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1、Unit 24 Others,Passage A Satellite Fundamentals Passage B Microwave Radio System Passage C Introduction of Color Image Segmentation,Passage A Satellite Fundamentals Satellite communication has become a part of everyday life in the late 1980s. An international telephone call is made as easily as loca

2、l 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 the world into our hom

3、es each night.,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 life, satellite-based communicatio

4、n 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 idea has subsequently blossomed

5、 into a sophisticated satellite network that spans the globe.,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 Echospacecraft launched in 1960, was not equi

6、pped 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 locations on the earth.,In contrast w

7、ith 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 station, a communications complex that transmit

8、ted 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.,During the span of years that separates Telstarfrom today satellites, there have been

9、 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 individual ground station for only a limited period of time e

10、ach day. The satellite appeared from below the horizon, raced across the sky, and then disappeared below the opposite horizon.,Since the ground station was cut off from the now invisible satellite, a station situated below the horizon had to be activated to maintain the communication link. In a diff

11、erent 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 series.,The latter type of satellite system would have entail

12、ed 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 rapid rate of speed, the spacecraft appeared to be stationary or fix

13、ed 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.,Simply stated a satellite in a geo-stationary orbital position appears to be fixed over one portion of the earth. An altitude of

14、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, it is stationary in the sky in relation to an observer on the earth.,The

15、 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 communications link between a station and the satellite. Once the stations antenna is properly al

16、igned, 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 different slot. Prior to this era, a ground stations antenna had to physically track a

17、 satellite as it moved across the sky.,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 the earth can be reached by satellite. This concept was the basis of Arthur Clarkes original

18、 vision of a globe-spanning communications network.,Notes 1 As the satellite received a signal from a ground or earth station, a communications complex that transmitted and/or received satellite signals, it relayed its own signal to earth. 當(dāng)衛(wèi)星接收了從地面或地球站發(fā)來(lái)的信號(hào)后， 衛(wèi)星上的通信設(shè)備將要發(fā)送的和（或）已接收的衛(wèi)星信號(hào)復(fù)合起來(lái)后再發(fā)往地球。 2

19、Today communications satellites, for the most part, have followed suit and are placed in what are called geo-stationary orbital positions or “slots”. 對(duì)于當(dāng)今的通信衛(wèi)星, 絕大部分都遵循此道, 放置在被稱之為相對(duì)地面靜止軌道的位置上。,Exercises 1. Fill in the blanks. (1) This capacity to exchange information on a global basis, be it a telep

20、hone 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 communication is a of a dream that stretches back to an era when the term satellite was only an i

21、dea conceived by a few individuals. (3) Rather, Echo was a huge myriad balloon functioned as a .,(4) 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 . (5) Todays communication satellites,

22、 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) In line with this capability, a satellite may be able to supp

23、ort trans- mission.,(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 implemented a flexible timetable to the impact of their d

24、ecisions.,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 uplink for our purposes will refer to the transmission from t

25、he 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 receive a stronger signal than the Midwest.( ),(5) I

26、f 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 launch and place a satellite in any slot it chooses sin

27、ce the slots are assigned on an international basis through the auspices of the International Communications Union.( ),3. Fill the blanks with the best choice. (1) The early satellites embodied designs. a. active b. passive c. both a and b d. none of them (2) communications satellite l

28、aunched 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 point on the earth can be reache

29、d by satellite. a. one b. two c. three d. four,(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 geo

30、graphical 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 b,,Passage B Microwave Radio System Forty years have passed since the res

31、earch and 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

32、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.1,Radio communication operating at medium or high frequencies was first applied to the fields, such as broadcasting, maritime

33、 mobile 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 radi

34、o-relay 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.

35、2 There is now a marked trend that the telecommunication network is moving gradually towards digital systems.,Analogue radio-relay systems The 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,In 1951, Bell System completed a transcontinental 4 GHz ra

37、dio-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 carries 2, 7

38、00 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.,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 network. It

40、 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.,Digital radio-relay system

41、s The 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 spectrum utiliza

42、tion.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 to the re

43、duction of the total system cost; , 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 transmission is attaina

44、ble; the flexibility in transmitting various kinds of signals, such as voice, data, and video, is available without mutual interactions among different signals.,The first full-scale commercial use of a digital radio-relay system with the above advantages commenced in 1969 by the introduction of a 2

45、 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 used for digital sy

46、stems 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 generally,applied to short-haul radio-relay systems. One example of a lon

47、g 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 reason, the digital technique

48、 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 use of the 110 GHz fre

49、quency band, which is the most valuable frequency resource.,Notes 1 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. 無(wú)線微波中繼的發(fā)展是以雷達(dá)技術(shù)為基礎(chǔ)的, 現(xiàn)在它在傳輸多路電話和電視信號(hào)方面起著

50、重要作用。 這是一個(gè)由and連接的并列句。 句中的it即為the microwave radio-relay system。 for multiplex telephone and television signals是介詞短語(yǔ), 作定語(yǔ)修飾transmission medium。 ,2 Recently, a rapid advance in semiconductor and pulse circuit technologies gave impetus to the research and development of digital radio-relay systems. 近來(lái)

