【正文】 （⑵每個(gè)衛(wèi)星以 L1 和 L2波段發(fā)射不同的頻率⑶a different datum and time reference system to GPS。 when a GPS signal passes through the ionosphere and troposphere it is refracted, causing the speed of the signal to be different from the speed of a GPS signal in space.（GPS 衛(wèi)星信號(hào)的傳播時(shí)間會(huì)因大氣影響而改變；當(dāng) GPS 信號(hào)穿過(guò)電離層和對(duì)流層時(shí)會(huì)發(fā)生折射，因?yàn)樾盘?hào)速度與在太空中信號(hào)的速度不同。）Space Segment: The GPS Constellation（太空部分： GPS 星座）The first GPS satellite was launched by the U .S. Air Force in early 1978.（第一顆 GPS 衛(wèi)星由美國(guó)空軍在 1978 年的早些時(shí)候發(fā)射的）There are now 24 satellites orbiting the earth at an altitude of about 10,900 miles.（現(xiàn)在有24 顆衛(wèi)星在繞地球大約 10900 英里的高度運(yùn)行）The high altitude insures that the satellite orbits are stable, precise and predictable, and that the satellites’ motion through space is not affected by atmospheric drag.（這么高的高度確保了衛(wèi)星軌道是穩(wěn)定、精確和可知的，并且確保了衛(wèi)星穿過(guò)太空的運(yùn)行不受到大氣阻力的影響）There are four satellites in each of 6 orbital planes.（6 個(gè)軌道平面中每個(gè)有 4 顆衛(wèi)星。）Differential Correction（差分改正）Differential correction is a method used to reduce the effects of selective available (SA) and other sources of GPS positioning error (differential correction cannot correct for multipath or receiver error。）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(duì)赤道面軌道有一個(gè)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(gè)系統(tǒng)將能夠完全的共同使用，這意味著一個(gè)地球上的使用者將可以使用任一接收機(jī)從屬于任一三個(gè)系統(tǒng)的任一衛(wèi)星組接收信號(hào)確定一個(gè)位置）BEIDOU（北斗）BEIDOU is a satellite positioning and navigating system developed and deployed independently by China.（北斗是一個(gè)由中國(guó)獨(dú)立發(fā)展和部署的衛(wèi)星定位和導(dǎo)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(gè)系統(tǒng)的太空部分由 3 顆（2 顆運(yùn)行＋1 顆備用）地球同步衛(wèi)星運(yùn)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(shí)起一直運(yùn)行穩(wěn)定）The spare one was successfully launched on May 25, 2003.（備用的一個(gè)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(guó)防目的而發(fā)展，這套系統(tǒng)也用于民用，將在我國(guó)許多經(jīng)濟(jì)部門提供相當(dāng)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(shí)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(guó)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(dòng)式定位系統(tǒng)，不需要從使用者向衛(wèi)星發(fā)送信號(hào)）But BEIDOU is an active positioning system.（但是北斗是一個(gè)主動(dòng)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(lái)定位和導(dǎo)航，使用者需要首先向衛(wèi)星發(fā)射信號(hào)，然后信號(hào)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(lái)的信號(hào)為基礎(chǔ)，計(jì)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(cè)數(shù)據(jù)獲取和定位計(jì)算都是由 GCC 來(lái)實(shí)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(jī)都有接收和發(fā)射或轉(zhuǎn)發(fā)信號(hào)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(gè)縮寫含義，地理信息系統(tǒng)。）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(jī)的人沒有意識(shí)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)。）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(jī)知道用了多少時(shí)間，用信號(hào)的傳播時(shí)間乘上光速（因?yàn)樾l(wèi)星信號(hào)以光速傳播，大約186,000 英里每秒）來(lái)計(jì)算距離。）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(cè)量GPS 接收機(jī)和衛(wèi)星的第一步，要求測(cè)出從衛(wèi)星到接收機(jī)的信號(hào)傳播時(shí)間。）Most recreational grade receivers cannot collect and store files that can be differentially corrected.（多數(shù)休閑級(jí)接收機(jī)不能收集和存儲(chǔ)可以用來(lái)差分改正的文件）How Accurate Is GPS?（ GPS 準(zhǔn)確度如何？）The accuracy that can be achieved using GPS depends on the type of equipment used, the time of observation, the positions of the satellites being used to pute positions, the model and the software you used.（使用 GPS 可以達(dá)到的準(zhǔn)確度依賴于使用裝置的類型、觀測(cè)時(shí)間、用來(lái)計(jì)算位置的衛(wèi)星的位置、你所使用的模式和軟件）In general, recreational and mapping grade receivers using C/A code without differential correction are accurate to between 30 and 100 meters.（大體上講，使用 C/A 碼沒有差分改正的休閑和繪圖級(jí)接收機(jī)精度在 30 到 100 米之間。為了提供一個(gè)很好的了解，下面給出的定義由蘭德（1989）和美國(guó)地質(zhì)調(diào)查局（美國(guó)地質(zhì)勘探局，1997） “. . . . a system of hardware, software, and procedures designed to support the capture, management, manipulation, analysis, modeling, and display of spatially referenced data for solving plex planning and management problems .”1“GIS 是一個(gè)由硬件，軟件，和程序設(shè)計(jì)，支持捕獲，管理，處理，分析，建模，并顯示空間參照的數(shù)據(jù)，以解決復(fù)雜的規(guī)劃和管理的問(wèn)題的一個(gè)系統(tǒng)。 it counteracts only the errors that are mon to both referen