Previous | Table of Contents | Next |
The satellite itself acts as an active relay much the same as a microwave relay. A satellite communications system involves three basic elements: the space segment, the signal element, and the ground segment. The space segment comprises the satellite and its launch vehicle. The signal element comprises the frequency spectrum over which the satellite communicates, and the ground segment comprises the earth station, antennae, multiplexer, and access element.
Advantages of Satellite Systems. The advantage of a satellite system can be seen in the transmission costs, which are not distance sensitive, and the costs for broadcasting, which are fixed whether there are one or 100 stations that receive the down signal. Another advantage is the high bandwidth that satellite signals are capable of supporting. Bit errors are random, making it possible to use statistical systems for more efficient error detection and correction. Some satellite service providers are described in the following sections.
American Mobile Satellite Corp. (AMSC). AMSC offers satellite-based mobile data services using its own L-band satellite. AMSC is owned by three major shareholders McCaw/AT&T, MTEL, and Hughes although its stock is publicly traded. In the US, AMSC offers service through its Virginia hub. Downlink services may come through the Washington DC international teleport for services sold through the Virginia hub. Pricing is competitive with terrestrial services. For example, a full-time 64K-bps link between Washington, DC, and Brussels, Belgium, would cost $1,350 per month.
OmniTracs. OmniTracs, a service of Qualcomm Inc., uses excess capacity on Ku-band U.S. satellites to provide a data-only mobile tracking service for large trucking companies. The OmniTracs service now has more than 50,000 terminals deployed in trucks in North America. Qualcomm plans to expand its service into Europe, Japan, and South America using excess capacity on existing Ku- and C-band satellites.
Globalstar. Globalstar is the name of a low earth orbit (LEO) system designed for mobile voice services by a joint venture of Loral and Qualcomm. Globalstar has recently extended its ownership to an entirely new set of investors who plan to use excess capacity on available satellites. A series of gateways around the globe will provide an integrated network into the public switched telephone network (PSTN) and the satellite links.
Odyssey. Odyssey, a system proposed by TRW, is composed of four satellites. The TRW system will use fewer satellites for nearly global coverage because the system will be higher in the sky. The Odyssey uses the TRW advanced bus (AB940) L-band dish for mobile-to-satellite links, an S-band dish for satellite-to-mobile links, and two small Ka-band antennae for satellite/ground station links. Each satellite will operate as a bent pipe system, with switching and processing performed at the ground stations using spread spectrum modulation.
Ellipso. Ellipso, proposed by Mobile Communications Holding Inc., is a high elliptical orbiting (HEO) system, consisting of six (although 24 are planned) small satellites deployed in three elliptical orbits. Two of these orbits, called Borealis, will be inclined at 116 degrees. One orbit will be equatorial, which will provide dependable access to users in the northern and southern hemispheres. The Ellipso satellites will be small and use a simple bent pipe design with L-band for uplinks and S-band for downlinks.
Orion Atlantic. Orion is an international partnership of eight companies that operates its own Ku-band satellite composed of 34 transponders. Orions focus is European business-to-business communications arrangements, as well as transatlantic connectivity. Services include cable distribution, business television, news and network backhauls, feeds, and standard business communications requirements. The service can support a full range of multimedia requirements, including telecommuting and interactive desktop video.
A unique mesh network provides completely independent service for international firms with multiple locations. Uplink/downlink services for 64Kbps access is in the range of about $2,000 per month for an enterprisewide LAN. A dedicated 64K-bps full service point-to-point link can be provisioned for about $1,400 per month for a 36-month contract. This service includes all equipment for rooftop-to-rooftop access, which is configured to support a dynamically allocated bandwidth service supporting both voice and data requirements. Installation for such a service would be about $10,000. Such an international connection is priced below regular internal terrestrial services and completely bypasses all monthly recurring local loop costs. A second system is planned that would cover a large part of Russia, the Middle East, Africa, and South America.
Previous | Table of Contents | Next |