Importance of a Common Framework for the Realization of Space-Time Reference Systems

General relativity establishes a distinction between proper quantities which are directly measurable and coordinate quantities which depend on some conventions. While basic mesurements are proper quantities (in nearly all cases either time or frequency is measured), coordinates are the necessary tool with which to study, by modeling, the interactions of physical phenomenon with the measurements. They are also the basic instrument for exchanging and summarizing the results of the measurements, mostly through the coordinates of objects (geodetic station, radio-source…) realizing a space reference.