Distance géocentrique

The Earth-centered, Earth-fixed coordinate system (acronym ECEF), also known as the geocentric coordinate system, is a cartesian spatial reference system that represents locations in the vicinity of the Earth (including its surface, interior, atmosphere, and surrounding outer space) as X, Y, and Z measurements from its center of mass. Its most common use is in tracking the orbits of satellites and in satellite navigation systems for measuring locations on the surface of the Earth, but it is also used in applications such as tracking crustal motion. The distance from a given point of interest to the center of Earth is called the geocentric distance, R = (X2 + Y2 + Z2)0.5, which is a generalization of the geocentric radius, R0, not restricted to points on the reference ellipsoid surface. The geocentric altitude is a type of altitude defined as the difference between the two aforementioned quantities: = R − R0; it is not to be confused for the geodetic altitude. Conversions between ECEF and geodetic coordinates (latitude and longitude) are discussed at geographic coordinate conversion. As with any spatial reference system, ECEF consists of an abstract coordinate system (in this case, a conventional three-dimensional right-handed system), and a geodetic datum that binds the coordinate system to actual locations on the Earth. The ECEF that is used for the Global Positioning System (GPS) is the geocentric WGS 84, which currently includes its own ellipsoid definition. Other local datums such as NAD 83 may also be used. Due to differences between datums, the ECEF coordinates for a location will be different for different datums, although the differences between most modern datums is relatively small, within a few meters. The ECEF coordinate system has the following parameters: The origin at the center of the chosen ellipsoid. In WGS 84, this is center of mass of the Earth. The Z axis is the line between the North and South Poles, with positive values increasing northward.