资源描述
Unit 24 Others,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 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 the world into our homes 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 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 idea has subsequently blossomed 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 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 locations on the earth.,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 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.,During the span of years that 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 individual 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.,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 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 series.,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 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“slots2.,Simply stated a satellite in a geo-stationary orbital 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,it is stationary in the sky in relation to an observer on the earth.,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 communications 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 different slot.Prior to this era,a ground stations antenna had to physically track a 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.
展开阅读全文