Forty-five years ago today — June 23, 1966 — the Passive Geodetic Earth Orbiting Satellite was launched by a Thor-Agena rocket from Vandenberg Air Force Base, California.
(Test inflation of PAGEOS. Note the relative sizes of the trucks and people in the lower right. The test took place in a blimp hangar in Weeksville, North Carolina. NASA image from Wikimedia Commons.)
The PAGEOS was an inflatable sphere exactly 100 feet (30.48 meters) in diameter, made of aluminized mylar. Being highly reflective, it was used as a tracking target by ground stations; triangulating on the spacecraft, since its orbit was known to a high degree of accuracy, allowed the ground stations to improve the accuracy of world survey maps.by