conventional terrestrial (CT)
The Conventional Terrestrial (CT) system is a geocentric Cartesian coordinate system. Coordinates in the CT system are normally denoted as (X, Y, Z) coordinates. The point (0, 0, 0) is defined as the centre of mass of Earth. The CT coordinate axes are aligned with the International Reference Pole and International Reference Meridian that are fixed with respect to Earth. The CT Z-axis passes through the geocentre and the International Reference Pole. The CT X-axis passes through the geocentre and a point on Earth's surface at zero latitude and zero longitude (the Greenwich meridian). The CT Y-axis is perpendicular to both the X and Z axes to form a right-handed Cartesian system.