Iridium satellite constellation

Coverage of Earth by the Iridium satellites which are arranged in 6 orbits of 11 satellites each.

The Iridium satellite constellation is a satellite constellation providing voice and data coverage to satellite phones, pagers and integrated transceivers over the 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.^[2] Satellites are in low Earth orbit at a height of approximately 485 mi (781 km) and inclination of 86.4°. Orbital velocity of the satellites is approximately 17,000 mph (27,000 km/h). Satellites communicate with neighboring satellites via K_a band 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.^[3]

Satellites

The satellites each contain seven Motorola/Freescale PowerPC 603E processors running at roughly 200 MHz,^[4] 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.^[5] 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^[6] and weighs about 1,500 lb (680 kg).^[7] 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.

In-orbit spares

Spare satellites are usually held in a 414 mi (666 km) storage orbit.^[2] These will be boosted to the correct altitude and put into service in case of a satellite failure. After the Iridium company emerged from bankruptcy the new owners decided to launch seven new spares, which would have ensured two spare satellites were available in each plane. As of 2009 not every plane has a spare satellite; however, the satellites can be moved to a different plane if required. A move can take several weeks and consumes fuel which will shorten the satellite's expected service life.

Significant orbital plane changes are normally very fuel-intensive, but orbital perturbations aid the process. The Earth's equatorial bulge causes the orbital right ascension of the ascending node (RAAN) to precess at a rate that depends mainly on the period and inclination. Iridium satellites have an inclination of 86.4°, so every satellite in a retrograde (inclination < 90°) orbit, their equator crossings steadily precess westward.

A spare Iridium satellite in the lower storage orbit has a shorter period so its RAAN moves westward more quickly than the satellites in the standard orbit. Iridium simply waits until the desired RAAN (i.e., the desired orbital plane) is reached and then raises the spare satellite to the standard altitude, fixing its orbital plane with respect to the constellation. Although this saves substantial amounts of fuel, it can be a time-consuming process.

Next-generation constellation

Iridium is currently developing, and is expected to launch beginning in 2016, Iridium NEXT, a second-generation worldwide network of telecommunications satellites, consisting of 66 satellites, with six in-orbit and nine on-ground spares. These satellites will incorporate features such as data transmission which were not emphasized in the original design.^[8] The original plan was to begin launching new satellites in 2014.^[9] Satellites will incorporate additional payload for Aireon, Inc.^[10] and perhaps cameras and sensors in collaboration with some customers and partners. Iridium can also be used to provide a data link to other satellites in space, enabling command and control of other space assets regardless of the position of ground stations and gateways.^[8] The constellation will provide L-band data speeds of up to 1.5 Mbit/s and high-speed Ka-Band service of up to 8 Mbit/s.^[11][12]

The existing constellation of satellites is expected to remain operational until Iridium NEXT is fully operational, with many satellites expected to remain in service until the 2020s. Iridium is planning for the next-generation of satellites to have improved bandwidth. This system will be backward compatible with the current system. In August 2008, Iridium selected two companies—Lockheed Martin and Thales Alenia Space—to participate in the final phase of the procurement of the next generation satellite constellation. On June 2, 2010 the winner of the contract was announced as Thales Alenia Space, in a $2.1 billion deal underwritten by Compagnie Française d'Assurance pour le Commerce Extérieur.^[13] Iridium additionally stated that it expected to spend about $800 million to launch the satellites and upgrade some ground facilities.^[14]

In June 2010, Iridium signed the largest commercial rocket launch deal ever at that time, a US$492 million contract with SpaceX to launch 70 Iridium NEXT satellites on seven Falcon 9 rockets from 2015–2017 via the Vandenberg AFB Space Launch Complex 3.^[15] The final two satellites will be orbited by a single launch^[16] of an ISC Kosmotras Dnepr-1.^[11] Technical issues and consequential demands from Iridium’s insurance delayed the launch of the first pair of Iridium NEXT satellites until April 2016.^[17]

Patents and manufacturing

The primary patent for the system is "Satellite cellular telephone and data communication system," United States Patent 5,410,728, and Motorola also generated hundreds of additional patents as they developed and built the Iridium system.

The main patents on the Iridium system, U.S. Patents 5,410,728 and 5,604,920, are in the field of satellite communications, and the manufacturer generated several hundred patents protecting the technology in the system. Satellite manufacturing initiatives were also instrumental in the technical success of the system. Motorola made a key hire of the engineer who set up the automated factory for Apple's Macintosh. He created the technology necessary to mass-produce satellites on a gimbal, taking weeks instead of months or years and at a record low construction cost of only US$5 million per satellite. At its peak during the launch campaign in 1997 and 1998, Motorola produced a new satellite every 4.3 days, with the lead-time of a single satellite being 21 days.^[18]

Launch campaign

Motorola used launch vehicles from three companies from three different countries—the Delta II from McDonnell Douglas; the Proton K from Krunichev in Russia; and the Long March IIC from China Aerospace Science and Technology Corporation.

Defunct satellites

Over the years several Iridium satellites have ceased to work and are no longer in active service, some are partially functional and have remained in orbit whereas others have tumbled out of control or have reentered the atmosphere.^[19]

Iridium 21, 27, 20, 11, 24, 71, 44, 14, 79, 69 and 85 all suffered from issues before entering operational service soon after their launch in 1997. Of which Iridium 27, 79 and 85 have since decayed out of orbit; Iridium 11, 14, 20 and 21 have been since renamed to Iridium 911, 914, 920 and 921 respectively since replacements of the same name were launched.^[20]

Iridium 33 collision

	Wikinews has news coverage of the 2009 satellite collision Russian and US satellites collide Burning debris from satellites spotted over several US cities:

At 16:56 UTC on February 10, 2009 Iridium 33 collided with the defunct Russian satellite Kosmos 2251.^[21] This was the first time two intact satellites collided.^[22] Iridium 33 was in active service when the accident took place but was one of the oldest satellites in the constellation, having been launched in 1997. The satellites collided at roughly 35,000 km/h (22,000 miles per hour)^[23]

Iridium moved one of its in-orbit spares, Iridium 91 (formerly known as Iridium 90) to replace the destroyed satellite,^[24] completing the move on March 4, 2009.

References

External links

• Iridium Satellite LLC corporate web site
• Iridium satellite tracking
• Iridium satellite phone descriptions
• Sky Rocket

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