### Terrestrial Reference System (TRS)

The TRS is a global Cartesian reference system fixed to the Earth’s body with its origin at the Earth’s center of mass (including the mass of the atmosphere). Such a system is commonly referred to as Earth-Centered Earth-Fixed (ECEF). The *z*-axis of this system coincides with the mean rotational axis of the earth. The *xy*- and *xz*-planes of this system are formed by the mean equatorial plane perpendicular to the *z*-axis and the mean meridian plane of Greenwich, respectively. The *x*-axis passes through the meridian plane and the *y*-axis is directed in a way that the system is right handed.

The addition of an ellipsoid as a reference surface to the Cartesian reference system allows the use of geodetic coordinates. The center of the ellipsoid coincides with the origin of the system and the semi-minor axis with the z-axis. The conversion from Cartesian xyz coordinates to ellipsoidal latitude, longitude, and height and vice versa can be done in a rigorous way.

This system co-rotates with the earth in its diurnal motion in space and therefore, positions of points anchored on the Earth’s solid surface have coordinates which undergo only small variations with time, due to geophysical effects such as tectonic or tidal deformations. Time variations of this system are continuously monitored with respect to a quasi-inertial system.

Figure 3. Geocentric earth-centered earth-fixed *x*-*y*-*z* reference system