1、Unit 24 Others Unit 24 Others Passage A Satellite Fundamentals Passage B Microwave Radio System Passage C Introduction of Color Image Segmentation Unit 24 Others Passage A Satellite FundamentalsSatellite communication has become a part of everyday life in the late 1980s.An international telephone ca
2、ll is made as easily as local call to a friend who lives down the block.We also see international events,such as an election in England and a tennis match in France,with the same regularity as local political and sporting events.In this case,a television news program brings the signals and sounds of
3、 the world into our homes each night.Unit 24 Others This capability to exchange information on a global basis,be it a telephone call or a news story,is made possible through a powerful communications tool-the satellite.For those of us who grew up at a time when the space age was not a part everyday
4、life,satellite-based communication is the culmination of a dream that stretches back to an era when the term satellite was only an idea conceived by a few inspired individuals.These pioneers included authors such as Arthur C.Clarke,who fostered the idea of a worldwide satellite system in 1945.This i
5、dea has subsequently blossomed into a sophisticated satellite network that spans the globe.Unit 24 Others The first generation of satellites was fairly primitive when compared with contemporary spacecraft.These early satellites embodied active and passive designs.A passive satellite,such as the Echo
6、spacecraft launched in 1960,was not equipped with a two-way transmission system.Rather,Echo was a huge aluminized myriad balloon that functioned as a reflector.After the satellite was placed in a low earth orbit,signals relayed to Echo reflected or bounced off its surface and returned to different l
7、ocations on the earth.Unit 24 Others In contrast with the Echo series,the Telstaractive communications satellite launched in 1962 carried receiving and transmitting equipment.It was an active participant in the reception-transmission process.As the satellite received a signal from a ground or earth
8、station,a communications complex that transmitted and/or received satellite signals,it relayed its own signal to earth.1 Telstar also paved the way for todays communications spacecraft since it created the worlds first international satellite television link.Unit 24 Others During the span of years t
9、hat separates Telstarfrom today satellites,there have been a number of improvements.For example,spacecraft such as Telstar and Echo were placed in low earth orbits.In this type of orbital position,a satellite traveled at such a great rate of speed that it was visible,and hence usable,to an individua
10、l ground station for only a limited period of time each day.The satellite appeared from below the horizon,raced across the sky,and then disappeared below the opposite horizon.Unit 24 Others Since the ground station was cut off from the now invisible satellite,a station situated below the horizon had
11、 to be activated to maintain the communication link.In a different scenario,it would have been necessary to launch a series of satellites to create a continuous satellite-based relay for any given earth station.As one satellite disappeared,it would have been replaced by the next satellite in the ser
12、ies.Unit 24 Others The latter type of satellite system would have entailed the development of a very complex and cumbersome earth and space-based network.Fortunately though,this problem was eliminated in 1963 and 1964 through the launching of the Syncom satellites.Rather than circling the earth at a
13、 rapid rate of speed,the spacecraft appeared to be stationary or fixed in the sky.Todays communications satellites,for the most part,have followed suit and are new placed in what are called geo-stationary orbital positions or“slots”2.Unit 24 Others Simply stated a satellite in a geo-stationary orbit
14、al position appears to be fixed over one portion of the earth.An altitude of 22,300 miles above the earths equator,a satellite travels at the same speed at which the earth rotates,and its motion is synchronized with the earth rotation.Even though the satellite is moving at an enormous rate of speed,
15、it is stationary in the sky in relation to an observer on the earth.Unit 24 Others The primary value of a satellite in a geo-stationary orbit is its ability to communicate with ground stations in its coverage area 24 hours a day.This orbital slot also simplifies the establishment of the communicatio
16、ns link between a station and the satellite.Once the stations antenna is properly aligned,only minor adjustments may have to be made in the antennas position over a period of time.The antenna is repositioned to a significant degree only when the station establishes contact with a satellite in a diff
17、erent slot.Prior to this era,a ground stations antenna had to physically track a satellite as it moved across the sky.Unit 24 Others Based on these principles,three satellites placed in equidistant positions around the earth can create a world-wide communications system in that almost every point on
