SECOR

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SECOR (Sequential Collation of Ranges) was a group of small U.S. military satellites in the 1960s designed to measure exact locations on Earth. It helped improve global maps and the placement of ground stations for other satellites, especially on isolated Pacific islands.

How SECOR worked
Each mission used four mobile ground stations: three at known locations and a fourth at an unknown point. Distances from the satellite to the three known stations let scientists determine the satellite’s position in space. Then the distance from the unknown station to that satellite was used to calculate the unknown station’s coordinates. The process was repeated many times to improve accuracy. Once the unknown station’s position was found, it became a known station, and one of the four stations moved to a new unknown point to start again.

Legacy
SECOR was a forerunner of later navigational satellite systems, such as Timation and the Global Positioning System (GPS). Fifteen dedicated SECOR satellites were built, and eight SECOR transponders flew on other satellites.

Satellites and versions
There were two versions of SECOR satellites: Type I and Type II. Type I satellites were spherical, about 50.8 cm in diameter, and weighed about 16.8 kg. They looked similar to some early other research satellites, had nine antennas (eight around the equator for distance measurements and one on top for telemetry), and used polished aluminum with a thin heat-control coating. Inside, batteries and voltage regulators were arranged to keep a stable power supply for accurate operation. A magnet inside helped align the satellite with Earth’s magnetic field, and despin coils slowed unwanted rotation.

Type II satellites were rectangular blocks, roughly 25.3 x 29.8 x 34.9 cm, almost fully covered in solar cells, with flexible-steel- tape antennas. They were more compact and designed to serve as secondary payloads on other missions, with antennas perforated to reduce shadowing on the solar panels.

Ground stations and data
The ground stations were portable, with separate shelters for radio equipment, data handling, and storage, plus power and air conditioning for all-weather operation. Data were recorded on magnetic tape and processed by a computer.

Transponders
SECOR transponders could be attached to host satellites, sharing power, antennas, and telemetry with the payloads they accompanied.

Data collection
During a satellite pass, roughly 70 measurements per second were recorded, and a typical pass yielded about 48,000 measurements. By combining data from several passes, engineers achieved precise position calculations. When enough measurements had established an unknown point, that station was moved to a new unknown location, and the process began again.

Note
EGRS, or Engineer Geodetic Research Satellite, was the Army Corps of Engineers’ acronym for the SECOR satellites.