大学英语四级模拟试卷第11组（阅读3）
Passage Three
Questions 31 to 35 are based on the following passage.
In a world that is becoming more and more interdependent, there is an ever-increasing need to link communications systems on various continents and to provide live international television coverage. This need is now being met by the communications satellites.
Communications satellites make use of technology that has been available for some time: the microwave radio relay. Microwave, which have a higher frequency than
ordinary radio waves, are used routinely in sending thousands of telephone calls and television programs across long distances. They give high-quality performance, and they can carry many messages at the same time.
But they has always been one problem in using radio relay in overseas communications. Although high-frequency waves can travel almost unlimited distances, they
travel only in straight lines. Since the curvature of the earth limits a microwave’s line-of-sight path to about 30 miles, good reception requires a series of relay towers spaced every 30 miles or so. Obviously it isn’t possible to built these towers across the ocean. But by sending signals high up into the sky and then bouncing them back again to a far-off spot, we can send microwave messages long distances.
As long ago as 1945, Arthur C. Clarke, an English science-fiction writer, proposed that manned “stationary” satellites be used to relay and broadcast electromagnetic communication signals. In 1945, of course, the idea of getting a satellite out into space seemed fantastic. But with ten years, satellites were close to reality. With the first launching of a satellite into orbit by the Soviet Union (Sputnik I) in 1957, the real development work on satellite communications began.
Shortly thereafter, two successful satellites were launched in the United States, Echo I and Telstar I.
The launching of the Telstar I satellite in 1962 marked a major step toward opening the era of commercial satellite communications. Echo I, a ten-story aluminum-coated balloon, was a “passive” target. it merely reflected weak signals back to the earth. But Telstar I was the first “active” satellite to pick up a broadband signal, amplify it, and transmit it back to the earth on a different frequency. The satellite’s transmission of transatlantic television thrilled millions.
A few months after Telstar I went into orbit, Relay, a medium-altitude satellite launched by the National Aeronautics and Space Administration (NASA), provided
the first satellite communication between North and South America. Relay was followed by the Telstar II satellite, and by NASA’s Syncom series and its successors——all of them high-altitude (23,000 miles) satellites whose orbits are synchronous with the rotation of the earth so that this positions, if they could be seen from the earth, would appear to be fixed in one spot.
Shortly before Telstar I was launched, the Un责任编辑:yechenglu
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