Precise measurements of satellite orbits by ranging (distance) and other technologies yield accurate determination of the geoid shape and its variations over time. The geoid is defined as the equipotential surface of Earth's gravity field which best fits global mean sea level. It describes the mass distribution within the earth, from which one can infer Earth’s dynamic structure. The new generation of gravity satellites are also very sensitive to short term changes in the geoid reflecting redistribution of the mass of the surface such as ice melt or large scale seasonal changes in water budget Geoid measurements are also crucial for calibrating GPS-determined height with a standard mean sea level datum.