ITRF2000: From Theory to Implementation

One of the main geodetic activities is to determine positions of points or objects over the Earth surface or nearby space. Meanwhile when dealing with geodetic observations, a certain Terrestrial Reference System (TRS) has to be adopted in which point positions will be expressed. Since there are many ways to realize the adopted TRS, the most challenging issue is to select the “best” one that preserves the actual quality of geodesy observations. Conversely, noting that the observations are, and should remain, insensitive to the adopted theoretical TRS, they should not be altered by the added pseudo observations (termed as constraints) permitting to specify a Terrestrial Reference Frame (TRF) as the “best” realization of the TRS. In this paper we will focus on the main important parameters needed to define a TRF (called datum definition), as well as the current achievement of Space Geodesy techniques, through the activities related to the International Terrestrial Reference Frame (ITRF). The use of minimum constraints approach is emphasized both in case of individual TRS realization by space geodesy techniques and in combination, allowing to precisely define the ITRF datum definition. Some important results of the latest ITRF solution, namely ITRF2000, will be used to illustrate the discussion over two axes: accuracy of the datum definition and the current quality of space geodesy solutions used in the ITRF2000 combination.