Geodetic Satellites

A geodetic satellite is a spacecraft built to support satellite geodesy, the branch of geodesy that uses artificial satellites to measure the size and shape of the Earth, the precise location of points on its surface, and the structure of its gravitational field. The simplest geodetic satellites are entirely passive: dense spherical spacecraft covered in retroreflectors that carry no onboard instruments or power system, relying instead on laser ranging from ground stations that time the round trip of reflected laser pulses to fix the satellite's orbit with extreme precision. Because a passive geodetic satellite has no propulsion or electronics to fail and orbits far enough from the atmosphere to resist orbital decay for centuries, these spacecraft serve as long lived, stable reference points for measuring gradual changes in the Earth's crust and gravity field rather than for any single observational mission.