LAGEOS
LAGEOS, or Laser Geodynamics Satellites, are a series of scientific research satellites designed to provide an orbiting benchmark for geodynamical studies of the Earth.The spacecraft are aluminum spheres covered with retro-reflectors, giving them the appearance of giant golf balls. They have no onboard sensors or electronics, and are not attitude controlled.
Measurements are made by transmitting pulsed laser beams from Earth ground stations to the satellites. The laser beams then return to Earth after hitting the reflecting surfaces; the travel times are precisely measured, permitting ground stations in different parts of the Earth to measure their separations to better than one inch in thousands of miles.
The LAGEOS satellites are able to determine positions of points on the Earth with extremely high accuracy due to the stability of their orbits.
The high mass-to-area ratio and the precise, stable (attitude-independent) geometry of the LAGEOS spacecraft, together with their extremely regular orbits, make these satellites the most precise position references available.
The LAGEOS mission consists of the following key goals:
- Provide an accurate measurement of the satellite's position with respect to Earth,
- Determine the planet's shape (geoid) and,
- Determine tectonic plate movements associated with continental drift.
There are two LAGEOS spacecraft, LAGEOS-1 launched in 1976, and LAGEOS-2 launched in 1992. As of 2004, both LAGEOS spacecraft are still in service.
An analysis of the laser-ranging data obtained by the two LAGEOS satellites, published in 1997, claimed to have found evidence of the frame-dragging effect predicted by Einstein's theory of general relativity with an accuracy of about 20 percent.