Several different types of global satellite communications systems are in various stages of development. Each system, either planned or existing, has a unique configuration optimized to support a unique business plan based on the services offered and the markets targeted.

In the last few years more than 60 global systems have been proposed to meet the growing demand for international communications services. More are being planned and these are in addition to a large number of new regional systems.

Some of the global systems intend to provide global phone service, filling in where ground-based wireless systems leave off or providing seamless connectivity between different systems. Others intend to provide global data connectivity, either for low-cost short message applications such as equipment monitoring, or for high-speed Internet access anywhere in the world.

The global phone systems will target two very different markets. The first is the international business user, who want the ability to use a single mobile wireless phone anywhere in the world. This is impossible today on terrestrial systems because mobile phone standards are different from region to region. The second market is unserved and underserved communities where mobile and even basic telecommunications services are unavailable.

Because global and regional satellite systems are relatively new in non-military communications, these market approaches still are untested and it is likely that economics, user acceptance rates, technical difficulties and other factors will cause adjustments in the business plans of many of these systems.

The Types of Satellite Systems

The design of a satellite system is closely tied to the market it is intended to serve and the type of communications services it is intended to offer. There are four general system designs, which are differentiated by the type of orbit in which the satellites operate: Geostationary Orbit (GEO), Low-earth Orbit, Medium-earth Orbit (MEO), and Highly Elliptical Orbit (HEO). Each of these has various strengths and weaknesses in its ability to provide particular communications services.

Outside of the well-defined GEO universe, the differences between these systems is often not absolute and the acronyms applied to a system can be confusing and sometimes misleading. Several systems, for example, are variously described as LEOs and MEOs. Constantly evolving technology along with newly developing markets and service definitions combine to blur the lines between one satellite system and another.

The definitions below are meant to describe the general characteristics of GEOs, MEOs, LEOs and HEOs. Although examples of commercial systems employing these satellites are given, keep in mind that each system has unique characteristics that may not match precisely the general descriptions. The same caution should be applied to ascribing a particular satellite type's limitations to any one commercial system, since each uses several strategies for minimizing or overcoming the limitations inherent in satellite designs. For example, some systems may employ more than one type of satellite.

GEOSTATIONARY (GEO)
GEO systems orbit the Earth at a fixed distance of 35,786 kilometers (22,300 miles). The satellite's speed at this altitude matches that of the Earth's rotation, thereby keeping the satellite stationary over a particular spot on the Earth. Examples of GEO systems include INTELSAT, Inmarsat, and PanAmSat.

Geostationary satellites orbit the Earth above the equator and cover one third of the Earth's surface at a time. The majority of communications satellites are GEOs and these systems will continue to provide the bulk of the communications satellite capacity for many years to come.

GEOs support voice, data, and video services, most often providing fixed services to a particular region. For example, GEO satellites provide back-up voice capacity for the majority of the U.S. long distance telephone companies and carry the bulk of nation-wide television broadcasts, which commonly are distributed via from a central point to affiliate stations throughout the country.

Until recently, the large antennae and power requirements for GEO systems limited their effectiveness for small-terminal and mobile services. However, newer high-powered GEO satellites using clusters of concentrated "spot beams" can operate with smaller terrestrial terminals than ever before and can support some mobile applications. GEO satellite coverage typically degrades beyond 20 degrees North Latitude and 20 degrees South Latitude.

GEO systems have a proven track record of reliability and operational predictability not yet possible for the more sophisticated orbital designs now being deployed. GEO systems are also less complicated to maintain because their fixed location in the sky requires relatively little tracking capability in ground equipment. In addition, their high orbital altitude allows GEOs to remain in orbit longer than systems operating closer to Earth. These characteristics, along with their high bandwidth capacity, may provide a cost advantage over other system types.

However, their more distant orbit also requires relatively large terrestrial antennae and high-powered equipment and are subject to transmission delays. In addition, since only a few large satellites carry the load for the entire system, a GEO satellite loss is somewhat more consequential than for the systems described below.

