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Earth Centered, Earth Fixed coordinates
Earth Centered, Earth Fixed coordinates in relation to latitude and longitude.

ECEF ("Earth-Centered, Earth-Fixed"), also known as ECR ("Earth Centered Rotational"), is a geographic coordinate system and Cartesian coordinate system, and is sometimes known as a "conventional terrestrial" system.[1] It represents positions as an X, Y, and Z coordinate. The point (0,0,0) is defined as the center of mass of the Earth,[2] hence the name Earth-Centered. Its axes are aligned with the International Reference Pole (IRP) and International Reference Meridian (IRM) that are fixed with respect to the surface of the Earth,[3][4] hence the name Earth-Fixed. This term can cause confusion since the Earth does not rotate about the z-axis (unlike an inertial system such as ECI), and is therefore alternatively called ECR.

The z-axis is pointing towards the north but it does not coincide exactly with the instantaneous Earth rotational axis.[3] The slight "wobbling" of the rotational axis is known as polar motion.[5] The x-axis intersects the sphere of the Earth at 0° latitude (Equator) and 0° longitude (Greenwich). This means that ECEF rotates with the earth and therefore, coordinates of a point fixed on the surface of the earth do not change. Conversion from a WGS84 Datum to ECEF can be used as an intermediate step in converting velocities to the North East Down coordinate system.

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