18、 the earth can be reached by satellite.This concept was the basis of Arthur Clarkes original vision of a globe-spanning communications network.Unit 24 Others Notes1 As the satellite received a signal from a ground or earth station,a communications complex that transmitted and/or received satellite s
19、ignals,it relayed its own signal to earth.当卫星接收了从地面或地球站发来的信号后，卫星上的通信设备将要发送的和（或）已接收的卫星信号复合起来后再发往地球。2 Today communications satellites,for the most part,have followed suit and are placed in what are called geo-stationary orbital positions or“slots”.对于当今的通信卫星,绝大部分都遵循此道,放置在被称之为相对地面静止轨道的位置上。Unit 24 Others
20、 Exercises1.Fill in the blanks.(1)This capacity to exchange information on a global basis,be it a telephone call or a news story,is made possible through a powerful communications tool .(2)For those of us who grew up at a time when the space age was not a part of everyday life,satellite-based commun
21、ication is a of a dream that stretches back to an era when the term satellite was only an idea conceived by a few individuals.(3)Rather,Echo was a huge myriad balloon functioned as a .Unit 24 Others(4)In a different scenario,it would have been necessary to launch a series of satellites to create a c
22、ontinuous satellite-based relay for any given .(5)Todays communication satellites,for the most part,have followed suit and are now placed in what are called orbital position or“”.(6)In order to create communications link,the satellite uses ,the satellite equipment that conducts the two-way relays.(7
23、)In line with this capability,a satellite may be able to support trans-mission.Unit 24 Others(8)With two-degree spacing standard,an earth station must be equipped with an accurate antenna that can receive a satellites signal without created by a neighboring spacecraft.(9)Fortunately though,the FCC i
24、mplemented a flexible timetable to the impact of their decisions.Unit 24 Others 2.True/False.(1)Spacecraft such as Telstar and Echo were placed in low earth orbits.()(2)The antenna is repositioned to a minor degree when the station establishes contact with a satellite in a different slot.()(3)The up
25、link for our purposes will refer to the transmission from the earth station to the satellite,while the downlink is the transmission from the satellite to the earth station.()(4)In the case of the United States,the transmission can be concentrated in one part of the country,and the East Coast could r
26、eceive a stronger signal than the Midwest.()Unit 24 Others(5)If an earth station pointed its receiving dish toward a specific satellite,it could not intercept a different satellites signal.()(6)The FCCs decision has had serious repercussions for organizations that use satellites.()(7)A nation can la
27、unch and place a satellite in any slot it chooses since the slots are assigned on an international basis through the auspices of the International Communications Union.()Unit 24 Others 3.Fill the blanks with the best choice.(1)The early satellites embodied designs.a.active b.passive c.both a and b d
28、.none of them(2)communications satellite launched in 1962 carried receiving and transmitting equipment.a.Echo b.Telstarc.Intelsat d.Intelsat(3)Based on these principles,satellites placed in equidistant positions around the earth can create a world-wide communications system in that almost every poin
29、t on the earth can be reached by satellite.a.one b.two c.three d.fourUnit 24 Others(4)The Intelsat satellites carried transponders that provided the satellite with a total average transmission capacity of voice circuits.a.12;4000 b.27;4000 c.2;240 d.48;100,000(5)A would fall on a narrowly defined ge
30、ographical zone.a.footprint b.spot beam c.both a and b d.none of them(6)A dish could receive this spot beam transmission within the confines of the reception area.a.smaller b.less expensive c.a or b d.a and bUnit 24 Others Passage B Microwave Radio SystemForty years have passed since the research an
31、d development of microwave technology were launched in the United States,the United Kingdom,Germany,France and other countries.At the beginning,research was mainly directed towards the development of radar technique.The results were first applied for military purposes.The development of the microwav
32、e radio-relay system was based on radar technique,and it now plays an important role as the transmission medium for multiplex telephone and television signals.1Unit 24 Others Radio communication operating at medium or high frequencies was first applied to the fields,such as broadcasting,maritime mob
33、ile and intercontinental communication,where its superiority was evident.It was only after the advent of microwave communication technology that it became possible for radio technology to be applied in full scale in the field where cable transmission had been dominant.Thus,the microwave radio-relay
