SECOR spacecraft are satellites used by the United States armed forces for geodesy measurements that precisely determined the locations of points on the Earth's surface. Experiments with these spacecraft led to the Timation and GPS Navstar systems.[1] SECOR 1 was launched on 11 January 1964 and SECOR 13 on 14 April 1969.[1][2] SECOR was an abbreviation for "Sequential Collation of Range".[3]