全球定位系统的介绍 --通信工程外文翻译

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译文题目：Introduction to the Global Positioning System

全球定位系统的介绍

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Introduction to the Global Positioning System

---------From “Corvallis Microtechnology, Inc. 1996”

Chapter One: What is GPS?

The Global Positioning System (GPS) is a location system based on a constellation of about 24 satellites orbiting the earth at altitudes of approximately 11,000 miles. GPS was developed by the United States Department of Defense (DOD), for its tremendous application as a military locating utility. The DOD's investment in GPS is immense. Billions and billions of dollars have been invested in creating this technology for military uses. However, over the past several years, GPS has proven to be a useful tool in non-military mapping applications as well.

GPS satellites are orbited high enough to avoid the problems associated with land based systems, yet can provide accurate positioning 24 hours a day, anywhere in the world. Uncorrected positions determined from GPS satellite signals produce accuracies in the range of 50 to 100 meters. When using a technique called differential correction, users can get positions accurate to within 5 meters or less.

Today, many industries are leveraging off the DOD's massive undertaking. As GPS units are becoming smaller and less expensive, there are an expanding number of applications for GPS. In transportation applications, GPS assists pilots and drivers in pinpointing their locations and avoiding collisions. Farmers can use GPS to guide equipment and control accurate distribution of fertilizers and other chemicals. Also，GPS is used for providing accurate locations and as a navigation tool for hikers, hunters and boaters.

Many would argue that GPS has found its greatest utility in the field of Geographic Information Systems (GIS). With some consideration for error, GPS can provide any point on earth with a unique address (its precise location). A GIS is basically a descriptive database of the earth (or a specific part of the earth). GPS tells you that you are at point X,Y,Z while GIS tells you that X,Y,Z is an oak tree, or a spot in a

stream with a pH level of 5.4. GPS tells us the "where". GIS tells us the "what". GPS/GIS is reshaping the way we locate, organize, analyze and map our resources.

Chapter Two: Trilateration - How GPS Determines a Location

In a nutshell, GPS is based on satellite ranging - calculating the distances between the receiver and the position of 3 or more satellites (4 or more if elevation is desired) and then applying some good old mathematics. Assuming the positions of the satellites are known, the location of the receiver can be calculated by determining the distance from each of the satellites to the receiver. GPS takes these 3 or more known references and measured distances and "triangulates" an additional position.

As an example, assume that I have asked you to find me at a stationary position based upon a few clues which I am willing to give you. First, I tell you that I am exactly 10 miles away from your house. You would know I am somewhere on the perimeter of a sphere that has an origin as your house and a radius of 10 miles. With this information alone, you would have a difficult time to find me since there are an infinite number of locations on the perimeter of that sphere.

Second, I tell you that I am also exactly 12 miles away from the ABC Grocery Store. Now you can define a second sphere with its origin at the store and a radius of 12 miles. You know that I am located somewhere in the space where the perimeters of these two spheres intersect - but there are still many possibilities to define my location.

Adding additional spheres will further reduce the number of possible locations. In fact, a third origin and distance (I tell you am 8 miles away from the City Clock) narrows my position down to just 2 points. By adding one more sphere, you can pinpoint my exact location. Actually, the 4th sphere may not be necessary. One of the possibilities may not make sense, and therefore can be eliminated.

For example, if you know I am above sea level, you can reject a point that has negative elevation. Mathematics and computers allow us to determine the correct point with only 3 satellites.

Based on this example, you can see that you need to know the following information in order to compute your position:

A) What is the precise location of three or more known points (GPS satellites)?

B) What is the distance between the known points and the position of the GPS receiver?

Chapter Three: How the Current Locations of GPS Satellites are Determined GPS satellites are orbiting the Earth at an altitude of 11,000 miles. The DOD can predict the paths of the satellites vs. time with great accuracy. Furthermore, the satellites can be periodically adjusted by huge land-based radar systems. Therefore, the orbits, and thus the locations of the satellites, are known in advance. Today's GPS receivers store this orbit information for all of the GPS satellites in what is known as an almanac. Think of the almanac as a "bus schedule" advising you of where each satellite will be at a particular time. Each GPS satellite continually broadcasts the almanac. Your GPS receiver will automatically collect this information and store it for future reference.

