Uuit21 Understanding the GPS () 认识GPS()
GPS Error Sources (GPS误差来源)
There are many sources of possible errors that will degrade the accuracy of position computed by a GPS receiver.（有许多或然误差【possible error】来源，这种误差会降低【degrade】GPS接收机计算的位置的准确度）【that从句修饰errors】
The travel time of GPS satellite signals can be altered by atmospheric effects; when a GPS signal pass 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卫星信号的传播时间会因大气影响而改变【alter】；当GPS信号穿过电离层【ionosphere】和对流层【troposphere】时会发生折射【refract】，因为信号速度与在太空中信号的速度不同。）
Sunspot activity also caus interference with GPS signals.（太阳黑子活动也对GPS信号产生干扰【interference干涉、干扰】）
Another source of error is measurement noi, or distortion of the signal caud by electric
al interference or errors inherent in the GPS receiver itlf.（另一个误差源是测量噪声，或者是由于电干扰【electrical interference】或GPS接收机自己固有的误差引起的信号失真【distortion】）【即，电干扰和接收机固有误差 都是失真原因】
Errors in the ephemeris data (the information about satellite orbits) will also cau errors in computed positions, becau the satellites weren’t really where the GPS receiver “thought” they were (bad on the information it received) when it computed the positions.（星历数据（关于卫星轨道的信息）误差也会引起位置的误差，因为当计算位置时，卫星不真正地在接收机“认为”它们在的地方（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 nanocond translates to 1 foot or 0.3 meters ur error on the ground.（卫星原子钟内的很小的变化【variation变奏、变化】（钟漂【drift漂流；应该是钟偏clock offt】）能转化成很大的位置误差；一纳秒【nanocond】的钟差能造成1英尺或0.3米的地面用户误差）
Multipath effects ari when signals transmitted from the satellites bounce off a reflective
surface before getting to the receiver antenna.（当从卫星发射的信号在到达接收机天线【antenna天线】之前，被一个反射面【reflective surface】反射【bounce off】时，多路径效应【Multipath effect】出现了【ari】）
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 t.（当这种情况发生时，接收机接收直线路径来的信号，也【as well as】接收延迟路线（多路径）来的信号。其效果与电视的重影【ghost】或重像类似）
How to Reduce GPS Errors（如何减小GPS的误差）
You’ve probably heard people talk about getting 2 to 5 meters accuracy with a GPS receiver, or even centimeter or millimeter accuracy.（你可能听说过有人说GPS接收机可以达到2到5米的准确度，甚至是厘米或毫米级精度）
Is there a way to cancel out the errors and get satisfied accuracy?（有方法低偿【cancel out】误差并获得满意的精度吗？）
The answer is yes, but the level of accuracy depends on the type of equipment you are using.（回答是肯定的，但是准确度的级别依赖于你使用的装备的类型）
The following discussion describes a technique ud to achieve 2 to 5 meters accuracy using mapping (resource) grade receivers.（下面的讨论描述了一种用于绘图（资源）级【grade】接收机的技术，能够获得2到5米的准确度）
Some mapping grade receivers are even capable of sub-meter accuracy, but the incread accuracy comes at a price.（一些绘图级的接收机甚至能够达亚米级精度，但是精度的提高是以很高代价带来的【at a price以很高代价adv.】
Survey grade receivers are the most accurate, capable of centimeter or even millimeter accuracy, depending on the equipment, but they u more advanced techniques to achieve this level of accuracy and, naturally, are more expensive.（测地级接收机最精确，凭【depend on】装备能够达到厘米甚至是毫米级精度，但是它们是使用更先进的技术才得到这样的精度水平，自然地，就更昂贵。）
Recreational grade receivers usually can receive real-time differential corrections, but they cannot store a file that can be differentially corrected using post-processing methods.（休闲【Recreational娱乐的、休闲的】级的接收机通常可以接收实时差分改正，但是它们不能存储文件，【that从句修饰file】以便可以用后处理【post-processing后处理】方法差分【differentially】改正。）
Differential Correction（差分改正）
Differential correction is a method ud to reduce the effects of lective 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 common to both reference and roving receivers).（差分改正是一种用来减小 选择可用性政策(SA)影响和其它GPS定位误差源 的方法（差分改正不能修正多路径效应或接收机误差；它只能抵消【counteract抵消】基准接收机【reference receiver】和流动接收机【roving receiver】所共有的误差）
It requires, in addition to your “roving” GPS receiver, a GPS receiver on the ground in a k
nown location to act as a static reference point.（这需要——除了【in addition to除……之外】你的“流动”GPS接收机之外 —— 一个GPS接收机在地面上一个已知的位置担当【act as】静态【static】参考点）【如果要直译的话，可以两边加破折号把中间的句子夹起来】
This type of tup is often called a GPS ba station. Since the ba station “knows” where it is, it can compute the errors in its position calculations (in reality, i桃树简笔画
t computes timing errors) and apply them to any number of roving receivers in the same general area.（这种设备经常被称为GPS基准站。由于基准站“知道”在哪儿，它就可以计算它自己的位置误差（实际上，它是计算 定时误差【timing error】），并将它们应用于【apply to将……应用于】许多【any number of许多】同一区域的流动接收机）
