## 1 Reference Systems--Relativistic Positioning vs. Localizing Systems
Basically, a'reference system' (RS) is a _set of procedures_ for creating a'reference frame' (RF) from which space and time coordinates can be ascribed to \"geographic\" locations belonging to a given, extended material body such as, for examples, Earth, Moon, Mars, etc. The RS named WGS84 ( _World Geodetic System 1984_) is the set of procedures used by the GPS for Earth. The set of procedures used by Galileo is named ITRS (_International Terrestrial Reference System_). Beside, the RF of the reference systems WGS84 is called merely _WGS84 Reference frame_ defined in a very similar way to the ITRF (_International Terrestrial Reference Frame_). For Galileo, the RF is named GTRF09v1 (_Galileo Terrestrial Reference Frame 2009 ver. 1_), which is a particular frame among those provided by the ITRS (_International Terrestrial Reference System_). The reference system WGS84 has three main features: a Cartesian system of coordinates also called _grid of [Cartesian] coordinates_ provided by the particular terrestrial frame called the 'WGS84 _Reference frame_' as indicated above, a reference oblate ellipsoid called the '_datum_' from which a zero altitude can be somehow defined for Earth's surface, and a gravitational, nominal _mean sea level_ equipotential surface called the '_geoid_' which is defined from the EGM96 (_Earth Gravitational Model_ 1996). Now, a positioning system (PS) is the physical realization of a grid of coordinates, i.e., a \"physical grid\". Let us note that the circles of latitude (parallels) and longitude (meridians) have no physical counterparts; unless we can find a very improbable way to draw lines with, for instance, colored pigments on the whole of Earth's surface. Actually, electromagnetic signals are used which \"draw\" a grid of coordinates varying in time called _emission grid_. Each coordinated \"line\" are then electromagnetically \"stamped\" by so-called _time stamps_. Actually, these grids of coordinates are physically achieved by signages in space and time which assume existence of physical media for digital informations. Such physical media are well known for road networks for instance with milestones. In the case of PSs, physical media are electromagnetic fields carrying digital informations, which are dates also known as 'time stamps' broadcast by clocks on board satellites. Now, what are the differences between \"classical\" and relativistic PSs? At the present days, all positioning systems (GPS, Galileo, Beidou, etc.) are classical PSs because the time stamps must be corrected for the relativistic effects such as time dilatation for instance. Without these corrections, the grids provided by current PSs which are grids of light coordinates, i.e., emission grids, do not correspond to the grids of null geodesics defined from the systems of reference (e.g., WGS84). More precisely, the process of positioning is a transitive process: the users are positioned relative to satellites of the PSs and the satellites are positioned with respect to the RF via ground stations. Then, the users are positioned in the RF. But, relativistic corrections must be made regularly over time between the space (satellites) and control (reference frame/ground stations) segments by uploading to the satellites, ephemerids, and orbits, corrected by ground calculations and from which the time stamps transmitted to users are modified. These calculation processes carried out in ground stations and uploaded to satellites are eliminated in the case of RPSs. It is through a process known as 'auto-location' [14], entirely relativistic and autonomous, that the correct ephemerids and orbits of satellites are continuously and permanently determined in time without intermediate calculations of relativistic corrections. Let us note that ground stations as \"ancillary ground satellites\" must be included as crucial parts in the auto-location process. Thus, the relationship between the space and control segments becomes entirely relativistic in nature and defines somehow a unique \"space-control segment\".