Iridium Satellite Constellation

The name Iridium comes from the atom of the same name that has 77 electrons which rotate around the nucleous. In fact, this satellite constellation was initially supposed to have 77 satellites. It was subsequently reduced to 66 to reduce costs, but the name has remained nonetheless.

The Iridium network is a global mobile communication system, capable of covering the entire surface of the Earth, designed to supply digital services (voice, data, fax and paging) through portable terminals, independent of the user's location in the world and of the availability of traditional telecommunications networks. The aeronautic Iridium service will also allow global access to commercial and private airline passengers.

  Each circle in the diagram corresponds to the coverage of one Iridium satellite

 

Global coverage will be supplied by a constellation of 66 satellites orbiting at a low altitude (780 Km from the Earth's surface) that offers the same transmission quality as land-based cellular networks, eliminating the typical delay of geostationary satellites. The satellites are arranged on 6 orbital levels and each level contains 11 operating satellites and 1 backup satellite.

As opposed to terrestrial mobile networks, where the only element that can move itself is the subscriber, satellite networks have a dual movement: that of the constellation which rotates around the Earth and that of the subscriber who moves from one point to another on Earth by land, sea or air.

The design of the site was carried out by Motorola, which in 1989 created the Motorola Satellite Communications Division (SATCOM) with headquarters at Chandler, Arizona, to develop this project. Within the SATCOM compound you will find:

• · The Iridium Satellite Control Facility (for the control and management of the satellites from Earth during the test phase)

• The Antenna Compound (a closed area that contains a huge antenna)

• The Integration and Test Laboratory (where all hardware, software and every component of the Iridium system is tested).
  
  
• The Satellite Manufacturing Centre (where the satellites are assembled).

   

Due to the elevated costs of this project (3.7 billion USD), Motorola made agreements with major Japanese electronics firms and many telephone companies from around the world to form a consortium called Iridium Inc., whose role was that of implementing and managing the system.

Motorola has been working on this project since 1990 and by itself in 1995 spent 100 million USD. From August, 1995 it will spend another 700 million for the construction of the satellites. The licence to construct, launch, and administer the Iridium system was granted to Motorola from the Federal Communications Commission (FCC) on January 31, 1995 in Washington DC.

In recent years, other mobile satellite network projects have been proposed, each with fewer satellites and without cross-links. Each Iridium satellite, as opposed to the other satellite networks in which satellites act as simple repeaters, will not be connected to the land stations and with mobile terminals, but instead will be interconnected to 4 other satellites, forming a true network. Thus they can avoid the use of intermediaries with the land stations for calls between Iridium users. In addition, this allows them to direct calls to and from mobile terminals through the other satellites to the earth station (gateway) that is closest to the subscriber's position. The terrestrial gateways are also interconnected through a satellite network and not through terrestrial links.

Another important difference from the other networks is the transmission technique: TDMA (Time Division Multiple Access) for Iridium and CDMA (Code Division Multiple Access) for the other networks. Motorola's decision to use the TDMA technique caused many problems because it cannot share the same band, which is already very limited, with the other satellite systems. The available band of 16 MHz was divided by the FCC in two parts: 8 MHz to Motorola for Iridium (the minimum band with which Iridium could efficiently operate) and 8 MHz to the other operators with the CDMA technique, which has a larger capacity and lets more operators simultaneously use the same spectrum.

Among all the satellite systems presented, the Iridium system is the most expensive and the most sophisticated, mainly because of the ISL (Inter Satellite Link) feature. The implementation of this function greatly increased the complexity of the network making it necessary to add further elements on board, such as a computer to manage the call routing from one satellite to another, additional transmitters, and more antennas. These additions increase the consumption of energy on board, creating the need for more resources. In addition, more fuel is needed to maneuver the satellites to the proper position because this system requires greater precision when pointing the antennas that have to communicate both with the other satellites and with the earth stations. This results in Iridium satellites being larger both in size and weight and thus more expensive to launch in orbit with respect to the competition.

In August 1995, Motorola announced that it raised 800 million USD from a group of international investors. This consortium is made up of telecommunications companies and large firms in the industry:

• Iridium Africa Corporation (Mawarid Overseas Company, one of the largest and most diverse companies in Saudi Arabia, involved in the construction, public services, and telecommunications sectors).