The Department of Defense constantly monitors the orbit of the satellites looking for deviations from predicted values. Any deviations (caused by natural atmospheric phenomenon such as gravity), are known as ephemeris errors. When ephemeris errors are determined to exist for a satellite, the errors are sent back up to that satellite, which in turn broadcasts the errors as part of the standard message, supplying this information to the GPS receivers.

By using the information from the almanac in conjuction with the ephemeris error data, the position of a GPS satellite can be very precisely determined for a given time.

Chapter Four: Computing the Distance Between Your Position and the GPS

Satellites

GPS determines distance between a GPS satellite and a GPS receiver by measuring the amount of time it takes a radio signal (the GPS signal) to travel from the satellite to the receiver. Radio waves travel at the speed of light, which is about 186,000 miles per second. So, if the amount of time it takes for the signal to travel from the satellite to the receiver is known, the distance from the satellite to the receiver (distance = speed x time) can be determined. If the exact time when the signal was transmitted and the exact time when it was received are known, the signal's travel time can be determined.

In order to do this, the satellites and the receivers use very accurate clocks which are synchronized so that they generate the same code at exactly the same time. The code received from the satellite can be compared with the code generated by the receiver. By comparing the codes, the time difference between when the satellite generated the code and when the receiver generated the code can be determined. This interval is the travel time of the code. Multiplying this travel time, in seconds, by 186,000 miles per second gives the distance from the receiver position to the satellite in miles.

Chapter Five: Four (4) Satellites to give a 3D position

In the previous example, you saw that it took only 3 measurements to "triangulate" a 3D position. However, GPS needs a 4th satellite to provide a 3D position. Why?? Three measurements can be used to locate a point, assuming the GPS receiver and satellite clocks are precisely and continually synchronized, thereby allowing the distance calculations to be accurately determined. Unfortunately, it is impossible to synchronize these two clocks, since the clocks in GPS receivers are not as accurate as the very precise and expensive atomic clocks in the satellites. The GPS signals travel from the satellite to the receiver very fast, so if the two clocks are off by only a small fraction, the determined position data may be considerably distorted.

The atomic clocks aboard the satellites maintain their time to a very high degree of

accuracy. However, there will always be a slight variation in clock rates from satellite to satellite. Close monitoring of the clock of each satellite from the ground permits the control station to insert a message in the signal of each satellite which precisely describes the drift rate of that satellite's clock. The insertion of the drift rate effectively synchronizes all of the GPS satellite clocks.

The same procedure cannot be applied to the clock in a GPS receiver. Therefore, a fourth variable (in addition to x, y and z), time, must be determined in order to calculate a precise location. Mathematically, to solve for four unknowns (x,y,z, and t), there must be four equations. In determining GPS positions, the four equations are represented by signals from four different satellites.

Chapter Six: The GPS Error Budget

The GPS system has been designed to be as nearly accurate as possible. However, there are still errors. Added together, these errors can cause a deviation of +/- 50 -100 meters from the actual GPS receiver position. There are several sources for these errors, the most significant of which are discussed below:

Atmospheric Conditions

The ionosphere and troposphere both refract the GPS signals. This causes the speed of the GPS signal in the ionosphere and troposphere to be different from the speed of the GPS signal in space. Therefore, the distance calculated from "Signal Speed x Time" will be different for the portion of the GPS signal path that passes through the ionosphere and troposphere and for the portion that passes through space.

As mentioned earlier, GPS signals contain information about ephemeris (orbital position) errors, and about the rate of clock drift for the broadcasting satellite. The data concerning ephemeris errors may not exactly model the true satellite motion or the exact rate of clock drift. Distortion of the signal by measurement noise can further increase positional error. The disparity in ephemeris data can introduce 1-5 meters of

positional error, clock drift disparity can introduce 0-1.5 meters of positional error and measurement noise can introduce 0-10 meters of positional error.

Ephemeris errors should not be confused with Selective Availability (SA), which is the intentional alteration of the time and ephemeris signal by the Department of Defense.

A GPS signal bouncing off a reflective surface prior to reaching the GPS receiver antenna is referred to as multipath. Because it is difficult to completely correct multipath error, even in high precision GPS units, multipath error is a serious concern to the GPS user.

