Iridium satellite constellation
The Iridium satellite constellation provides L band voice and data information coverage to satellite phones, satellite messenger communication devices and integrated transceivers. Iridium Communications owns and operates the constellation, additionally selling equipment and access to its services. It was conceived by Bary Bertiger, Raymond J. Leopold and Ken Peterson in late 1987 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, required for global coverage, and additional spare satellites to serve in case of failure. Satellites are placed in low Earth orbit at a height of approximately and inclination of 86.4°. The nearly polar orbit and communication between satellites via Ka band inter-satellite links provide global service availability, regardless of the position of ground stations and gateways.
In 1999, The New York Times quoted a wireless market analyst, regarding people having "one number that they could carry with them anywhere" as "expensive... There never was a viable market."
Due to the shape of the original Iridium satellites' reflective antennas, the first generation satellites focused sunlight on a small area of the Earth surface in an incidental manner. This resulted in a phenomenon called Iridium flares, whereby the satellite momentarily appeared as one of the brightest objects in the night sky and could be seen even during daylight. Newer Iridium satellites do not produce flares.
Overview
The Iridium system was designed to be accessed by small handheld phones, the size of a cell phone. While "the weight of a typical cell phone in the early 1990s was 10.5 ounces" Advertising Age wrote in mid 1999 that "when its phone debuted, weighing and costing $3,000, it was viewed as both unwieldly and expensive."An omnidirectional antenna was intended to be small enough to be mounted on the planned phone, but the low handset battery power was insufficient for contact with a satellite in geostationary orbit, above the Earth; the normal orbit of communications satellites, in which the satellite appears stationary in the sky. In order for a handheld phone to communicate with them, the Iridium satellites are closer to the Earth, in low Earth orbit, about above the surface. With an orbital period of about 100 minutes a satellite can only be in view of a phone for about 7 minutes, so the call is automatically "handed off" to another satellite when one passes beyond the local horizon. This requires a large number of satellites, carefully spaced out in polar orbits to ensure that at least one satellite is continually in view from every point on the Earth's surface. At least 66 satellites are required, in 6 polar orbits containing 11 satellites each, for seamless coverage.
Orbit
Orbital velocity of the satellites is approximately. Satellites communicate with neighboring satellites via Ka band inter-satellite links. Each satellite can have four inter-satellite links: one each to neighbors fore and aft in the same orbital plane, and one each to satellites in neighboring planes to either side. The satellites orbit from pole to same pole with an orbital period of roughly 100 minutes. This design means that there is excellent satellite visibility and service coverage especially 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 six orbital planes spaced 30° apart, with 11 satellites in each plane. The original concept was to have 77 satellites, which is where the name Iridium came from; the element iridium has the atomic number 77, and the satellites evoked the Bohr model image of electrons orbiting around the Earth as its nucleus. This reduced set of six planes is sufficient to cover the entire Earth surface at every moment.History
The Iridium satellite constellation was conceived in the early 1990s as a way to reach high Earth latitudes with reliable satellite communication services. Early calculations showed that 77 satellites would be needed, hence the name Iridium, after the metal with atomic number 77. It turned out that just 66 were required to complete the blanket coverage of the planet with communication services.First generation
The first-generation constellation was developed by Iridium SSC, and financed by Motorola. The satellites were deployed in 1997–2002. All the satellites needed to be in orbit before commercial service could begin.Iridium SSC employed a globally diverse fleet of rockets to get their 77 satellites into orbit, including launch vehicles from the United States, Russia, and China. 60 were launched to orbit on twelve Delta II rocket carrying five satellites each; 21 on three Proton-K/DM2 rocket with seven each, two on one Rokot/Briz-KM rocket carrying two; and 12 on six Long March 2C/SD rocket carrying two each. The total setup cost for the first-generation fleet was approximately.
The first test telephone call was made over the network in 1998, and full global coverage was complete by 2002. However, although the system met its technical requirements, it was not a success in the market. Poor reception from inside buildings, bulky and expensive handsets, and competition with the conventional cellular phone contributed to its failure. Insufficient market demand existed for the product at the price points on offer from Iridium as set by its parent company Motorola. The company failed to earn revenue sufficient to service the debt associated with building out the constellation and Iridium went bankrupt, one of the largest bankruptcies in US history at the time.