34、system using analogue modulation grew into a very important transmission medium in the overall telecommunications networks in many countries.Recently,a rapid advance in semiconductor and pulse circuit technologies gave impetus to the research and development of digital radio-relay systems.2 There is
35、 now a marked trend that the telecommunication network is moving gradually towards digital systems.Unit 24 Others Analogue radio-relay systemsThe first microwave radio-relay system was established in 1947 by Bell Telephone Laboratories(BTL)between New York and Boston(300 km).It carried 480 channels
36、operating in the 4 GHz band,using frequency modulation and heterodyne repeater scheme.The fact that most of the present microwave systems use the same modulation and repeater method is a clear indication of the excellent foresight of BTL.3 Unit 24 Others In 1951,Bell System completed a transcontinen
37、tal 4 GHz radio-relay system between New York and San Francisco.It carried 600 telephone channels or one television signal.After that,many countries in every region of the world undertook the development of microwave radio-relay systems.The present highest capacity microwave system in operation carr
38、ies 2,700 telephone channels.Recently,work has started for developing a system with a capacity of 3,600 channels.Microwave systems are now playing a very important role in the telecommunications networks of almost all countries.In vast countries,such as the United States and Canada,microwave systems
39、 carry the major portion of the long-distance traffic.Unit 24 Others On the other hand,in Europe,which consists of many small or medium countries,microwave systems in the network seem to be slightly less important.In many developing countries,microwave radio-relay systems play the key role in the ne
40、twork.It is quite logical because the microwave system has the following features:the initial investment required for the construction of transmission routes is low;the construction can be completed within a relatively short period;it is suitable for television transmission.Unit 24 Others Digital ra
41、dio-relay systemsThe digital systems for which preliminary researches were conducted in the 1940s were unfortunately superseded by the rapidly developed frequency modulation(FM)systems,mainly because of the lack of high-speed pulse technique for multiplexing and the ineffectiveness in frequency spec
42、trum utilization.4The study on microwave digital transmission systems was resumed in the early 1960s.According to various investigations on the microwave digital systems,their features can be summed up as follows:the realization of low-cost high-speed PCM terminal equipment contributes a great deal
43、to the reduction of the total system cost;Unit 24 Others the almost complete pulse regenerative repeating can be achieved provided the carrier-to-noise power ratio is above the threshold level,and the accumulation of noise due to multiple repeating is negligible.Therefore high-quality long-haul tran
44、smission is attainable;the flexibility in transmitting various kinds of signals,such as voice,data,and video,is available without mutual interactions among different signals.Unit 24 Others The first full-scale commercial use of a digital radio-relay system with the above advantages commenced in 1969
45、 by the introduction of a 2 GHz digital radio transmission system carrying 240 telephone channels,which was developed in Japan.5 Since then,development has continued in various countries all over the world,especially in the United States,Canada,Italy,United Kingdom,France and Japan.Frequency bands u
46、sed for digital systems are mainly 2,11 and 13 GHz at the present stage.The 2 GHz band is generally applied to small capacity systems about 10 Mbps and the 11 or 13 GHz band to medium capacity systems at 50 or 100 Mbps.6 These frequencies are generallyUnit 24 Others applied to short-haul radio-relay
47、 systems.One example of a long haul digital system is the 6 GHz band system of 45 Mbps which has been operational for some years in the United States.There is a marked tendency that the future telecommunications network in general will gradually evolve into the integrated digital network.For this re
48、ason,the digital technique applied to radio-relay systems is being actively studied all over the world.The target which is still to be pursued in the future development of the digital technique is the more efficient use of the frequency spectrum.In particular,efforts should be continued for a better
49、 use of the 110 GHz frequency band,which is the most valuable frequency resource.Unit 24 Others Notes1 The development of the microwave radio-relay system was based on radar technique,and it now plays an important role as the transmission medium for multiplex telephone and television signals.无线微波中继的
50、发展是以雷达技术为基础的,现在它在传输多路电话和电视信号方面起着重要作用。这是一个由and连接的并列句。句中的it即为the microwave radio-relay system。for multiplex telephone and television signals是介词短语,作定语修饰transmission medium。Unit 24 Others 2 Recently,a rapid advance in semiconductor and pulse circuit technologies gave impetus to the research and developm