Chapter Seven: Measuring GPS Accuracy

As discussed above, there are several external sources which introduce errors into a GPS position. While the errors discussed above always affect accuracy, another major factor in determining positional accuracy is the alignment, or geometry, of the group of satellites (constellation) from which signals are being received. The geometry of the constellation is evaluated for several factors, all of which fall into the category of Dilution Of Precision, or DOP.

DOP is an indicator of the quality of the geometry of the satellite constellation. Your computed position can vary depending on which satellites you use for the measurement. Different satellite geometries can magnify or lessen the errors in the error budget described above. A greater angle between the satellites lowers the DOP, and provides a better measurement. A higher DOP indicates poor satellite geometry, and an inferior measurement configuration.

Some GPS receivers can analyze the positions of the satellites available, based upon the almanac, and choose those satellites with the best geometry in order to make the DOP as low as possible. Another important GPS receiver feature is to be able to ignore or eliminate GPS readings with DOP values that exceed user-defined limits. Other GPS receivers may have the ability to use all of the satellites in view, thus minimizing the DOP as much as possible.

全球定位系统的介绍

----摘自Corvallis Microtechnology公司，1996

第一章：什么是GPS？

全球定位系统（ＧＰＳ）是一种基于２４颗高度大约１１０００英里的地球轨道卫星的定位系统。ＧＰＳ是美国国防部（ＤＯＤ）因为其在军事定位装置方面巨大的应用而开发的。国防部对ＧＰＳ的投入是极大的。已经有数十亿美元的投资为了开发这种军事应用技术。然而，过去一段时间以来，ＧＰＳ已经被证实是在测绘非军事地图应用方面十分有用的工具。

GPS卫星的轨道足够高以避免以土地为基础的系统相关的问题，还可以在世界上任何地方提供每天24小时准确的定位。在裸眼可视位置GPS卫星信号产生的定位精度为５０到１００米。当使用差分技术时，用户可以得到精度为5米以下的定位。

今天，许多行业都促使国防部改变。由于GPS单位正变得更小，更便宜，GPS 的应用正在不断增加。在交通运输应用方面，GPS协助飞行员和司机准确定位它们的位置，避免碰撞。农民可以使用GPS引导设备并且控制化肥和其他化学品的准确分布。此外，GPS用于提供准确的位置，并作为徒步旅行者，猎人和船民的导航工具。

很多人认为，GPS已经在地理信息系统（GIS）领域发挥最大的应用。在考虑到一些误差的情况下，GPS可以提供地球上任何一点的唯一一个地址（它的精确位置）。 GIS从根本上说是地球的一个描述的数据库（或地球的特定部分）。GPS会告诉你，你是在点X，Y，Z，而GIS告诉你，X，Y，Z点是一棵橡树，或河流中的一个pH值5.4的点。 GPS告诉我们，“在哪里”， GIS告诉我们“是什么”。 GPS/ GIS正在重塑定位，管理，分析，并且映射我们的资源。

第二章：三边定位——GPS是怎样定位的

简单地说，GPS是基于卫星测距 -计算接收器和3颗或更多颗卫星（4个或更多，如果需要高程）之间的距离，然后用一些以前的正确数字进行计算。假设

卫星的位置是已知的，通过确定每个卫星到接收机的距离，可以计算出接收机的位置。 GPS用这3个或更多的已知的参考和测量距离然后“三角测量”出额外的位置。

作为一个例子，假设我要你根据我给你提供的很少的一些线索让你在一个固定的位置找到我。首先，我告诉你，我离你的房子正好是10英里远。你会知道我在一个以你的房子为圆心，半径10英里的球形边界的地方。只有这些信息，你很难找到我，因为在这球形的边界上有无数的位置点。

第二，我告诉你，我也正好离ABC杂货店12英里远。现在，你可以画出一个以杂货店为圆心，半径12英里的球形。你知道，我所在的地方，在两个球形空间的周长交叉的地方 - 但我的位置还是有很多的可能性。

添加更多的范围将进一步减少可能的地点。事实上，第三个圆心和距离（我告诉你是8英里远的城市时钟）使我的位置缩小到只有2点。再增加一个范围，你可以找出我的确切位置。其实，第四球体可能不是必需的。其中一个可能是没有意义的，并因此可以被消除。

