7. Measurements from space

Satellite orbits are influenced by the gravity field. This is one of the forces acting on the satellite, preventing it from following the perfectly elliptical trajectory predicted by Kepler's laws for an ideal Earth with no lateral density heterogeneities. Analysis of these orbit perturbations, known as orbitography, provides a better understanding of the forces acting on the satellite, and in particular the gravity field. Since the 1970s, we have been able to determine the long spatial wavelengths (typically in excess of 500 km) of the Earth's gravity field by accumulating observations on numerous dedicated satellites (e.g. Starlette) and non-dedicated satellites (e.g. Spot satellites).