LESSON 11

COMMUNICATIONS SATELLITES

I. Consider the following questions in the group of four. A spokesperson will report on your discussion to the whole group.

1. What is a communications satellite?

2. What kind of satellites do you know?

3. What for are satellites used in television?

4. How satellites are put into orbit?

II. Read the text below and check its comprehension.

Communications Satellites

A communications satellite is any earth-orbiting spacecraft that provides communication over long distances by reflecting or relaying radio-frequency signals.

History and Development

Some of the first communications satellites were designed to operate in a passive mode. Instead of actively transmitting radio signals, they served me-rely to reflect signals that were beamed up to them by transmitting stations on the ground. Signals were reflected in all directions, so they could be picked up by receiving stations around the world. Echo 1, launched by the United States in 1960, consisted of an aluminized plastic balloon 30 m (100 ft) in diameter. Launched in 1964, Echo 2 was 41 m (135 ft) in diameter. The capacity of such systems was severely limited by the need for powerful transmitters and large ground antennas.

Satellite communications currently make exclusive use of active systems, in which each satellite carries it own equipment for reception and transmis-sion. Score, launched by the United States in 1958, was the first active com-munications satellite. It was equipped with a tape recorder that stored mes-sages received while passing over a transmitting ground station. These mes-sages were retransmitted when the satellite passed over a receiving station. Telstar 1, launched by American Telephone and Telegraph Company in 1962, provided direct television transmission between the United States, Europe, and Japan and could also relay several hundred voice channels. Launched into an elliptical orbit inclined 45 ° to the equatorial plane, Telstar could only relay signals between two ground stations for a short period during each revolution, when both stations were in its line of sight.

Hundreds of active communications satellites are now in orbit. They receive signals from one ground station, amplify them, and then retransmit them at a different frequency to another station. One frequency band used, 500 MHZ wide, is divided into repeater channels of various bandwidths (located at 6 GHZ for upward, or uplink, transmission and 4 GHZ for downward, or downlink, transmission). A band at 14 GHZ (uplink) and 11 or 12 GHZ (downlink) is also much in use, mostly with fixed (non-mobile) ground stations. An 80-MH Z-wide band at about 1.5 GHZ (up-and downlink) is used with small, mobile ground stations (ships, land vehicles, and aircraft). Solar energy cells mounted on large panels attached to the satellite provide power for reception and transmission.

Geosynchronous Orbit

A satellite in a geosynchronous orbit follows a circular orbit over the equator at an altitude of 35,800 km (22,300 mi) completing one orbit every 24 hours, in the time that it takes the earth to rotate once. Moving in the same direction as the earth's rotation, the satellite remains in a fixed position over a point on the equator, thereby providing uninterrupted contact between ground stations in its line of sight. The first communications satellite to be placed in this type of orbit was Syncom 2, launched by the National Aeronautics and Space Administration (NASA) in 1963. Most of those that followed were also placed in geosynchronous orbit.

Commercial Communications Satellites

Deployment and operation of communications satellites on a commercial basis began with the founding of the Communications Satellite Corporation (COMSAT) in 1963. When the International Telecommunications Satellite Organization (INTELSAT) was formed in 1964, COMSAT became the U.S. member. Based in Washington, D.C., INTELSAT is owned by more than 120 nations. Intelsat 1, known as Early Bird, launched in 1965, provided either 2400 voice circuits or one two-way television channel between the United States and Europe. During the 1960s and 1970s, message capacity and transmission power of the Intelsat 2, 3, and 4 generations were progressively increased by beaming the satellite power only to the earth and segmenting the broadcast spectrum into transponder units of a certain bandwidth. The first of the Intelsat 4s, launched in 1971, provided 4000 voice circuits. With the Intelsat 5 series (1980), introduction of multiple beam operation resulted in additional increases in capacity. A satellite's power could now be concentrated on small regions of the earth, making possible smaller-aperture, lower-cost ground stations. An Intelsat 5 satellite can typically carry 12,000 voice circuits. The Intelsat 6 satellites, which entered service in 1989, can carry 24,000 circuits and feature dynamic on-board switching of telephone capacity among six beams, using a technique called SS-TDMA (satellite-switched time division multiple access). By the early 1990s, Intelsat had 15 satellites in orbit, providing the world's most extensive telecommunications system. Other systems also provide international service in competition with Intelsat. By 1997, all regulatory restraints to such competition will have been lifted. The growth of international systems has been paralleled by domestic and regional systems, such as the U.S.Telstar, Galaxy, and Spacenet programs and Europe's Eutalsat and Telecom.

Services

Commercial satellites provide a wide range of communications services. Television programs are relayed internationally, giving rise to the phenome-non known as the "global village". Satellites also relay programs to cable tele-vision systems as well as to homes equipped with dish antennas. In addition, very small aperture terminals (VSATs) relay digital data for a multitude of business services. Intelsat satellites now carry over 100,000 telephone cir-cuits, 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 reduc-ing the size of ground stations that provide telephone service.

