Cosmos 1884 was a Soviet Global Navigation Satellite System (GLONASS) satellite launched from the Baikonur cosmodrome aboard a Proton rocket. Originally established in order to locate the Soviet Union's civil aircraft and its merchant and fishing vessels, the signals were used by many American GPS system receivers as a complement/backup to the GPS system itself. The operational system contained 21 satellites in 3 orbital planes, with 3 on-orbit backups. Each satellite was identified by its slot number, which defined the orbital plane (1-8, 9-16, 17-24) and the location within the plane. The 3 orbital planes were separated 120 degrees, and the satellites within the same orbit plane by 45 degrees. The orbits were roughly circular with an inclination of about 64.8 degrees, a semi-axis of 25,440 km, and a period of 11h 15m 44s.