SECOR

SECOR (Sequential Collation of Ranges)[1] was a series of small United States Armed Forces satellites launched in the 1960s for geodesy measurements that precisely determined the locations of points on the Earth's surface, particularly of isolated islands in the Pacific Ocean.

This data allowed for improved global mapping and precise positioning of ground stations for other satellites.

The Type 1 SECOR satellites were spherical, 50.8 cm (20.0 in) in diameter, in construction similar to Vanguard III and the early SOLRAD/Galactic Radiation and Background (GRAB) spacecraft.

Their surface was made of polished aluminium covered with a thin layer of silicon monoxide to help with thermal regulation.

These devices were used to stop undesired rotation induced by launch vehicle separation, and by passing near the magnetic poles of Earth.

The current in the coils generated a magnetic field of its own that opposed Earth's, thus slowing the satellite's rotation.

The initial braking took several days because of the weak magnetic field at the typical SECOR orbital altitudes.

While the specific implementation was done on a case-by-case basis, the general idea was to place a transponder either outside/inside of a host satellite, sharing power, antennas, and telemetry with other experiments.

Lighter weight, solid-state electronics equipment was eventually developed to replace the initial units.

[3] The accuracy of the position calculations was enhanced by the great amount of data obtained in each satellite pass.

Because depending on the exact angle between the satellite and a ground station, the precision could have variation, data was collected during several passes.