There has been an explosive growth in the use of wireless communication systems in recent years. The demand for such wireless services as mobile cellular telephony, radio paging, and other personal communication devices has been spiraling steadily upward. It is projected that by 2001, there will be nearly 300 million wireless subscribers throughout the world. Even while the wireless industry of the terrestrial cellular market is expanding rapidly, especially in the United States and Europe, there are still an estimated three billion people living in China, India, Pakistan, and the Philippines who have no phone at home due to harsh geographical conditions.

As we approach the new millennium, satellite-based communication systems will be the next frontier for this industry. They will assume a vital role in infrastructure, securing telecommunication links during disasters and supporting humanity's space-based efforts.

A new epoch in space-based wireless communications has already begun with the deployment of two low-Earth-orbiting (LEO) communication satellite systems: Iridium and Globalstar. The Motorola-led Iridium consortium successfully launched the last five satellites in its strong network during the last year. The entire Iridium network integrates terrestrial phone systems and satellites.

Several other satellite systems having global or broad geographical coverage will join this new arena within the next 3 to 4 years, thereby complementing and extending existing terrestrial wireless services. Users of conventional terrestrial cellular services, business people, travelers, maritime vessels, aeronautical and industrial facilities, journalists, government agencies, the Coast Guard and emergency-related organizations, others on the go, and people living in sparsely populated areas will be able to communicate with each other via these services.

Satellite-based mobile communication systems are characterized by the distance of their satellites from Earth. LEO satellites are typically located 310 miles (500 kms) to 932 miles (1,500 kms) above the planet, whereas medium-Earth-orbit (MEO) versions are located from 3,100 miles (5,000 kms) to 7,456 miles (12,000 kms) above the planet. Geosynchronous Earth-orbit satellites, located 22,245 miles (35,800 kms) above the equator, move in synchronism with Earth's rotation. While GEO satellites seem to be stationary to an Earthbound observer, LEO and MEO satellites appear to be in constant movement.

GEO SYSTEMS

GEO satellites have been used in such commercial communication services as television broadcasting and long-distance telephone trunking, and for maritime communication services since 1965. With the exception of the polar regions, global coverage can be provided by three GEO satellites equally spaced above the equator. In the past, GEO satellites were not viable for hand-held phone communications due to the lengthy signal propagation delay and large power loss. Advances in space technology, however, allow satellites equipped with large-aperture phased array antennas to increase their transmitting power, thereby making the GEO approach viable for delivering telephony to hand-held phones in vast areas of the world.

Several operators have opted for regional GEO systems, which typically require a single satellite. The Asia Cellular Satellite System (ACeS), which is being developed by a three-company consortium from Indonesia, Thailand, and the Philippines, will provide services in 26 Southeast Asian countries, including Japan, China, India, and Pakistan. ACeS's satellite, positioned over the equator at 118 degrees east longitude above the island of Borneo, offers mobile phone, facsimile, data, and paging services. Hence, many people in this region who have no access to terrestrial communication links will one day be able to roam anywhere they wish and still keep in touch with each other.

Another regional GEO satellite, Thuraya, will furnish mobile satellite services to 1.8 billion people in 58 countries ranging from the Middle East and North Africa to eastern Europe, Turkey, the Indian subcontinent, and Central Asia. Thuraya will be positioned over the equator at 44 degrees east above the Somali coast. The program, a consortium of 14 telecommunications organizations in various Arab countries, is run out if its headquarters in the Thuraya Satellite Communications Co., located in Abu Dhabi, United Arab Emirates.

The Thuraya system will use a time-division-multiple-access (TDMA) scheme and support 13,750 voice channels. Hughes Network Systems will supply the dual-mode handsets. The satellite will connect calls from users to other users through its 256 reconfigurable spot beams. The company presently envisions an air-time price of US $0.50 per minute for system users.

LEO SYSTEMS

LEO satellites at very low altitudes differ from GEO satellites in two main ways: they are close enough to receive hand-held device signals with a very small propagation delay, and they form cellular towers in the sky. The major disadvantage of LEO systems, when compared with GEO systems, is that they require more satellites with a smaller size and lighter weights to provide global coverage. Complicated ground-based tracking systems are needed to control LEO satellites.

The Iridium system operated by the Motorola-led international consortium of 20 telecommunications and industrial companies is the first LEO system to turn the promise of global wireless service into a reality. By integrating ground-based cellular infrastructures with 66 LEO satellites and thus forming a cross-linked grid 485 miles (780 kms) above the Earth, Iridium provides such global telecommunications as telephony, data, and pager services.

Each satellite in the Iridium constellation rotates around the Earth within a period of approximately 100 minutes at one of the six orbital planes of 86.4 degrees inclination. With one global telephone number and an Iridium satellite phone, you can contact anyone on the planet. The satellite's on-board processor processes calls placed by hand-held subscribers or forwarded by gateways and routes to other Iridium satellites in the constellation or to gateways on the ground. This inter-satellite networking capability and direct access of hand-held subscribers to the satellite is a significant distinguishing feature of the Iridium system.

