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

Abstract

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.The paper addresses the issue of how the realization of space-time references may be affected by the choice of coordinate conventions. Some techniques used in space geodesy and astrometry will be studied, stressing how the model chosen, or other assumptions made explicitly or implicitly, influence the results (coordinate quantities). Particular emphasis will be on the issues on time. Indeed the prospects in time metrology are such that new definitions should be drawn to make the best use of the next generation of clocks and be able to compare them. In all fields, however, it is of ever increasing importance for all users to deal with properly defined quantities and common conventions, since the measurement uncertainty in any technique is bound to decrease. For this reason, the Bureau International des Poids et Mesures and the International Astronomical Union have created in 1997 the Joint Committee on relativity for space-time reference systems and metrology, which has among its tasks “to establish definitions and conventions to provide a coherent relativistic frame for all activities in space-time references and metrology at a sufficient level of uncertainty”.KeywordsFrequency StandardSpace GeodesyInternational AstronomicalBarycentric SystemAtomic Time ScaleThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.