• Iridium Andes Caribe Venezuela (a private, Venezuelan investors consortium with experience in food markets, communications, construction, finance and retail sales)
• Iridium Canada (made up of BCE Mobile Communications Inc., BCE Telecom International Inc., the largest Canadian telecommunications companies and Motorola Canada Ltd.)
• Iridium China Ltd (made up of China Aerospace Inc., owner of the China Great Wall Industry Corporation, the commercial branch of the Chinese Armed Forces which supplies the Long March IIC, capable of putting two Iridium satellites into orbit for every launch, and China Aerospace International Holdings Ltd a diversified Hong Kong firm)
• Iridium India Telecom Limited (formed from a group of financial institutions including Industrial Development Bank of India, Infrastructure Leasing & Financial Services Ltd., Ex-Im Bank of India, Industrial Credit and Investment Corporation of India, General Insurance Corporation, Housing Development Finance Corporation, Life Insurance Corporation, Shipping Credit and Investment Corporation of India, and Unit Trust of India)
• Iridium Middle East Corporation (made up of Mawarid and Saudi Binladin Group, one of the largest industrial groups in Saudi Arabia)
• Iridium SudAmerica Corporation (made up of Iridium Andes-Caribe, INEPAR, a diversified Brazilian group operating in the telecommunications sector, the electrical energy control devices and services sector, mass transportation, vehicle distribution, and financial markets, and Motorola Inc.)
• Khrunichev Enterprise (a company owned by the Russian Federation, with interests in aerospace design and production. It has spent the last 30 years in the production of launch vehicles, orbiting stations, and other space equipment. It will supply the Proton rockets for the satellite launches)
• Korea Mobile Telecommunications Corporation (formed by Korea Telecom and Sungkyong Business Group)
• Lockheed (the leading American company in space systems and defence)
• Motorola (USA)
• Nippon Iridium Corporation (Daiwa Securities Co., Kansai Cellular Telephone Company, Kyushu Cellular Telephone Company, Hokkaido Cellular Telephone Company, Hokuriku Cellular Telephone Company, Industrial Bank of Japan, Long-Term Credit Bank of Japan, Mitsubishi Corporation, Mitsui & Co. Ltd., The Sanwa Bank Ltd., SECOM Co. Ltd., Shikoku Cellular Telephone Company, Sony Corporation, Tohoku Cellular Telephone Company, Ushio Inc., Kyocera, supplier of electronic components and DDI, an independent telecommunications company, Chugoku Cellular Telephone Company)
• Pacific Electric Wire & Co. Ltd (the largest Taiwanese producer of electric cables)
• Raytheon Company (an American firm which will be responsible for the design and construction of the antennas for the Iridium satellites that will communicate with mobile handsets)
• Sprint (an international telecommunications company, the only one to have an entirely digital fibre-optic network operating in the United States)
• STET (a firm that supplies telephone, telegraphic, telex, data, and mobile services in Italy; main shareholder in Telecom Italia and Telecom Italia Mobile. In addition, STET builds communications equipment for civil and military use, and provides research and consulting services. One of the two Iridium control centres will be managed by them in Rome)
• Thai Satellite Communication Co. (United Communications Industry Co. Ltd., one of the largest cellular operators in Thailand)
• VEBACOM GmbH (one of the four largest German telecommunications firms)

Each of the aforementioned firms will not only have a piece of Iridium, but they will also build and own the terrestrial gateways and act as the local distributors for satellite service in the markets that are assigned at the moment they enter the consortium. They will need to choose service providers in the assigned territories for selling services to the public, possibly linked with a land-based cellular service. In particular, one of the two gateways capable of controlling the whole satellite constellation will be built in Italy by STET, one of the more important investors in this project, financing 1.6 billion USD. Lockheed will be responsible for the construction of certain parts of the Iridum satellite, in particular the controls for positioning, propulsion, environment, and the construction of the solar panels. Software Technologies Inc. (STI) will be responsible for the tracking and the control of the satellite network. The software they created, called COMET, translates the position of the satellite, the operational data, and air-traffic control, in a graphical environment. It operates on Sun SparcStations with the Solaris operating system.

