More than 20 years have passed since the American technicians at Bell Labs demonstrated that radio cellular technology could be used to offer mobile communication services to a mass market. Today, on the eve of the new millennium, the telecommunications industry is preparing to face a dramatic expansion phase and growth rates, which up until now, have never been experienced.

In the last couple of years, satellite communications have been the subject of growing attention on the part of numerous telecommunications operators attracted by the enormous potential they offer; in fact, they are described as one of the most versatile means of transmission to supply international, regional, and domestic telecommunications services to the world. At the end of 1994, there were 108 telecommunication satellites in service around the globe representing a total capacity of more than 2400 transponders and by the end of 1997 that number reached 3500.

Mobile satellite telephone systems represent a global communication network designed to supply digital services (voice, data, fax, paging, and localisation) independent of the user's location in the world and of the traditional telecommunication network's availability, through the use of portable terminals at a moderate cost.

Millions of people in the world have not yet been reached by terrestrial communication systems. With satellite systems, developing countries and remote areas of the world can be served at a moderate cost and with modest infrastructure. In addition, satellite systems represent a wide variety of other applications. For example, the ability to track people who need to travel frequently from one continent to another and who need to always remain in contact with their home office, emergency communications in remote areas, backup support systems in case of a natural disaster that wipes out the terrestrial network, and for personal use.

To supply global coverage for handheld terminals with the same transmission quality as terrestrial cellular networks, more satellites (constellations) are necessary, preferably at a low or medium altitude (LEO, MEO and HEO). These serve to mitigate the lateness of typical propagation found in geostationary (GEO) satellites and to make the connection using little power. The use of constellation satellites of a low to medium altitude, which move at a speed different from the earth's, make it necessary to continually handover from one satellite to another; in fact, each mobile terminal is under the coverage of a single satellite only for brief periods. In contrast, terrestrial cellular network handovers are due to the mobility of the subscriber who moves outside the coverage area. Satellite telephone systems are mainly suitable for zones that are free of aerial obstructions; thus when trying to use the service inside urban-area buildings, the power emitted becomes weak. For this reason, the terminals sold by all the companies will be the dual-standard type (constellation standard + the national standard), that is, capable of connecting also to the existing terrestrial digital networks such as GSM, DCS, DAMPS, etc. In the future there are plans for tri-standard telephones.

Up until the beginning of the 90's, global mobile communications via satellite were dominated almost exclusively by a single organization, INMARSAT (INternational MARitime SATellite Organization). INMARSAT is an international non-profit organisation with operations based in London, made up of numerous international operators (76 in March 1995). Initially born with the mission of supplying voice and data transmission via ether to ships at any location on the globe, it then extended its services to mobile users on the earth's surface or in the air. The first terminals for connecting to INMARSAT satellites were the size of an overnight bag and cost about 70.000.000 Lira. Today they are smaller in size and cost about 30.000.000 Lira. Communication tariffs are around 9000 Lira per minute. Not having been designed specifically to offer mobile telephone systems, the INMARSAT system has a very complicated access procedure. You need to recognise the satellite that sees you and set its frequency on your handset, after having correctly pointed the antenna in the right direction. In addition, it only allows you to make and receive calls of a limited duration.

In 1990, Motorola presented its own mobile satellite system called IRIDIUM with the intention of becoming the first commercial operator in the world to offer global mobile communications via satellite. The IRIDIUM system planned to launch 77 LEO (Low Earth Orbit) second-generation satellites that would have made up a ring capable of supplying constant global communications in every part of the earth's surface. Telecommunications industry experts, used to geostationary systems with a maximum of five satellites, expressed great scepticism about the fact that such a system could offer competitive service. The managers of IRIDIUM shared some of these worries. In a subsequent version of their project, they reduced the number of planned satellites to 66, thereby reducing both operating and construction costs.

Subsequent to Motorola's presentation of the Iridium system, many other satellite communication systems were planned, but only a few saw the light of day. In fact, the boom in demand for mobile services seen in all countries and the overwhelming acceptance of the GSM digital standard has caused the satellite communications world to suffer. The diffusion of a digital standard for cellular communications like GSM and the capability it offers to roam internationally with ample territorial coverage, even in remote regions of the world, represents strong competition for satellite telephony. In the meantime, the progress made by the technology in the areas of digital signal expansion, satellite engineering, and the miniaturisation of the components made many of the satellite projects much more feasible.

INMARSAT, impeded by its monopolistic structure between national telephone administrations, and forced to face a saturated market, like that of mobile maritime communications, created a corporation called IGC (ICO Global Communications). It had the specific task of managing and maintaining its own system for personal satellite phones at a medium altitude (MEO) called INMARSAT-P; meanwhile, the existing INMARSAT was to continue to offer satellite services to ships and planes.

Another fundamentally important issue for the development of mobile communications via satellite is an economic one. The financial market is cautiously observing this complex technology that requires huge initial investments and a division of the risk amongst more stakeholders. Even the most extravagant among all the proposed projects is technically feasible, but the financial feasibility is another story; and since the number of aspiring players is larger than what the market seems able to bear, it will be economics rather than technology that decides which systems will be able to take off. Other problems come from authorities, mainly from developing countries, that are against systems that pass over their territorial networks. Regional satellite systems have been proposed both by the ASC (Afro-Asian Satellite Corporation) and the AMT (Asia Mobile Telecommunications). These would be based on the use of huge GEO satellites which require less investment due to the use of a smaller number of satellites, and which would cover smaller areas. The regions that are less well equipped with telecommunications include Africa, Asia, and Australia, areas considered by global operators as the major targeted markets.

This persuaded the operators to rethink about their position on the market segmentation of mobile users.

A fundamental issue for the implementation of a satellite network is the frequency band to use, which must be available at the same frequency throughout the world. To use the band, you need to obtain licences in every nation in which you plan to operate. This necessity induced the mobile communication satellite operators to make agreements with local telephone companies, not only to obtain the necessary licences but also to facilitate the commercialisation of the service. In the last year there have been numerous requests for licences to provide satellite communication services on the part of consortiums created ad hoc by telecommunications companies, aerospace firms, and space electronics companies.

All the various satellite mobile networks differ from Iridium in their transmission technique:
TDMA (Time Division Multiple Access) for Iridium and CDMA (Code Division Multiple Access) for the other networks.
The TDMA technique permits multiple access through the time division of the frequencies in small fractions of a second, doubling the capacity of the system compared to an analog one.
The CDMA technique (developed by Qualcomm), is much more flexible than TDMA and permits, with its 4.4 trillion different codes, the use of the same spectrum at the same moment by different systems, while at the same time increasing the capacity twenty-fold compared to an analog one.
With the TDMA technique, two different systems cannot use the same spectrum at the same time because a unique computer must carry out the time division in order to synchronise correctly. Motorola's decision to use the TDMA technique caused several problems because it does not allow the sharing of the same band, which is already limited enough, with other satellite systems. Adjacent or overlapping satellites with different systems cannot use the same frequencies/channels; otherwise they would interfere with the radio connections. The available band assigned of 16 MHz (1610 - 1626,5 MHz for the uplink and 2483,5 - 2500 MHz for the downlink) was divided by the FCC (Federal Communications Commission) in two parts: 8 MHz for Iridium (which according to Motorola is the minimum band with which the Iridium system can efficiently operate) and 8 MHz for the other operators that operate with the CDMA technique.

Among the various mobile satellite systems proposed, those that appear to be feasible are: