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Concept
SECOR
Summary
SECOR (Sequential Collation of Ranges) 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.
Any SECOR satellite could be linked to four mobile ground stations: three were placed in accurately determined known locations, and a fourth one was placed in an unknown location. By measuring a satellite's distance from the three known stations, its position in space was determined. Then, the distance between the unknown ground station and the previously determined satellite's position was used to compute the unknown ground station's coordinates. This process was repeated many times, to enhance the accuracy of the measurement. Once the unknown station's position was accurately determined, it became a known station. Then one of the four stations was moved to a new unknown point, and the process began again.
SECOR was a predecessor to navigational satellite systems such as Timation and Navstar-GPS (Global Positioning System).
Fifteen dedicated SECOR satellites were made, and eight SECOR transponders flew as a subsystem of other satellites.
Two versions of SECOR satellites were built, Type I and Type II. They had several differences, but they also shared many features. Therefore, a detailed description is provided for Type I, and a differences-only summary for Type II.
The Type 1 SECOR satellites were spherical, in diameter, in construction similar to Vanguard III and the early SOLRAD/Galactic Radiation and Background (GRAB) spacecraft. The satellites massed in average, with most of it being batteries and voltage regulators. Their surface was made of polished aluminium covered with a thin layer of silicon monoxide to help with thermal regulation. There were nine collapsible antennas, eight around the equator for distance measurement and one atop the sphere for telemetry and command.
Official source
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