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The Iridium satellite constellation is a satellite constellation providing voice and data coverage to satellite phones, pagers and integrated transceivers over Earth's entire surface. Iridium Communications owns and operates the constellation and sells equipment and access to its services. It was originally conceived by Bary Bertiger, Dr. Ray Leopold and Ken Peterson in late 1987 (and protected by patents by Motorola in their names in 1988) and then developed by Motorola on a fixed-price contract from July 29, 1993 to November 1, 1998 when the system became operational and commercially available. The constellation consists of 66 active satellites in orbit, and additional spare satellites to serve in case of failure.〔 Satellites are in low Earth orbit at a height of approximately and inclination of 86.4°. Orbital velocity of the satellites is approximately . Satellites communicate with neighboring satellites via inter-satellite links. 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. The over-the-pole orbital design produces "seams" where satellites in counter-rotating planes next to one another are traveling in opposite directions. Cross-seam inter-satellite link hand-offs would have to happen very rapidly and cope with large Doppler shifts; therefore, Iridium supports inter-satellite links only between satellites orbiting in the same direction. The constellation of 66 active satellites has 6 orbital planes spaced 30 degrees apart, with 11 satellites in each plane (not counting spares). The original concept was to have 77 satellites, which is where the name Iridium came from, being the element with the atomic number 77 and the satellites evoking the Bohr model image of electrons orbiting around the Earth as its nucleus. This reduced set of 6 planes is sufficient to cover the entire Earth's surface at every moment. Because of the shape of the Iridium satellites' reflective antennas, the satellites focus sunlight on a small area of the Earth's surface in an incidental manner. This results in an effect called Iridium flares, where the satellite momentarily appears as one of the brightest objects in the night sky and can even be seen during daylight.〔(【引用サイトリンク】url=http://www.satobs.org/iridium.html )〕 == Satellites == The satellites each contain seven Motorola/Freescale PowerPC 603E processors running at roughly 200 MHz,〔(【引用サイトリンク】title=How the Iridium Network Works )〕 connected by a custom backplane network. One processor is dedicated to each cross-link antenna ("HVARC"), and two processors ("SVARC"s) are dedicated to satellite control, one being a spare. Late in the project an extra processor ("SAC") was added to perform resource management and phone call processing. The cellular look down antenna has 48 spot beams arranged as 16 beams in three sectors. The four inter-satellite cross links on each satellite operate at 10 Mbit/s. Although optical links could have supported a much greater bandwidth and a more aggressive growth path, microwave cross links were favored because the bandwidth was more than sufficient for the desired system. Nevertheless, a parallel optical cross link option was carried through a critical design review, and ended when the microwave cross links were shown to support the size, weight and power requirements allocated within the individual satellite's budget. Iridium Satellite LLC has stated that their second generation satellites would also use microwave, not optical, inter-satellite communications links. Such cross-links are unique in the satellite telephone industry, as other providers do not relay data between satellites; Globalstar and Inmarsat both use a bent-pipe architecture without cross-links. The original design as envisioned in the 1960s was that of a completely static "dumb satellite" with a set of control messages and time-triggers for an entire orbit that would be uploaded as the satellite passed over the poles. It was found that this design did not have enough bandwidth in the space-based backhaul to upload each satellite quickly and reliably over the poles. Moreover, fixed, static scheduling would have left more than 90% of the satellite links idle at all times. Therefore, the design was scrapped in favor of a design that performed dynamic control of routing and channel selection late in the project, resulting in a one-year delay in system delivery. Each satellite can support up to 1100 concurrent phone calls at 2400 bit/s〔(【引用サイトリンク】title=How the Iridium Network Works )〕 and weighs about . The Iridium System presently operates within a 1618.85 to 1626.5 MHz band adjacent to the 1610.6–1613.8 MHz Radio Astronomy Service (RAS) band. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Iridium satellite constellation」の詳細全文を読む スポンサード リンク
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