although further analysis will be necessary to deliver a more precise error budget, our findings demonstrate that the most recent realizations of the ITRS are more accurate than the previous in terms of origin and scale rate definition. The current level of ITRF2008 accuracy is likely to be at the level of 0.5 mm/year along each origin component and better than 0.3 mm/year in the scale rate according to the most recent studies.