The constellation continued operation following the bankruptcy of the original Iridium corporation. A new entity emerged to operate the satellites and developed a different product placement and pricing strategy, offering communication services to a niche market of customers who required reliable services of this type in areas of the planet not covered by traditional geosynchronous orbit communication satellite services. Users include journalists, explorers, and military units.
No new satellites were launched 2002–2017 to replenish the constellation, even though the original satellites based on the LM-700A model had been projected to have a design life of only 8 years.
Second generation
The second-generation Iridium-NEXT satellites began to be deployed into the existing constellation in January 2017. Iridium Communications, the successor company to Iridium SSC, has ordered a total of 81 new satellites being built by Thales Alenia Space and Orbital ATK: 66 operational units, nine on-orbit spares, and six ground spares.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.
, the original plan had been to begin launching new satellites in 2014.
The design was complete by 2010, and Iridium stated that the existing constellation of satellites would remain operational until Iridium NEXT is fully operational, with many satellites expected to remain in service until the 2020s, while the NEXT satellites would have improved bandwidth. The new system was to be backward-compatible with the current system. In June 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. Iridium additionally stated that it expected to spend about $800 million to launch the satellites and upgrade some ground facilities. This expenditures were partially financed with debt: between 2010 and 2017, the debt of Iridium Communications Inc. grew from $37.4 millions to $1,455.6 millions.
SpaceX was contracted to launch all the Iridium NEXT satellites. All the Iridium NEXT launches have taken place using a Falcon 9 rocket launch from Vandenberg Space Force Base in California. Deployment of the constellation began in January 2017, with the launch of the first ten Iridium NEXT satellites. Most recently, on January 11, 2019, SpaceX launched an additional ten satellites, bringing the number of upgraded satellites in orbit to 75.
Original Iridium constellation
The satellites each contained seven Motorola/Freescale PowerPC 603E processors running at roughly 200 MHz, connected by a custom backplane network. One processor was dedicated to each cross-link antenna, and two processors were dedicated to satellite control, one being a spare. Late in the project an extra processor was added to perform resource management and phone call processing.The cellular look down antenna had 48 spot beams arranged as 16 beams in three sectors. The four inter-satellite cross links on each satellite operated at 10 Mbit/s. Optical links could have supported a much greater bandwidth and a more aggressive growth path, but microwave cross links were chosen because their 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 stated that their second generation satellites would also use microwave, not optical, inter-satellite communications links. Iridium's cross-links are unique in the satellite telephone industry as other providers do not relay data between satellites; Globalstar and Inmarsat both use a transponder 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 favour 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 concurrent phone calls at bit/s and weighs about. The Iridium System presently operates within a dedicated band segment from to MHz and shares with Globalstar a band segment from to MHz. These segments are part of the wider L band, adjacent to the Radio Astronomy Service band segment from to MHz.
The configuration of the Satellite concept was designated as Triangular Fixed, 80 Inch Main Mission Antenna, Light-weight. The packaging design of the spacecraft was managed by Lockheed Bus Spacecraft team; it was the first commercial satellite bus designed at the Sunnyvale Space Systems Division in California. The TF80L configuration was considered a non-conventional, innovative approach to developing a satellite design that could be assembled and tested in five days. The TF80L design configuration was also instrumental in simultaneously solving fundamental design problems involving optimization of the communications payload thermal environment and RF main mission antenna performance, while achieving the highest payload fairing packaging for each of the three main launch vehicle providers.
The first spacecraft mock-up of this design was built in the garage workshop in Santa Clara, California for the Bus PDR/CDR as a proof-of-concept model. This first prototype paved the way for the design and construction of the first engineering models. This design was the basis of the largest constellation of satellites deployed in low Earth orbit. After ten years of successful on-orbit performance, the Iridium team celebrated the equivalent of cumulative years of on-orbit performance in 2008. One of the engineering Iridium satellite models was placed on permanent exhibit in the National Air and Space Museum in Washington, D.C.