Terrestrial reference frame requirements within GGOS perspective

One of the main objectives of the promising and challenging IAG project GGOS (Global Geodetic Observing System) is the availability of a global and accurate Terrestrial Reference Frame for Earth Science applications, particularly Earth Rotation, Gravity Field and geophysics. With the experience gained within the activities related to the International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF), the combination method proved its efficiency to establish a global frame benefiting from the strengths of the various space geodetic techniques and, in the same time, underlining their biases and weaknesses. In this paper we focus on the limitation factors inherent to each individual technique and to the combination, such as the current status of the observing networks, distribution of the co-location sites and their quality and accuracy of the combined frame parameters. Results of some TRF and EOP simultaneous combinations using CATREF software will be used to illustrate the current achievement and to help drawing up future goals and improvements in the GGOS framework. Beyond these technical aspects, the overall visibility and acceptance of ITRS/ITRF as international standard for science and applications is also discussed.