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Ground stations have been set up in order to avoid the problem of double time lag.
A．Y
B．N
C．NG
答案
更多“Ground stations have been set up in order to avoid the problem of double time lag.A．YB．NC．”相关的问题
第1题
The Flying Phone Box
A surprise for air travelers
The strides made in satellite-communications technology in recent years are at last yielding benefits for air travelers, particularly executives who need to keep in touch with their offices. Five airlines— three of them based in the Asia-Pacific region—have recently equipped aircraft to offer outbound (向外的)telephone calls to anywhere in the world using a new satellite system. And in the not-too-distant future, businessmen will be able to send faxes in flight, too.
Air Canada, Qantas of Australia, United Airlines of the US and Japan Air Lines have at least one satellite-communication aircraft apiece. Hong Kong's Cathay Pacific Airways is also about to offer the service and several other airlines are expected to follow. The calls, dialed directly and charged to a credit card, will cost about US $ 9 ～ 10 per minute.
The earlier flying phones
Phone communications from aircraft are not entirely new. For some time, telephones have been available to passengers on US internal flights, for example, and on private jets.
But these systems have used cellular phones operating on very high frequency radio links. These systems—the same links that keep pilots in touch with flight controllers—have either limited range or low quality and reliability, particularly over vast oceans such as the Pacific. By contrast, the new satellite service will provide clear links instantly to destinations around the world.
Technological obstacles
Several technological problems had to be resolved before the service could be introduced. One was to define an effective technical standard, Airinc 741, that guarantees compatibility among different manufacturers' avionics systems—essential if the service was to be truly global.
Another was to make reliable antennas(天线) small enough and light enough to mount on aircraft of different types. And a third was to perfect voice-coding algorithms(规则系统), which tell machinery how to break down voice signals into digital signals for transmission at the standard rate of 9,600 bits a second.
A further drawback, for any type of call routed by satellite, is an uncomfortable time lag between words being spoken and received. Because of the distance that signals have to travel, there is a delay of about a quarter of a second. With airborne calls, there is the potential for a double delay as signals arriving via satellite at a ground station are passed on via another satellite to their destination.
To avoid this problem, ground stations have been equipped to identify calls from aircraft and route them to coaxial or fibre-optic cables for the second lag of their journey. Surface links, though long, are still considerably shorter than the 72,000-km round-trip into space. Most calls will therefore not suffer the double delay.
The International Maritime Satellite Organization (Inmarsat) approved the initial sitcom system for commercial service last November 'after two years of trials. Besides the handful of aircraft operated by the five major carriers who have signed on to the system, about two dozen company jets have fitted with the necessary equipment.
A new technology
The technology has three components: antennas and associated equipment on board planes, communications satellites, and ground stations.
In the course of development, antennas have lust both their dish shape and several kilograms. Even so, antenna and avionics weigh as much as two occupied scats. One obstacle to further weight savings, however, is that lighter antennas are more easily confused by the plethora(过多) of satellites now circling the Earth.
The satellites themselves are provided by Inmarsat, a London-based cooperative of 64 nations. One operational satellite and several 'back-ups
A．Y
B．N
C．NG
第2题
A．Common citizens have the right to bear arms.
B．T.V. and radio stations and newspapers have the right to report the details of crimes.
C．Every citizen has the right to kill others in order to protect himself.
D．Everybody has the fight to learn how to use weapons.
第3题
第4题
According to the passage, chimpanzees can't build power stations because______.
A．they don't have a larynx which helps to pronounce sounds
B．they cannot cooperate with each other
C．they don't have a communication system
D．they don't have speech like the humans
第5题
Advertising on radio continues to grow because______.
A．more local radio stations have been set up
B．modem technology makes it more entertaining
C．it provides easy access to consumers
D．it has been revolutionized by Internet radio
第6题
Public radio and television stations, on the 【33】______ hand, do not have advertisements and people do not have to 【34】______ to watch them. These stations gain their money 【35】______ the government, private companies, and from some of the 【36】______ who watch or listen to their programs. The 【37】______ government and some large corporations give 【38】______ , large gifts on money, to the public stations. Small businesses and people also 【39】______ money to their local public radio and television stations.
ABC, CBS, and NBC are the three 【40】______ commercial radio and television 【41】______ in the United States. Most local commercial radio and TV stations 【42】______ their programs from one of these national networks. 【43】______ example, each network had a TV news program in the evening, 【44】______ the local stations broadcast in addition to their 【45】______ local news programs.
【26】
A．that
B．this
C．it
D．which
第7题
根据下列文章,请回答 1～21 题。
Text
Most radio and television stations in the United States are commercial stations,________26is to say, they earn their money from _______ 27or commercials. Private companies purchase radio and television_______28from the commercial stations in order to_______29their products. Cable television stations are also _______ 30stations, though they do not usually have advertisements. _______31watch cable stations, people must pay the cable TV company a certain amount of money each _______ 32
ublic radio and television stations, on the_______ 33hand, do not have advertisements and people do not have to _______ 34to watch them. These stations gain their money_______ 35the government, private companies, and from some of the36who watch or listen to their programs. The _______ 37government and some large corporations give _______ 38, large gifts on money, to the public stations. Small businesses and people also_______39money to their local public radio and television stations.
ABC,CBS,and NBC are the three_______40commercial radio and television _______ 41in the United States. Most local commercial radio and TV stations _______ _______42their programs from one of these national networks. _______ 43example, each network has a TV news program in the evening, _______ 44the local stations broadcast in addition to their_______45local news programs.
第 1 题
A．that
B．this
C．it
D．which
第8题
Advertising on radio continues to grow because ___________.
A) more local radio stations have been set up
B) modern technology makes it more entertaining
C) it provides easy access to consumers
D) it has been revolutionized by Internet radio.
第9题
A.that
B.this
C.it
D.which
第10题
Public radio and television stations, on the【C8】______hand, do not have advertisements and people do not have to【C9】______to watch them. These stations gain their money from the【C10】______, from private companies, and from some of the【C11】______who watch or listen to their programs. The【C12】______government and some large corporations give【C13】______, large gifts on money, to the public stations. Small businesses and people also【C14】______money to their local public radio and television stations.
ABC , CBS, and NBC are the three【C15】______commercial radio and television【C16】______in the United States. Most local commercial radio and TV stations【C17】______their programs from one of these national networks.【C18】______example, each network had a TV news program in the evening,【C19】______the local stations broadcast in addition to their【C20】______local news programs.
【C1】
A．that
B．this
C．it
D．which
