【正文】 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(shí)時(shí)，三維坐標(biāo)，導(dǎo)航和授時(shí)的功能。這些高精度原子鐘能夠使 GPS 提供最準(zhǔn)確的時(shí)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(gè)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(gè)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(bào)軌道和實(shí)際軌道之間的偏差被傳回衛(wèi)星。）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）來計(jì)算距離。在實(shí)際當(dāng)中，需要第四個(gè)觀測值來修正時(shí)間誤差）Uuit21 Understanding the GPS (II) 認(rèn)識(shí) GPS(II)GPS Error Sources (GPS 誤差來源 )There are many sources of possible errors that will degrade the accuracy of position puted by a GPS receiver.（有許多或然誤差來源， 這種誤差 會(huì)降低GPS 接收機(jī)計(jì)算的位置的準(zhǔn)確度）The travel time of GPS satellite signals can be altered by atmospheric effects。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(jī)可以達(dá)到 2 到 5 米的準(zhǔn)確度，甚至是厘米或毫米級(jí)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(zhǔn)確度的級(jí)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(jí)接收機(jī)的技術(shù)，能夠獲得 2 到 5 米的準(zhǔn)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(jí)的接收機(jī)甚至能夠達(dá)亞米級(jí)精度，但是精度的提高是以很高代價(jià)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(jí)接收機(jī)最精確，憑裝備能夠達(dá)到厘米甚至是毫米級(jí)精度，但是它們是使用更先進(jìn)的技術(shù)才得到這樣的精度水平，自然地，就更昂貴。由于基準(zhǔn)站“知道”在哪兒，它就可以計(jì)算它自己的位置誤差（實(shí)際上，它是計(jì)算 定時(shí)誤差），并將它們應(yīng)用于將……應(yīng)用于】許多同一區(qū)域的流動(dòng)接收機(jī)）This requires that the base and rover receivers “ see” the same set of satellites at the sametime.（這需要基準(zhǔn)接收機(jī)和流動(dòng)接收機(jī)同時(shí)“看到”相同一批衛(wèi)星）The base station, depending upon how it is configured, can correct roving GPS receiver data in one (or both) of two ways.（基準(zhǔn)站，依照如何配置，可以用兩種中的一種方法（或同時(shí)用兩種）對(duì) GPS 流動(dòng)接收機(jī)的數(shù)據(jù)進(jìn)行修正）⑴in the first method, called realtime differential correction or realtime differential GPS (DGPS), the base station transmits (usually via radio link) error correction messages to other GPS receivers in the local area.（第一種方法，稱為實(shí)時(shí)差分改正或?qū)崟r(shí)差分 GPS（DGPS），基準(zhǔn)站發(fā)射（通常經(jīng)由無線電聯(lián)系）誤差改正信息到其它當(dāng)?shù)氐?GPS 接收機(jī)）In this case, the positions you read on your GPS receiver while you are out collecting data are the corrected positions.（這樣，當(dāng)你外出收集數(shù)據(jù)時(shí)在你的 GPS 接收機(jī)上讀出的位置就是改正后的位置。）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)。（⑴雙頻（L1 范圍：15971617 MHz；L2 范圍：12401260 MHz）⑵each satellite transmits a different frequency on L1 and L2。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(dǎo)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。”2 “ . . . . a puter system capa。）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(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