General Principles for the Realization of Reference Systems for Earth Dynamics
A reference system is useful to express conveniently the location of a point and to study kinematic and dynamic phenomena. Although the definition of a reference system is theoretically very simple, its realization becomes very fifficult since it must be achieved by measurements reduced and expressed in this system.A referenoe system can be materialized by direct measurements of the coordinates or, more frequently, indirectly by measuring differential quantities and calculating the coordinates “de proche en proche.” The accuracy of the system definition cannot be better than the accuracy of the measurements themselves.For Earth dynamics we need a reference system related to the Earth, but since observations of other bodies such as galaxies, stars, planets, and satellites also may be required, the reference systems in which the positions of these bodies are expressed must also be related to the selected terrestrial system.To optimize the selection of a reference system one must consider both the phase of the operations for the establishment of the system and the phase of the utilization of the system, because the establishment of a reference system is not the final goal.