【正文】 it emitted the same code（.當(dāng) GPS 接收機(jī)收到一個(gè)從衛(wèi)星來的碼時(shí)，它可以在它的內(nèi)存條中回憶并“記起”它發(fā)出的相同碼）This little “ trick” allows the GPS receiver to determine when the signal left the satellite.（這個(gè)小小的“技巧”使得 GPS 接收機(jī)能夠確定信號是何時(shí)離開衛(wèi)星的）Once the receiver has the distance measurements, it’s basically a problem of geometry（.一旦接收機(jī)測出了距離，基本上就只有幾何上的問題了）If it “ knows” where the four satellites are, and how far it is from each satellite, it can pute its location through trilateration. Here’s an illustration of how it works.（如果知道了 4 顆衛(wèi)星的位置，距每顆衛(wèi)星的距離，就可以通過三邊測量來計(jì)算它的位置。這里有個(gè)例子說明它如何工作的）The GPS receiver “ locks on” to one satellite and calculates the range to be 12,000 miles.（GPS 接收機(jī)“鎖定”一顆衛(wèi)星并計(jì)算出距離為 12,000 英里）This fact helps narrow the receiver location down, but it only tells us that we are somewhere on a sphere which is centered on the satellite and has a 12,000 mile radius.（這把接收機(jī)的位置限定下來，但是它只能告訴我們在一個(gè)以這個(gè)衛(wèi)星為中心，半徑 12,000 英里的球面上）Many of the locations on that sphere are not on earth, but out in space.（這個(gè)球面的許多位置不在地球上，而在太空中）Now,consider that the receiver picks up a signal from a second satellite and calculates the range between the receiver and the satellite to be 10,000 miles.（現(xiàn)在，考慮這個(gè)接收機(jī)從第二顆衛(wèi)星獲得一個(gè)信號，計(jì)算出接收機(jī)到這顆衛(wèi)星的距離是 10,000英里）That means we are also somewhere on a sphere with a 10,000 mile radius with the second satellite at the center.（這就意味著我們也在一個(gè)以第二顆衛(wèi)星為中心、半徑 10,000 英里的球面上的某處）We must, therefore, be somewhere where these two spheres intersect.（因此，我們肯定在這兩個(gè)球面相交的某處。）When the two spheres intersect, a circle is formed, so we must be somewhere on that circle.（當(dāng)這兩個(gè)球面相交時(shí)，一個(gè)圓圈形成了，因此我們肯定在這個(gè)圓圈的某處）If the receiver picks up another satellite, says at 11,000 miles away, another sphere is formed, and there are only two points where the three spheres intersect.（如果接收機(jī)“拾取”了另一個(gè)衛(wèi)星，比方說11,000 英里以外，另一個(gè)球面形成了，三個(gè)球面相交只有兩個(gè)交點(diǎn)）Usually the receiver can discard one of the last two points because it is nowhere near the earth.（通常接收機(jī)可以丟棄最后這兩點(diǎn)中的一個(gè)點(diǎn)，因?yàn)樗x地球差的 遠(yuǎn)）So, we’re left with one point which is the location of the GPS practice, a fourth measurement is needed to correct for clock error（.這樣我們留下一個(gè)點(diǎn)，它是 GPS 接收機(jī)的位置。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 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。 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)星信號的傳播時(shí)間會(huì)因大氣影響而改變；當(dāng) GPS 信號穿過電離層和對流層時(shí)會(huì)發(fā)生折射，因?yàn)樾盘査俣扰c在太空中信號的速度不同。）Sunspot activity also causes interference with GPS signals（.太陽黑子活動(dòng)也對 GPS 信號產(chǎn)生干擾）Another source of error is measurement noise, or distortion of the signal caused by electrical interference or errors inherent in the GPS receiver itself.（另一個(gè)誤差源是測量噪聲，或者是由于電干擾或 GPS 接收機(jī)自己固有的誤差引起的信號失真）Errors in the ephemeris data (the information about satellite orbits) will also cause errors in puted positions, because the satellites weren’ t really where the GPS receiver “thought” they were (based on the information it received) when it puted the positions.（星歷數(shù)據(jù)（關(guān)于衛(wèi)星軌道的信息）誤差也會(huì)引起位置的誤差，因?yàn)楫?dāng)計(jì)算位置時(shí)，衛(wèi)星不真正地在接收機(jī)“認(rèn)為”它們在的地方（基于它接受的信息所知道））Small variations in the atomic clocks (clock drift) on board the satellites can translate to large position errors。 a clock error of 1 nanosecond translates to 1 foot or 0 .3 meters user error on the ground.（衛(wèi)星原子鐘內(nèi)的很小的變化（鐘漂）能轉(zhuǎn)化成很大的位置誤差；一納秒的鐘差能造成 1 英尺或 米的地面用戶誤差）Multi path effects arise when signals transmitted from the satellites bounce off a reflective surface before getting to the receiver antenna.（當(dāng)從衛(wèi)星發(fā)射的信號在到達(dá)接收機(jī)天線 之前，被一個(gè)反射面反射時(shí)，多路徑效應(yīng)出現(xiàn)了）When this happens, the receiver gets the signal in straight line path as well as delayed path(multiple paths). The effect is similar to a ghost or double image on a TV set（.當(dāng)這種情況發(fā)生時(shí)，接收機(jī)接收直線路徑來的信號，也接收延遲路線（多路徑）來的信號。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(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.（一些繪圖級的接收機(jī)甚至能夠達(dá)亞米級精度，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.（測地級接收機(jī)最精確，憑裝備能夠達(dá)到厘米甚至是毫米級精度，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。）Recreational grade receivers usually can receive realtime differential corrections, but they cannot store a file that can be differentially corrected using postprocessing methods.（休閑級的接收機(jī)通常可以接收實(shí)時(shí)差分改正，但是它們不能存儲(chǔ)文件，以便可以用后處理方法差分改正。）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。 it counteracts only the errors that are mon to both reference and roving receivers).（差分改正是一種用來減小 選擇可用性政策(SA)影響和其它 GPS 定位誤差源 的方法（差分改正不能修正多路徑效應(yīng)或接收機(jī)誤差；它只能抵消基準(zhǔn)接收機(jī)和流動(dòng)接收機(jī)所共有的誤差）It requires, in addition to your “ roving” GPS receiver, a GPS receiver on the ground in a known location to act as a static reference point.（這需要——除了你的“流動(dòng)”GPS 接收機(jī)之外 —— 一個(gè) GPS 接收機(jī)在地面上一個(gè)已知的位置擔(dān)當(dāng)參考點(diǎn)）This type of setup is often called a GPS base station. Since the base station “ knows” where it is, it can pute the errors in its position calculations (in reality, it putes timing errors) and apply them to any number of