51、, 半導(dǎo)體和脈沖電路技術(shù)的迅速發(fā)展, 推動(dòng)了數(shù)字無(wú)線中繼系統(tǒng)的研究和發(fā)展。 本句的主語(yǔ)為advance, 而in semiconductor and pulse circuit technologies是介詞短語(yǔ), 修飾advance。 ,3 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. 目前, 多數(shù)微波系統(tǒng)均采用同樣的調(diào)制和中繼方法,

52、 這一事實(shí)清楚地表明BTL很有先見(jiàn)之明。 本句中that most of the present microwave systems use the same modulation and repeater method是定語(yǔ)從句, 修飾主語(yǔ)中的主語(yǔ)that fact。 ,4 The digital systems for which preliminary researches were conducted in the 1940s were unfortunately superseded by the rapidly developed frequency modulation (FM

53、) systems, mainly because of the lack of high-speed pulse technique for multiplexing and the ineffectiveness in frequency spectrum utilization. 在20世紀(jì)40年代, 由于缺乏高速脈沖復(fù)用技術(shù)以及數(shù)字系統(tǒng)的頻譜利用率不高, 初期進(jìn)行的數(shù)字系統(tǒng)研究不幸由快速發(fā)展的頻率調(diào)制系統(tǒng)代替。 for which preliminary researches were conducted in the 1940s是一個(gè)定語(yǔ)從句, 修飾the digital syst

54、ems。 because of之后應(yīng)接短語(yǔ), 它有兩個(gè)并列的短語(yǔ), 即the lack of ...和the ineffectiveness in ...。 ,5 The first full-scale commercial use of a digital radio-relay system with the above advantages commenced in 1969 by the introduction of a 2 GHz digital radio transmission system carrying 240 telephone channels, which w

55、as developed in Japan. 1969年, 日本開(kāi)發(fā)了具有240條話音信道的2GHz數(shù)字無(wú)線傳輸系統(tǒng), 從而使具有上述優(yōu)點(diǎn)數(shù)字無(wú)線中繼系統(tǒng)全面投入商用。 carrying 240 telephone channels是分詞短語(yǔ)作定語(yǔ), 修飾system。 ,6 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. 2 GHz頻帶通常應(yīng)用

56、于小容量系統(tǒng), 其速率約為10 Mbps; 11或13 GHz頻帶用于中等容量系統(tǒng), 其速率約為50或100 Mbps。 在and the 11 or 13 GHz band to medium capacity systems at 50 or 100 Mbps句中, band后面省略了applied, 這種省略使文章較為簡(jiǎn)潔。 ,Exercises 1. Fill in the blanks. (1) This capacity to exchange information on a global basis, be it a telephone call or a news story

57、, 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 communication is a of a dream that stretches back to an era when the term satellite was only an idea conceived by a few

58、individuals.,(3) Rather, Echo was a huge myriad balloon functioned as a . (4) 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 . (5) Todays communication satellites, for the most part, have f

59、ollowed suit and are now placed in what are called orbital position or “ ”.,2. True/False. (1) Spacecraft such as Telstar and Echo were placed in low earth orbits.( ) (2) The uplink for our purposes will refer to the transmission from the earth station to the satellite, while the downlink is

60、the transmission from the satellite to the earth station.( ) (3) The early satellites embodied active and passive designs. ( ) (4) Telstarcommunications satellite launched in 1962 carried receiving and transmitting equipment.( ),(5) Based on these principles, three satellites placed in equidistant

61、positions around the earth can create a world-wide communications system in that almost every point on the earth can be reached by satellite.( ),,Passage C Introduction of Color Image Segmentation Image segmentation refers to partitioning an image into different regions that are homogeneous or “simi

62、lar” in some image characteristics. It is an important facet of human visual perception. Humans use their visual sense to partition their environment into distinct objects to help recognize these objects, classify them, guide their movements, and for almost every other visual task. It is a complex p

63、rocess that includes many interacting components. The analysis of color, shape, motion, and texture of objects is usually involved in this process. For the human visual system, this segmentation process is a spontaneous, natural activity.,Unfortunately, it is not easy to create artificial algorithms

64、 whose performance is comparable to that of the human visual system. As Marr has suggested that one of the major obstacles to the successful development of theories on segmentation has been a tendency to underestimate the complexity of the problem because the human performance is mediated by methods

65、 that are largely subconscious. Because of this, segmentation of images is weakened by various types of uncertainty making the simplest techniques ineffective.,Segmentation is usually the first task of any image analysis process, and thus, subsequent tasks rely heavily on the quality of segmentation

66、. For this reason, a considerable care is taken to improve the probability of a successful segmentation.,Image segmentation has taken a central place in numerous applications, including, but not limited to, multimedia databases, color image and video transmission over the Internet, digital broadcasting, interactive TV, video-on-demand, computer-based training, distance education, video-conferencing and tele-medicine, and, with the development of the hardware and communications infrastructure, to