IRIDIUM 91, IRIDIUM 90, IRIDIUM 94, IRIDIUM 95, and IRIDIUM 96 are additions to the fleet of 73 (66 + 7 spares) IRIDIUM satellites. They were launched by a Delta 2 rocket from Vandenberg AFB at 17:44 UT on 2002 February 11. The fleet enables links between phones from anywhere to anywhere on the globe. Currently, its main user is the US Department of Defense (DOD). The geomagnetism community has found the data from the housekeeping magnetometers of the fleet usable. The parameters of the nearly circular orbits of all five were similar: period 98.0 min, altitude 660 km, and inclination 86.6 deg.
Iridium 91, 90, 94, 95, and 96 are part of a commercial communications network comprised of a constellation of 66 LEO spacecraft. The system uses L-Band to provide global communications services through portable handsets. A total of 125 spacecraft were built by Lockheed for more than $700M. Commercial service began in 1998. The system employs 15-20 ground stations with a master control complex in Landsdowne, VA, a backup in Italy, and a third engineering center in Chandler, AZ. The spacecraft are 3-axis stabilized, with a hydrazine propulsion system. They have 2 solar panels with 1-axis articulation. The system employed L-Band using FDMA/TDMA to provide voice at 4.8 kbps and data at 2400 bps with a 16 dB margin. Each satellite had 48 spot beams for Earth coverage and used Ka-Band for crosslinks and ground commanding.
The constellation of 66 active satellites has 6 orbital planes spaced 30 degrees apart, with 11 satellites in each plane (not counting spares). Each satellite can have four inter-satellite links: two to neighbors fore and aft in the same orbital plane, and two to satellites in neighboring planes to either side. The satellites orbit from pole to pole with an orbit of roughly 100 minutes. This design means that there is excellent satellite visibility and service coverage, even at the North and South poles, where there are few customers.
The original concept was to have 77 satellites. This is where the name Iridium came from; Iridium is the element with atomic number 77. Because of the unique shape of the Iridium satellites' reflective antennae, the satellites focus sunlight on a small area of the Earth's surface. This results in an effect called "Iridium flares", where the satellite momentarily appears as one of the brightest objects in the night sky.
On 10 February 2009, at 16:56 GMT, the Iridium-33 satellite, which was operational at the time, collided with the defunct Russian Cosmos 2251 satellite. This was the first, and so far the only, collision, between two intact satellites. The collision created thousands of pieces of space debris.
Facts in Brief
Launch Date: 2002-02-11
Launch Vehicle: Delta II
Launch Site: Vandenberg AFB, United States