tisdag 13 augusti 2013

Iridium satellite constellation


Iridium satellite constellation

From Wikipedia, the free encyclopedia
An Iridium satellite
The Iridium satellite constellation is a large group of satellites providing voice and data coverage to satellite phones, pagers and integrated transceivers over Earth's entire surface.Iridium Communications Inc. owns and operates the constellation and sells equipment and access to its services. It was originally developed in 1992, and subsequently implemented in October of 1999.
The constellation consists of 66 active satellites in orbit, and additional spare satellites to serve in case of failure.[1] 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 Ka bandinter-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 at the North and South poles, where there are few customers. 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. (The element with the atomic number 66 is dysprosium.) This reduced set of 6 planes is sufficient to cover the entire Earth's surface at every moment.
Because of the unique shape of the Iridium satellites' reflective antennas, 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.[2]

Satellites[edit source | editbeta]

Comparison of GPSGLONASSGalileo andCompass (medium earth orbit) satellite navigation system orbits with the International Space StationHubble Space Telescope andIridium constellation orbits, Geostationary Earth Orbit, and the nominal size of the Earth.[a] TheMoon's orbit is around 9 times larger (in radius and length) than geostationary orbit.[b]
The satellites each contain seven Motorola/Freescale PowerPC 603E processors running at roughly 200 MHz,[3] 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.[4] The four inter-satellite cross links on each satellite operate at 10 Mbit/s. The inventors of the system had previously worked on a government study[citation needed] in the late 1980s that showed that microwave cross links were simpler and had fewer risks than optical cross links. 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. In recent press releases[citation needed], 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.
The original design envisioned a completely static 1960s "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. 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[5] and weighs about 1,500 lb (680 kg).[6] 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.
Owing to the highly reflective antennas, Iridium satellites cause a phenomenon known as Iridium flares, watched by enthusiasts and sometimes visible in daylight.[7][8]

In-orbit spares[edit source | editbeta]

Spare satellites are usually held in a 414 mi (666 km) storage orbit.[1] 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 like every satellite in a prograde (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, this can be a time-consuming process.

Next-generation constellation[edit source | editbeta]

Iridium is currently developing, and is expected to launch beginning in 2015, Iridium NEXT, a second-generation worldwide network of telecommunications satellites, consisting of 66 satellites and six in-orbit and nine ground spares. These satellites will incorporate features such as data transmission which were not emphasized in the original design.[9] The original plan was to begin launching new satellites in 2014.[10] Satellites will incorporate additional payload such as 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.[9] 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 andThales 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 $2.9 billion deal underwritten by Compagnie Française d'Assurance pour le Commerce Extérieur.[13]
In June 2010, Iridium signed the largest commercial rocket launch deal ever, a US$492 million contract with SpaceX to launch tens of Iridium NEXT satellites on multiple Falcon 9 launchers in 2015-2017 from Vandenberg AFB Space Launch Complex 3.[14] The 66 operational satellites in the constellation, plus six on-orbit spares, 70 satellites will be put in orbit by seven launches of 10 satellites each on the Falcon 9, plus two of the 800 kilograms (1,800 lb) Iridium NEXT satellites on a single launch[15] of the an ISC KosmotrasDnepr rocket, beginning in 2015 and completing the refresh of the entire constellation by 2017, as of August 2012.[11]

Patents and manufacturing[edit source | editbeta]

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 forApple'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.[16]

Launch campaign[edit source | editbeta]

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. The original constellation of 66 satellites, plus six spares, was launched in 12 months and 12 days, between May 5, 1997, and May 17, 1998, with an astounding success rate of 15 out of 15 successful launches and all 72 satellites put into the intended orbits. In one 13-day period (late-March to early-April 1998) they successfully put 14 satellites into orbit.[citation needed]
The most recent launches took place in 2002 when a total of seven spare satellites were launched.[citation needed]

Defunct satellites[edit source | editbeta]

Over the years several Iridium satellites have ceased to work and tumbled out of control, some have reentered the atmosphere while other partially functional satellites have remained in orbit. However these satellites are not in active service.[17]

Iridium 28[edit source | editbeta]

Iridium 28 failed in July 2008 and was replaced with the in-orbit spare Iridium 95.[18]

Iridium 33[edit source | editbeta]

At 16:56 UTC on February 10, 2009 Iridium 33 collided with the defunct Russian satelliteKosmos 2251.[19] This was the first time two intact satellites collided.[20] 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 22,000 miles per hour; roughly 32 times faster than a bullet in flight.[21]
Iridium moved one of its in-orbit spares to replace the destroyed satellite,[22] completing the move on March 4, 2009.

Notes[edit source | editbeta]

  1. ^ Orbital periods and speeds are calculated using the relations 4π²R³ = T²GM and V²R = GM, where R = radius of orbit in metres, T = orbital period in seconds, V = orbital speed in m/s, G = gravitational constant ≈ 6.673×10−11 Nm²/kg², M = mass of Earth ≈ 5.98×1024 kg.
  2. ^ Approximately 8.6 times when the moon is nearest (363 104 km ÷ 42 164 km) to 9.6 times when the moon is farthest (405 696 km ÷ 42 164 km).

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