The International Mobile Satellite Organization (INMARSAT), founded in 1979 as the International Maritime Satellite Organization, is a mobile tele-communications network, providing digital data links, telephone, and facsi-mile transmission, or fax, service between ships, offshore facilities, and shore-based stations throughout the world. It is also now extending satellite links for voice and fax transmission to aircraft on international routes.

Recent Technical Advances

Communications satellite systems have entered a period of transition from point-to-point high-capacity trunk communications between large, costly ground terminals to 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 onboard 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 toward each of the stations. Beam widths can be adjusted to cover areas as large as the entire United States or as small as a state like Maryland. Two stations far enough apart can receive different messages transmitted on the same frequency. Satellite antennas have been designed to transmit several beams in different directions, using the same reflector.

A new method for interconnecting many ground stations spread over great distances is scheduled to be tested in 1993, with the launch of NASA's ACTS (Advanced Communications Technology Satellite). Known as the hopping spot beam technique, it combines the advantages of frequency reuse, spot beams, and TDMA. By concentrating the energy of the satellite's transmitted signal, ACTS can use ground stations that have smaller antennas and reduced power requirements.

The concept of multiple spot beam communications was successfully demonstrated in 1991 with the launch of Italsat, developed by the Italian Research Council. With six spot beams operating at 30 GHZ (uplink) and 20 GHZ (downlink), the satellite interconnects TDMA transmissions between ground stations in all the major economic centers of Italy. It does this by demodulating uplink signals routing them between up- and downlink beams, and combining and remodulating them for downlink transmission.

The application of laser technology to satellite communications has been studied for over a decade. Laser beams can be used to transmit signals between a satellite and earth, but the rate of transmission is limited because of absorption and scattering by the atmosphere. Lasers operating in the blue-green wavelength, which penetrates water, have been used for communication between satellites and submarines.

Comprehension Check Up

1. What is the function of satellites? 2. What is the advantage of geosyn-chronous orbit? 3. What for were the first communications satellites design-ned? 4. When did deployment and operation of communications satellites begin? 5. In what way were message capacity and transmission power of the Intelsat 2,3,4 increased? 6. What is a satellite’s power now? 7. What are satellites’ services? 8. What does INMARSAT provide? 9. What is ‘frequency reuse’? 10. What technique combines the advantages of frequency reuse, spot beams and TDMA? 11. Can laser beams be used to transmit signals between a satellite and earth?

III. Choose the best answer.

1. The word severely means …

a) separately; b) strictly; c) normally; d) definitely

2. To follow a circular orbit implies that one should

a) not poke his nose into NASA’s affairs; b) move in a circle; c) watch how the satellite is put into orbit; d) neglect an elliptical orbit

3. To make exclusive use suggests that…

a) one uses things made by a distinguished designer; b) the use of things is very important; c) one uses these things only; d) smb. makes nice things for rich people

4. The best explanation of the word launch might be

a) lift-off; b) to send into the sky or space; c) to have smth. to eat at one’s lunch; d) set smb./smth. free

5. The word mobile means

a) Phone; b) not fixed in one position; c) fashionable; d) cool

6. Downward transmission refers to

a) the process of going down; b) the process of broadcasting from the underground; c) the process of a going down broadcasting; d) the process of making smth. on earth

7. To be in sight means

a) smth. worth seeing; b) presence in one’s view; c) to visit a well-known place; d) not to be blind

8. The verb to segment means …

a) to divide; b) to make equal parts; c) to divide into segments; d) to separate

9. The verb to relay refers to…

a) the process of being switched on or off; b) making smth. in another way; c) the process of sending out by an electrical arrangement or apparatus; d) laying again

10. Is scheduled to be tested suggests that …

a) tests should be made according to the time-table; b) smth. must be checked; c) smth. is planned to be checked at a definite future time; d) smth. should be checked quickly

IV. Use the right word from those given below.

Beam up, pick up, reflect, amplify, attach to, uninterrupted, deployment, message capacity, enhance, assign, ensure, application.

1. By … a single carrier frequency in each satellite repeater, time division multiple access … the most efficient use of the satellite’s onboard power supply. 2. The capacity of existing facilities was …. 3. The satellite provides … contact between ground stations in its line of sight. 4. Signals were … in all directions, so they could be … by receiving stations around the world. 5. Solar energy cells mounted on large panels … the satellite provide power for reception and transmission. 6. Operation and … of communications satellites began in 1963. 7. The … of laser technology to satellite communications has been studied for over a decade. 8. … was progressively increased. 9. The first communications satellites served to reflect signals that were … to them by transmitting stations on the ground. 10. Each ground station is … a time slot on the same channel for use in transmitting its communications.

V. Are the following statements True or False?

1. Commercial satellites provide a wide range of communications services.

2. Echo 1 was the first active communications satellite.

3. A satellite in a geosynchronous orbit follows an elliptical orbit.

4. Frequency reuse allows satellites to communicate with a number of ground stations.

5. Laser beams can’t be used to transmit signals between a satellite and earth because of absorption and scattering by the atmosphere.

VI. Express the main idea of each part in one sentence.

VII. Summarize the text and be ready to retell it.

VIII. Tell us about the advantages of using communications satellites.