The spacecraft measurements of a planet's concentrations of mass. Planets are not perfectly round, having variations in mass, as in a mountain range. A spacecraft uses its radio and ground stations to measure the Doppler shifts in its orbit. After removing factors in the shifts due to planetary movement, solar wind, and other phenomena, the remaining differenced Doppler measurement indicates the variations in mass distribution at and below the planet's surface. NASA's Jet Propulsion Laboratory pioneered the technique, which aids the accurate navigation of spacecraft and helps geologists locate the Earth's petroleum and mineral deposits.
Space probes have also carried out gravity-field surveys, including the Mars Global Surveyor mission and the Magellan spacecraft survey of the planet Venus.
Subjects: Astronomy and Astrophysics.