• PRO: GEO systems have significantly greater available bandwidth than the LEO and MEO systems described below. This permits them to provide two-way data, voice and broadband services that may be unpractical for other types of systems.
• PRO: Because of their capacity and configuration, GEOs are often more cost-effective for carrying high-volume traffic, especially over long-term contract arrangements. For example, excess capacity on GEO systems often is reserved in the form of leased circuits for use as a backup to other communications methods.
• CON: GEO systems, like all other satellite systems, require line-of-sight communication paths between terrestrial antennae and the satellites. But, because GEO systems have fewer satellites and these are in a fixed location over the Earth, the opportunities for line of sight communication are fewer than for systems in which the satellites "travel" across the sky. This is a significant disadvantage of GEO systems as compared to LEO and MEO systems, especially for mobile applications and in urban areas where tall buildings and other structures may block line-of-sight communication for hand-held mobile terminals.
• CON: Some users have expressed concern with the transmission delays associated with GEO systems, particularly for high-speed data. However, sophisticated echo cancellation and other technologies have permitted GEOs to be used successfully for both voice and high-speed data applications.

LEO systems fly about 1,000 kilometers above the Earth (between 400 miles and 1,600 miles) and, unlike GEOs, travel across the sky. A typical LEO satellite takes less than two hours to orbit the Earth, which means that a single satellite is "in view" of ground equipment for a only a few minutes. As a consequence, if a transmission takes more than the few minutes that any one satellite is in view, a LEO system must "hand off" between satellites in order to complete the transmission. In general, this can be accomplished by constantly relaying signals between the satellite and various ground stations, or by communicating between the satellites themselves using "inter-satellite links."

In addition, LEO systems are designed to have more than one satellite in view from any spot on Earth at any given time, minimizing the possibility that the network will loose the transmission. Because of the fast-flying satellites, LEO systems must incorporate sophisticated tracking and switching equipment to maintain consistent service coverage. The need for complex tracking schemes is minimized, but not obviated, in LEO systems designed to handle only short-burst transmissions.

The advantage of the LEO system is that the satellites' proximity to the ground enables them to transmit signals with no or very little delay, unlike GEO systems. In addition, because the signals to and from the satellites need to travel a relatively short distance, LEOs can operate with much smaller user equipment (e.g., antennae) than can systems using a higher orbit. In addition, a system of LEO satellites is designed to maximize the ability of ground equipment to "see" a satellite at any time, which can overcome the difficulties caused by obstructions such as trees and buildings.

There are two types of LEO systems, Big LEOs and Little LEOs, each describing the relative mass of the satellites used as well as their service characteristics.

Little LEO satellites are very small, often weighing no more than a human being, and use very little bandwidth for communications. Their size and bandwidth usage limits the amount of traffic the system can carry at any given time. However, such systems often employ mechanisms to maximize capacity, such as frequency reuse schemes and load delay tactics.

Little LEO systems support services that require short messaging and occasional low-bandwidth data transport, such as paging, fleet tracking and remote monitoring of stationary monitors for everything from tracking geoplatonic movements to checking on vending machine status. The low bandwidth usage may allow a LEO system to provide more cost effective service for occasional-use applications than systems that maximize their value based on bulk usage. Examples of Little LEO systems include Orbcomm, Final Analysis and Leo One.

Big LEO systems are designed to carry voice traffic as well as data. They are the technology behind "satellite phones" or "global mobile personal communications system" (GMPCS) services now being developed and launched.

Most Big LEO systems also will offer mobile data services and some system operators intend to offer semi-fixed voice and data services to areas that have little or no terrestrial telephony infrastructure. Smaller Big LEO constellations also are planned to serve limited regions of the globe. Examples of Big LEO systems include Iridium, Globalstar and the regional Constellation and ECO-8 systems.

An emerging third category of LEO systems is the so-called "super LEOs" or "mega LEOs," which will handle broadband data. The proposed Teledesic and Skybridge systems are examples of essentially Big LEO systems optimized for packet-switched data rather than voice. These systems share the same advantages and drawbacks of other LEOs and intend to operate with inter-satellite links to minimize transmission times and avoid dropped signals.