The system operates at four different links and in four different frequency bands. Each satellite in the constellation is connected by radio transmission to four others at frequencies between 23.18-23.38 GHz. Hand-held users can communicate directly with the satellite in the 1.616-1.626 GHz band. Links between the satellite and ground gateways also operate in 19.4-19.6 GHz (downlink) and 29.1-29.3 GHz (uplink) frequency range. Iridium handsets are dual-mode, working both as a typical cellular telephone and as a satellite telephone. Both TDMA and frequency-division-multiple-access (FDMA) technologies are embedded in the handsets, as in cellular GSM handsets. The satellites are controlled by a master control center located in Lansdowne, Virginia, USA.

Iridium launched its final satellites in early May 1998. The system is now operational and offers a wide variety of services to travelers, aeronautical industries, and military and governmental organizations. It provides voice, facsimile, and data communications for the cockpit and at passenger seats across all aviation segments. According to a recent press release by Stratos, an Iridium service provider, the American government had a contract with it and Hughes Global Services to obtain access to multi-network Iridium satellite services.

Another LEO satellite system that will provide global voice and data services is Globalstar. This system is global in nature, except for the polar regions. It is a constellation of 48 satellites orbiting with a period of 113 minutes in eight circular planes, and is inclined at 52 degrees at an altitude of 879 miles (1,414 kms). Global-star's satellites are less complicated and cheaper than their Iridium counterparts. They have no on-board processor or intersatellite links (Iridium does), and thus act like well-established reflectors in the sky relaying signals directly to ground gateways. Rather than directly connecting one caller to another by satellite, calls are first routed to gateways and then uplinked to the satellite. The satellite then downlinks this received call to another gateway.

The primary owners of Globalstar are Loral Space and Communications Ltd., and Qualcomm Inc. The system is operated and serviced by 12 telecommunications companies. In order to avoid communication linkage drops, three or four 16-foot to 20-foot (5-meter to 6- meter) dish antennas are installed at Globalstar gateways. Unlike Iridium, Globalstar handsets utilize code-division-multiple-access (CDMA) technology. Globalstar's unique capability is that signals from three or four visible satellites are combined at the gateways, and the strongest one is chosen to maximize power efficiency and eliminate call interruption. Therefore, satellites will be seamlessly added to and removed from calls in progress, as they are constantly moving in and out of view. So far, Globalstar has put 24 satellites in orbit. "With only two more successful launches of four satellites each, Globalstar will have the coverage required to initiate a regional roll-out of service in September," says Bernard L. Schwartz, chairman and chief executive officer of Globalstar.

OTHER LITTLE LEOs and MEOs

Other LEOs, notbly Orbcomm and Teledesic, provide such telecommunication services as broadband Internet access, videoconferencing, and multimedia; however, they do not allow phone calls. These satellites are relatively small compared with those of Iridium and Globalstar, and were designed for two-way data communications. Orbcomm is a consortium of Orbital Science Corp., Canada's Teleglobe Inc., and Malaysia's Technology Resources Industries Bhd. Orbcomm's total of 36 little LEOs will travel in two different circular orbits: one is located 460 miles (740 kms) above the Earth with 70 degrees inclination in pair, and the other is located at 523 miles (825 kms) in planes of eight with 45 degrees inclination. The Teledesic system will consist of 288 little LEO satellites in 12 polar orbital planes. It will provide data rates of 64 Mb/s, data rates 2,000 times faster than standard telephone modems. The system's operation frequency will be 27.5 GHz in the uplink (from user to satellite), and 28.5 GHz in the downlink (from satellite to user). The company plans to start commercial service in 2002. Its investors are Microsoft founder Bill Gates, cellular phone pioneer Craig McCaw, Boeing, and the AT&T Corp.

ICO Global Communications will feature 10 operational MEO satellites located at an altitude of 6,434 miles (10,355 kms) in 45 degrees and 135 degrees inclined orbits. ICO's ground network will consist of 12 ground stations with multiple antennas distributed strategically around the globe. Its gateways will function in ways similar to those of Global- star. The ICO system will launch its full service in 2000. ICO satellites are derived from an existing Hughes GEO satellite, and are four times heavier than Iridium satellites: 6,063 pounds (2,750 kgs) in orbit. ICO satellites travel more slowly than LEO satellites, thereby reducing the need for frequent handovers from one satellite to another.

Ellipso, an MEO system owned by Mobile Communications Holding Inc., Lockheed Martin Corp., Harris Corp., and three others from Australia and South Africa, will become fully operational in 2001. With 17 satellites in three orbital planes, it can provide almost complete global coverage. Seven equally spaced satellites located above the equator at an altitude of 5,008 miles (8,060 kms) will serve a 25 degrees north and a 55 degrees south latitude region. Another 10 satellites will orbit in two elliptical orbits inclined at 116 degrees. Each satellite will be able to handle 3,000 simultaneous phone calls.

REFERENCES

• Big LEO/MEO/GEO Market and Financial Review (1998).
• Glenister, Simon. "Iridium to Offer Aeronautical Service." Integrating Global Air Traffic Management, ICAO/ISC (June 1998). (See also: www.
• iridium.com/english/industry/wero/medialarticle_index.html.)
• http://www.globalstar.com.
• http://www.iridium.com.
• Miller, Barry. "Satellites Free the Mobile Phone." IEEE Spectrum Magazine (March 1998): 26-35.