The first occultation experiment was proposed when JPL was characterizing the precise refraction effects of Earth's atmosphere, with its known structure and composition, for the purpose of tracking spacecraft. Incidentally, occultation may be one method of identifying extrasolar planets . Some Earth-based telescopes, and proposed orbiting instruments, are equipped with an occulting disk to block out the light of the central star and facilitate examining the star's planetary disk. Scattered light in the instrument limits its effectiveness, but studies exist for orbiting a separate large occulting disk to improve the experiment's effectiveness from Earth-based or orbiting telescopes. Gravity Field Surveying Planets are not perfectly spherical.
Terrestrial planets are rough-surfaced, and most planets are at least slightly oblate.
Thus they have variations in their mass concentrations, sometimes associated with mountain ranges or other features visible on the surface. These differences are due to the variation of mass distribution on and beneath the surface. There are two reasons for surveying the gravity field of a planet. First, highly accurate navigation in orbit at a planet requires a good model of variations in the gravity field, which can be obtained by such a survey. They are extremely valuable in determining the nature and origin of features identifiable in imaging data.