Summary of LEO Pros and Cons

• PRO: The transmission delay associated with LEO systems is the lowest of all of the systems.
• CON: The small coverage area of a LEO satellite means that a LEO system must coordinate the flight paths and communications hand-offs a large number of satellites at once, making the LEOs dependent on highly complex and sophisticated control and switching systems.
• PRO: Because of the relatively small size of the satellites deployed and the smaller size of the ground equipment required, the Little LEO systems are expected to cost less to implement than the other satellite systems discussed here.
• CON: LEO satellites have a shorter life span than other systems mentioned here. There are two reasons for this: first, the lower LEO orbit is more subject to the gravitational pull of the Earth and second, the frequent transmission rates necessary in LEO systems mean that LEO satellites generally have a shorter battery life than others.

MEO systems operate at about 10,000 kilometers (between 1,500 and 6,500 miles) above the Earth, which is lower than the GEO orbit and higher than most LEO orbits. The MEO orbit is a compromise between the LEO and GEO orbits. Compared to LEOs, the more distant orbit requires fewer satellites to provide coverage than LEOs because each satellite may be in view of any particular location for several hours. Compared to GEOs, MEOs can operate effectively with smaller, mobile equipment and with less latency (signal delay).

Although MEO satellites are in view longer than LEOs, they may not always be at an optimal elevation. To combat this difficulty, MEO systems often feature significant coverage overlap from satellite to satellite, which in turn requires more sophisticated tracking and switching schemes than GEOs.

Typically, MEO constellations have 10 to 17 satellites distributed over two or three orbital planes. Most planned MEO systems will offer phone services similar to the Big LEOs. In fact, before the MEO designation came into wide use, MEO systems were considered Big LEOs. Examples of MEO systems include ICO Global Communications and the proposed Orblink from Orbital Sciences.

• PRO: MEO systems will require far fewer satellites than LEOs, reducing overall system complexity and cost, while still requiring fewer technological fixes to eliminate signal delay than GEOs.
• CON: MEO satellites, like LEOs, have a much shorter life expectancy than GEOs, requiring more frequent launches to maintain the system over time.
• PRO: MEO systems's larger capacity relative to LEOs may enable them to be more flexible in meeting shifting market demand for either voice or data services.
• CON: MEO systems, as well as some Big LEOs, targeted at the voice communications market may have a disadvantage when compared with cellular and other terrestrial wireless networks. A satellite signal is inherently weaker and is more subject to interference than those of terrestrial systems, thus requiring a larger antenna than a traditional mobile phone. By contrast, the trend in the mobile phone market is toward smaller and smaller phones.

HIGHLY ELLIPTICAL ORBIT (HEO)
HEO systems operate differently than LEOs, MEOs or GEOs. As the name implies, the satellites orbit the Earth in an elliptical path rather than the circular paths of LEOs and GEOs. The HEO path typically is not centered on the Earth, as LEOs, MEOs and GEOs are. This orbit causes the satellite to move around the Earth faster when it is traveling close to the Earth and slower the farther away it gets. In addition, the satellite�s beam covers more of the Earth from farther away, as shown in the illustration.

The orbits are designed to maximize the amount of time each satellite spends in view of populated areas. Therefore, unlike most LEOs, HEO systems do not offer continuous coverage over outlying geographic regions, especially near the south pole.
Several of the proposed global communications satellite systems actually are hybrids of the four varieties reviewed above. For example, all of the proposed HEO communications systems are hybrids, most often including a GEO or MEO satellite orbital plane around the equator to ensure maximum coverage in the densely populated zone between 40 degrees North Latitude and 40 degrees South Latitude. Examples of HEO systems include Ellipso and the proposed Pentriad.

Summary of HEO Pros and Cons

• PRO: The HEO orbital design maximizes the satellites' time spent over populated areas, thus requiring fewer satellites than LEOs and providing superior line-of-sight in comparison to most LEOs or GEOs.
• CON: Coverage of a typical HEO system is not as complete as other orbital designs, although they will provide good coverage over most population centers.