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.^[2]

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 ground stations were transportable. They consisted of three shelters, one for radio equipment, one for data handling, and one for storage. Generators and air conditioning for the electronic equipment were included.

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

• Determine the distance between the satellite and the stations several times.
• Compute the position of the satellite relative to the three known stations by trilateration.
• Compute the possible positions of the unknown ground station that could yield the measured distance.
• Repeat the cycle, narrowing down the set of possible positions with each satellite pass until the accuracy is deemed acceptable.

First, the distances between each station and a satellite were obtained and recorded. The distances between the known positions and this satellite were used to determine the satellite's location in space, and then, the previously measured distance from the fourth station and the calculated satellite position were used to calculate the coordinates of the fourth station with respect to the three known stations.^[3]

The accuracy of the position calculations was enhanced by the great amount of data obtained in each satellite pass. At about 70 measurements per second, and considering the time the satellite was visible from the ground station, a typical satellite pass yielded approximately 48,000 measurements. Because depending on the exact angle between the satellite and a ground station, the precision could have variation, data was collected during several passes. This allowed data from the best passes to be selected, while still conserving a great amount of redundancy.

Once enough measurements had been made to ensure the unknown position was precisely established, one of the stations was moved to a different place and became the new unknown position. In this way, former measurements helped establish new positions.^[2]

Sources differ in details such as names and launch dates. When there are inconsistencies, sources are provided for both.

EGRS (Engineer Geodetic Research Satellite) was the U.S. Army Corps of Engineers acronym for the SECOR satellites.