Satellite Communications

Commercial satellites provide a wide range of communications services. Television programmes are relayed internationally, giving rise to the phenomenon known as the “global village”. Satellites also relay programmes to cable television systems as well as to homes equipped with dish antennas (see Satellite Television). In addition, very small aperture terminals (VSATs) relay digital data for a multitude of business services. Intelsat satellites now carry over 100,000 telephone circuits, with growing use of digital transmission. Digital source-coding methods have resulted in a ten-fold reduction in the transmission rate needed to carry a voice channel, thus enhancing the capacity of existing facilities and reducing the size of ground stations that provide telephone service.

The International Maritime Satellite Organization (INMARSAT), founded in 1979, is a mobile telecommunications network providing digital data links, telephone, and facsimile transmission (fax) service between ships, offshore facilities, and shore-based stations throughout the world. It also provides satellite links for voice and fax transmission to aircraft on international routes.

Communications satellite systems have entered a period of transition from the use of point-to-point, high-capacity trunk communications between large, costly ground terminals towards that of multipoint-to-multipoint communications between small, low-cost stations. The development of multiple access methods has both hastened and facilitated this transition. With TDMA, each ground station is assigned a time slot on the same channel for use in transmitting its communications; all other stations monitor these slots and select the communications directed to them. By amplifying a single carrier frequency in each satellite repeater, TDMA ensures the most efficient use of the satellite’s on-board power supply.

A technique called frequency reuse allows satellites to communicate with a number of ground stations using the same frequency by transmitting in narrow beams pointed towards each of the stations. Beam widths can be adjusted to cover areas as large as the entire United States or as small as Belgium. Two stations far enough apart can receive different messages transmitted on the same frequency and the spatial separation between sites prevents interference by one beam with the next. Satellite antennas have been designed to transmit several beams in different directions, using the same reflector.

In addition to frequency reuse, multiple spot beams have other advantages. These tighter, more focused beams permit use of significantly smaller and less expensive earth station antennas. Concentrating satellite power over a smaller area increases penetration through atmospheric moisture and mitigates rain fade.

Multiple spot beam antennas are widely used today, both on broadcasting satellites and on satellites used as communications relays. A number of signals transmitted simultaneously are angled at different positions in the focal region area of the transmitting antenna so as to achieve differing coverage “footprints” on the ground. For broadcasting, this technique can focus signals on specific reception areas (to maximize use of the power available), while communications satellites use multiple spot beams to interconnect fixed and mobile ground stations at various locations, for instance for television news gathering and outside broadcasts. In this operation the satellite demodulates the uplink signals, routes them between uplink and downlink beams, then combines and remodulates them for downlink transmission.

Some satellites use laser beams instead of microwave radio for communication. Lasers operating in the blue-green wavelength, which penetrates water, have been used for communication between satellites and submarines. At higher powers the same laser technology has potential military use, since even a few seconds of exposure from a low-powered laser can disrupt space-based sensors by literally “blinding” them. Ground-based anti-satellite weapons can direct energy into space to stop enemies with orbital cameras from spying on one’s own weapons and troops during combat. The United States has fired trial weapons of this kind into space for test purposes.

Traditionally, all communications satellites operated in high orbit, so as to achieve the widest possible coverage. Some newer satellites operate in low Earth orbit (LEO); although these orbits are non-geostationary, by employing constellations of satellites, every point on the Earth can always have coverage from one of the satellites. This allows companies to offer a truly global service to mobile phone users by means of base stations aboard satellites in LEO, although the cost is inevitably high. Other applications for satellites travelling in LEO include remote sensing, surveying, weather monitoring, and spying as from this height extremely detailed images of Earth's surface can be captured.

Plans for a global, broadband “Internet-in-the-Sky” offering worldwide access to telecommunications services such as broadband Internet, videoconferencing, high-quality voice and other digital data services have not yet materialized. They are probably not far off but cost remains a key factor. Growing numbers of companies in the meantime are providing broadband Internet access to business users in mature markets by means of spot beams. As prices fall, these services will also appeal to home users outside connection range by terrestrial means.