There are some important investors missing from this list, among which German, English, and French telephone companies, and the two national Japanese telephone companies, who have made agreements with Globalstar, another important constellation still under construction.

Iridium satellites will be constructed at Motorola's SATCOM division at Chandler, Arizona, where a mass production process was implemented. Instead of having technicians who work around a single satellite, Iridium satellites are placed on trolleys and will be assembled in pieces passing from one station to another within the 1100 sq.m factory. Motorola insists that this production process allows three satellites to be constructed simultaneously and finished in three weeks, as opposed to the three years normally needed with past technology. The satellites can be launched into orbit about 8 days after construction. The first launch, scheduled for 1996, and each one thereafter will be the responsibility of three main carriers:

• McDonnell Douglas, which will use Delta II missiles capable of transporting 5 satellites each flight.

• Khrunichev State Research and Production Space Center, which will use Proton missiles, capable of transporting 7 satellites each flight.

• China Great Wall Industry Corporation, which will use Long March 2C/SD missiles, capable of transporting 2 satellites each flight

Currently, Motorola makes the only terminals that can be used on the Iridium network. An Iridium-system alphanumeric pager can receive messages with up to 66 characters and uses normal alkaline batteries that should last about 1 month. Iridium Dual-Standard telephones weigh about 200 grams, have about 1 hour of battery-life during conversation and 24 hours in standby, and come equipped with the RS-232 interface for data/fax transmission. It uses a smart card that must be inserted in the phone just like GSM/DCS handsets From an Iridium telephone you can contact any other phone on earth and can interface with all the existing terrestrial cellular systems (dual-standard) such as GSM, DCS, and AMPS where there are roaming agreements.

On March 25, 1996, Iridium Inc. became a member of the GSM MoU Association (Memorandum of Understanding), an association comprised of GSM operators, born in 1995 to promote the GSM technology. Its goal is to achieve international roaming, free circulation of approved GSM devices, advanced security characteristics, and unified billing among operators. Through this affiliation, Iridium will obtain roaming agreements with GSM operators around the world to supply its own subscribers with the ability to roam globally on a GSM network in addition to the satellite service. The technical compatibility between the GSM network and Iridium guarantees an efficient exchange of administrative information between GSM operators and those of Iridium.

Iridium subscribers will be billed according to the cell phone tariffs of the GSM/DCS operator when connected to the terrestrial network. When roaming with the satellite constellation, the subscriber will be billed at Iridium's rates.

The Iridium system represents a cost-saving alternative for developing countries and rural areas where the infrastructure costs for a telephone system are prohibitive. For these areas Mobile Exchange Units (MXUs) have been planned. They will function as sophisticated switchboards, sharing access to the Iridium system with common PSTN (Public Switched Telephone Network) telephones. An alternative would be to install public telephones with Iridium transceivers incorporated, working on electricity or solar power.

In addition, the Iridium system continues to supply service during earthquakes, hurricanes, floods and other natural disasters even if one or more earth stations (gateway) are destroyed. This type of uninterrupted service is guaranteed by the ISL (InterSatellite Link) with which calls can be directed through the satellite toward any other gateway. This, coupled with the fact that communications are not influenced by atmospheric conditions, makes Iridium an important means of communication in case of emergency.

Before Iridium is completely operative and commercial service begins (September 1998), more than 300 gigabytes of data must be transferred to the system, needed to control the whole network.

The SATCOM Satellite Control Facility, found at Motorola's general headquarters at Chandler, Arizona, will be responsible to monitor and control the movement of the satellites up until the launch of the first 40. After the test phase, there will be only two general control centres, from which they can administer and monitor the whole network at any given time. The first one is called the Master Control Facility (MCF) and is located at Landsdowne in Virginia next to Washington DC. The second one is called the Backup Control Facility (BCF), located at Rome, Italy and will be managed by Nuova Telespazio. There will also be three other earth stations located in Hawaii and Canada, called Telemetry, Tracking, and Control Centers (TTACs), which are connected directly to the MCF and will be responsible for positioning the satellites in the right orbit after being launched. All the other earth stations will work exclusively as gateways. Those found in the United States include stations in Hawaii, Florida, and Alaska. In February 1996, construction began in Japan for the new gateways.

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