A global gravity field model is a mathematical representation of the real gravity pull of the Earth over its entire surface. As our planet is not perfectly spherical, has an uneven distribution of mass (due to features such as mountains, ocean trenches and dense rock) and rotates permanently, the gravity varies slightly from place to place and over time. These mathematical models enable scientists and engineers to calculate different gravity-related quantities at any given location or elevation on and outside Earth.

To precisely determine the variations in the Earth’s gravity, geodesists use a mathematical model of the Earth as a reference body. This model is called the reference ellipsoid and is a mathematical apparatus of the planet. The reference ellipsoid has the same size and flattening as the Earth, contains the same mass as the Earth and rotates at the same rate as the Earth. These four properties make it possible to infer the ellipsoid’s gravity field, commonly known as the normal gravity field. Unlike the real Earth, the mass distribution inside the ellipsoid is assumed to be homogeneous. Geodesists therefore focus on identifying the differences in gravity between the real Earth and the ellipsoid.