【正文】 hs). The effect is similar to a ghost or double image on a TV set（.當這種情況發(fā)生時，接收機接收直線路徑來的信號，也接收延遲路線（多路徑）來的信號。）The first step in measuring the distance between the GPS receiver and a satellite requires measuring the time it takes for the signal to travel from the satellite to the receiver.（測量GPS 接收機和衛(wèi)星的第一步，要求測出從衛(wèi)星到接收機的信號傳播時間。Unit 20 Understan ding the GPS（認識 GPS）What Is GPS?（什么是 GPS）The global Positioning System (GPS) operated by the U .S. Department OF Defense (DOD)is a satellitebased system that can be used to locate positions anywhere on the earth（.全球定位系統(tǒng)（GPS），由美國國防部管理，是一個基于人造衛(wèi)星的系統(tǒng)，可以用來在全球任何地方定位）GPS provides continuous (24 hours/day), realtime, 3dimensional positioning, navigation and timing worldwide.（GPS 提供全世界范圍內(nèi)的全天候，實時，三維坐標，導航和授時的功能。）Once the receiver knows how much time has elapsed, the travel time of the signal multiplies the speed of light (because the satellite signals travel at the speed of light, approximately 186,000 miles per second) to pute the distance.（一旦接收機知道用了多少時間，用信號的傳播時間乘上光速（因為衛(wèi)星信號以光速傳播，大約186,000 英里每秒）來計算距離。其效果與電視的重影或重像類似）How to Reduce GPS Errors（如何減小 GPS 的誤差）You’ve probably heard people talk about getting 2 to 5 meters accuracy with a GPS receiver, or even centimeter or millimeter accuracy（.你可能聽說過有人說 GPS 接收機可以達到 2 到 5 米的準確度，甚至是厘米或毫米級精度）Is there a way to cancel out the errors and get satisfied accuracy?（有方法低償誤差并獲得滿意的精度嗎？） The answer is yes, but the level of accuracy depends on the type of equipment you are using.（回答是肯定的，但是準確度的級別依賴于你使用的裝備的類型）The following discussion describes a technique used to achieve 2 to 5 meters accuracy using mapping (resource) grade receivers.（下面的討論描述了一種用于繪圖（資源）級接收機的技術(shù)，能夠獲得 2 到 5 米的準確度）Some mapping grade receivers are even capable of submeter accuracy, but the increased accuracy es at a price.（一些繪圖級的接收機甚至能夠達亞米級精度，但是精度的提高是以很高代價帶來的Survey grade receivers are the most accurate, capable of centimeter or even millimeter accuracy, depending on the equipment, but they use more advanced techniques to achieve this level of accuracy and, naturally, are more expensive.（測地級接收機最精確，憑裝備能夠達到厘米甚至是毫米級精度，但是它們是使用更先進的技術(shù)才得到這樣的精度水平，自然地，就更昂貴。）Many people using recreational grade receivers don’ t realize they cannot get highly accurate readings using them autonomously (without differential correction)（.許多用休閑類接收機的人沒有意識到他們直接使用（沒有差分改正）它們不能取得高精度的讀數(shù)。GALILEO（伽利略）Satellite navigation users in Europe today have no alternative other than to take their positions from US GPS or Russian GLONASS satellites.（現(xiàn)在，歐洲衛(wèi)星導航的使用者除了使用美國的GPS 或俄羅斯的GLONASS 衛(wèi)星定位，沒有其它選擇了。）By placing satellites in orbits at a greater inclination to the equatorial plane than GPS, GALILEO will achieve better coverage at high latitudes.（由于比起 GPS，其衛(wèi)星放置相對赤道面軌道有一個更大的傾角，GALILEO 將在高緯度獲得更好的覆蓋）This will make it particularly suitable for operation over northern Europe, an area not well covered by GPS.（這將使得它特別適合北歐地區(qū)使用，該地區(qū) GPS 覆蓋率不很好）GPS,GLONASS and GALILEO, the three systems will be fully interoperable, which means that a user on Earth will be able to determine a position with any receiver picking up signals from any bination of satellites belonging to any of the three systems.