【正文】 Unit 20 Understan ding the GPS（認(rèn)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(biāo)，導(dǎo)航和授時的功能。）Any person with a GPS receiver can access the system, and it can be used for anyapplication that requires location coordinates.（任何擁有一臺 GPS 接收機(jī)的人都可以使用這套系統(tǒng)，可以應(yīng)用于任何需要位置坐標(biāo)的工作中）The GPS system consists of three segments: ⑴ the space segment: the GPS satellitesthemselves, ⑵the control segment, operated by the U .S. military, and ⑶the user segment,which includes both military and civilian users and their GPS equipments（.GPS 由三個部分組成：⑴太空部分：GPS 衛(wèi)星自己，⑵地面控制部分，由美國軍方控制，⑶用戶部分，包括軍方和民用用戶和他們的裝備。）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)星由美國空軍在 1978 年的早些時候發(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 個軌道平面中每個有 4 顆衛(wèi)星。）Each plane is inclined 55 degrees relative to the equator, which means that satellites crosstie equator tilted at a 55 degrees angle.（每個平面相對于赤道面傾斜55 度，這就意味著這些衛(wèi)星以 55 度角穿過赤道）The system is designed to maintain full operational capability even if two of the 24satellites fail.（這套系統(tǒng)被設(shè)計成即使 24 顆衛(wèi)星的 2 顆故障也能維持全工作能力）These 24 satellites make up a full GPS constellation.（這 24 顆衛(wèi)星組成了一個完整 的GPS 星座）The GPS satellites are powered primarily by sunseeking solar panels, with nicad batteries providing secondary power.（GPS 衛(wèi)星主要由太陽定向太陽能電池板提供動力，鎳鎘蓄電池提供備用動力）Each GPS satellite has four atomic clocks on board, only one of which is in use at a time. These highly accurate atomic clocks enable GPS to provide the most accurate timing system that exists. （每一個 GPS 衛(wèi)星有四個原子鐘，某一時刻只有一臺在使用。這些高精度原子鐘能夠使 GPS 提供最準(zhǔn)確的時間系統(tǒng)）Control Segment: . DOD Monitoring（控制部分：美國國防部監(jiān)控）The U .S. Department Of Defense maintains a master control station at Falcon Air Force Base in Colorado Springs.（美國國防部維持了一個主控站，在科羅拉多斯普林斯的佛肯空軍基地）There are four other monitor stations located in Hawaii, Ascension Island, Diego Garcia and Kwajalein.（還有 4 個監(jiān)控站座落在夏威夷、阿松森島、迭哥加西亞島、卡瓦加蘭島）The DOD stations measure the satellite orbits precisely.（DOD 站精確測量衛(wèi)星軌道）Any discrepancies between predicted orbits and actual orbits are transmitted back to the satellites.（任何預(yù)報軌道和實際軌道之間的偏差被傳回衛(wèi)星。）The satellites can then broadcast these corrections, along with the other position and timing data, so that a GPS receiver on the earth can precisely establish the location of each satellite it is tracking.（然后衛(wèi)星可以廣播這些改正值 ——連同位置和時間數(shù)據(jù)一起，以便地球上的 GPS 接收機(jī)可以精確確定它所跟蹤的每個衛(wèi)星的位置）User Segment: Military and Civilian GPS Users （用戶部分：軍方和民用 GPS 用戶）The U .S. military uses GPS for navigation, reconnaissance, and missile guidance systems.（美國軍方把 GPS 用在導(dǎo)航、偵察和導(dǎo)彈制導(dǎo)系統(tǒng)上Civilian use of GPS developed at the same time as military uses were being established, and has expanded far beyond original expectations.（GPS 的民用在軍用建立起之時就發(fā)展起來了，并且已經(jīng)發(fā)展的遠(yuǎn)遠(yuǎn)超過起初的期望）There are civilian applications for GPS in almost every field, from surveying to transportation to natural resource management to agriculture.（GPS 的民用用途幾乎 用在每個領(lǐng)域，從測量到交通到自然資源管理到農(nóng)業(yè)）Most civilian uses of GPS, however, fall into one of three categories: navigation, positioning and timing.（然而， GPS 的大部分民用用途，分為三種類型：導(dǎo)航、定位和授時。）How Does GPS Work?（ GPS 如何工作）A GPS receiver calculates its position by a technique called satellite ranging, which involves measuring the distance between the GPS receiver and the GPS satellites it is tracking.（GPS 接收機(jī)利用一項被稱為衛(wèi)星測距的技術(shù)，計算它自己的位置，涉及到 GPS 接收機(jī)到它所跟蹤的 GPS 衛(wèi)星的距離測量）The range (the range a receiver calculates is actually a pseudorange, or an estimate orange rather than a true range) or distance is measured as elapsed transit time.（距離（接收機(jī)測的距離實際上是偽距，或者說是距離估值，而不是真實距離）是用傳播時間來測的）The position of each satellite is known, and the satellites transmit their positions as part of the “ messages” they send via radio waves.（每個衛(wèi)星的位置已知，并且衛(wèi)星將它們的位置作為它們用無線電波發(fā)出的“訊文”的一部分傳送出去）The GPS receiver on the ground is the unknown point, and must pute its position based on the information it receives from the satellites.（地面 GPS 接收機(jī)是未知點，必需基于它從衛(wèi)星收到的這些信息計算它的位置。）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(jī)和衛(wèi)星的第一步，要求測出從衛(wèi)星到接收機(jī)的信號傳播時間。）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。）Distance measurements to four satellites are required to pute a 3dimensional (latitude, longitude and altitude) position.（計算一個三維（緯度、精度和高程）位置需要4 顆衛(wèi)星的距離值）In order to measure the travel time of the satellite signal, the receiver has to know when the signal left the satellite and when the signal reached the receiver.（為了測出衛(wèi)星信號的傳播時間，接收機(jī)需要知道信號是什么時候離開衛(wèi)星的和什么時候到達(dá)接收機(jī)的）Knowing when the signal reaches the receiver is easy, the GPS receiver just “ checks” its internal clock when the signal arrives to see what time it is（.確定信號到達(dá)接收機(jī)的時間容易，GPS 接收機(jī)只需在信號到達(dá)時“檢查”自己內(nèi)置的鐘，看看是什么時間）But how does it “know” when the signal left the satellite?（但是怎樣“知道”信號何時離開衛(wèi)星的呢？）All GPS receivers are synchronized with the satellites so they generate the same digital code at the same time.（所有 GPS 接收機(jī)與衛(wèi)星是同步的，因此它們在同一時間產(chǎn)生同樣的信號碼）When the GPS receiver receives a code from a satellite, it can look back in its memory bank and “remember” when