（GPS,GLONASS 和 GALILEO，這三個系統(tǒng)將能夠完全的共同使用，這意味著一個地球上的使用者將可以使用任一接收機從屬于任一三個系統(tǒng)的任一衛(wèi)星組接收信號確定一個位置）BEIDOU（北斗）BEIDOU is a satellite positioning and navigating system developed and deployed independently by China.（北斗是一個由中國獨立發(fā)展和部署的衛(wèi)星定位和導航系統(tǒng)）The space segment of this system consists of 3 (2 operational +1 spare) geosynchronous satellites orbiting 360,000km high above the Earth.（這個系統(tǒng)的太空部分由 3 顆（2 顆運行＋1 顆備用）地球同步衛(wèi)星運行于距地球 360000 公里的軌道上）The two operational satellites were both launched in 2000 and they have operated stably since then.（兩顆工作衛(wèi)星都在 2000 發(fā)射，它們從那時起一直運行穩(wěn)定）The spare one was successfully launched on May 25, 2003.（備用的一個在 2003 年 3 月25 日被成功發(fā)射）Although developed for defense purpose, the system will also be applied in civilian uses and will afford considerable advantages in many sectors of the economy of our country.（盡管為國防目的而發(fā)展，這套系統(tǒng)也用于民用，將在我國許多經(jīng)濟部門提供相當可觀的利益）Compared with Global Navigation Satellite Systems (GPS, GLONASS and GALILEO),BEIDOU should in fact be called a Local Navigation Satellite System.（與全球?qū)Ш叫l(wèi)星系統(tǒng)（GPS,GLONASS 和 GALILEO）相比，北斗實際應(yīng)該被叫做區(qū)域?qū)Ш叫l(wèi)星系統(tǒng)）It covers the territory and territory sea of China and around longitude 70~140 east, latitude 5~55 north.（它覆蓋了中國的領(lǐng)土和領(lǐng)海以及周邊地區(qū)——東經(jīng) 70－140，北緯 5 －55）The positioning principles of BEIDOU and GNSS are different.（北斗與 GNSS的定位原理不同）GPS, GLONASS and GALILEO are all passive positioning systems that do not require the transmission of signals from users to satellites.（GPS，GLONASS 和 GALILEO 都是被動式定位系統(tǒng)，不需要從使用者向衛(wèi)星發(fā)送信號）But BEIDOU is an active positioning system.（但是北斗是一個主動式定位系統(tǒng)）To make use of this system for positioning or navigating, users need to transmit signals to the satellites at first and then signals are retransmitted to the Ground Control Center.（用這套系統(tǒng)來定位和導航，使用者需要首先向衛(wèi)星發(fā)射信號，然后信號被轉(zhuǎn)送至地面控制中心）GCC putes out the users’ position based on the signals transmitted from the satellites.（GCC 以從衛(wèi)星傳來的信號為基礎(chǔ)，計算出使用者的位置）At last, the users get their position information via the satellites.（最后，經(jīng)由衛(wèi)星使用者得到它們的信息）Observation data acquisition and position putation are both carried out by GCC（.觀測數(shù)據(jù)獲取和定位計算都是由 GCC 來實現(xiàn)So it is required that both satellites and receivers own the functions of receiving andsending or retransmitting the signals.（因此，這就需要衛(wèi)星和接收機都有接收和發(fā)射或轉(zhuǎn)發(fā)信號的功能）Unit 23 GIS Basics（GIS 的基礎(chǔ)）Definitions of GIS“GIS” is an acronym meaning of Geographic Information System. In order to provide a good understanding of GIS, the following two definitions given by Rhind (1989) and the United States Geological Survey ( USGS , 1997 ) respectively are presented first地理信息系統(tǒng)” 是一個縮寫含義，地理信息系統(tǒng)。）In response, Europe will spend billions assembling a civil satellite system called Galileo ,scheduled to be operation by 2008.（作為反應(yīng)，歐洲將花費數(shù)十億裝配一個民用衛(wèi)星系統(tǒng)稱為“Galileo”，預期在 2008 年運轉(zhuǎn)For the benefit of students to get some knowledge on the petition in space orbit, brief introduction to GLONASS, GALILEO and BEIDOU is provided as follows.（為了使學生們得到一些太空軌道上的競爭的