例如，如果你知道我是海平面上，你可以拒绝一个海拔为负的点。数学和计算机让我们能够只用3颗卫星确定正确的点。

基于这个例子，你可以明白，你需要知道以下信息，以便计算你的位置：A）3个或更多的已知点（GPS卫星）的精确位置是什么？

B）已知点的位置和GPS接收器之间是多少距离？

第三章：如何确定GPS卫星当前位置

GPS卫星在高度为11000公里的轨道绕地球飞行。国防部可以非常准确地预测卫星的路径与时间的关系。此外，卫星还可以定期通过巨大的陆基雷达系统进行调整。因此，轨道，卫星的位置，都是预先已知的。今天的GPS接收器存储所有的GPS卫星的轨道信息，被称为星历。你的星历像“巴士时刻表”，提醒你每颗卫星在一个特定的时间点在什么地方。每个GPS卫星不断广播星历。你的GPS

接收器会自动收集这些信息，并将其存储以供将来参考。

国防部不断地监视卫星的轨道预测值的偏差。任何偏差（由于自然的大气现象，如重力），被称为星历误差。当被确定为存在卫星星历误差，误差被发送备份到该卫星，依次将错误作为标准的消息的一部分广播，提供这种信息给GPS 接收机。

通过使用星历表的信息误差数据，可以很精确地在一个给定的时间确定GPS 卫星的位置。

第四章：计算你的位置和GPS卫星之间的距离

GPS通过测量卫星和接收机之间无线电信号（GPS信号）传输所花费的时间来确定卫星和接收机的距离。无线电波以光速传播，这是大约每秒186,000英里的速度。因此，如果信号从卫星到接收机的时间量是已知的，卫星到接收机的距离（距离=速度×时间）可以被确定。如果信号发送和收到的时间确定，信号的传输时间才能确定。

为了做到这一点，卫星和接收机使用非常精确的同步时钟，以便它们在完全相同的时间，生成相同的代码。从卫星接收的代码可以跟接收器产生的代码进行比较。通过比较代码，卫星产生代码时与接收机接收后产生代码之间的时间差可以被确定。这个时间间隔为代码的旅行时间。每秒186,000英里的速度乘以这个时间间隔就是接收机位置与卫星之间的距离。

第五章：四颗卫星定位3D位置

在前面的例子中，你看到了，只用了3个测量“三角测量”三维位置。然而，GPS需要第四个卫星提供三维位置。为什么？

三个测量值可以用来定位一个点，假设GPS接收机和卫星时钟是精确和连续的同步的，从而使计算出阿里的距离非常精确。不幸的是，这两个时钟是不可能同步的，因为GPS接收机中的时钟是不像非常精确和昂贵的卫星原子钟那样精确的。GPS信号从卫星到接收机的速度非常快，所以，如果两个时钟是不同步的，

即使只有一小部分，所确定的位置数据也会大大失真。

船上的卫星的原子钟保持自己的时间在一个非常高的精确度。然而，卫星与卫星之间的时钟速率总是会有轻微的变化。从地面关闭监测每颗卫星的时钟使得控制站能在每个卫星的信号中插入精确地描述了该卫星的时钟漂移率的消息。有效的漂移速率的插入同步了的所有GPS卫星的时钟。

相同的程序不能被施加到一个GPS接收器中的时钟上。因此，第四个变量（除了为x，y和z），时间，必须确定，以计算一个精确的位置。在数学上，解决四个未知数（x，y和z，和t），必须有四个方程。在确定GPS定位上，四个方程表示从四个不同卫星的信号。

第六章：GPS误差

GPS系统已被设计为尽可能接近尽可能准确。然而，仍然有误差。这些错误加在一起可能会导致实际的GPS接收器的位置偏差为+ / - 50 -100米。这些错误的来源有几个，其中最重要的如下：

大气环境

电离层和对流层都折射GPS信号。这会导致从空间中传输的GPS信号的速度与电离层和对流层中的GPS信号的速度是不同的。因此，从“信号速度×时间”的计算的距离将是不同的GPS信号的路径，穿过电离层和对流层的部分，其通过空间的部分。

正如前面提到的，GPS信号包含有关下列内容的信息的星历表（轨道位置）的错误，和有关的广播卫星的时钟漂移的速率。星历误差的数据可能不完全模拟真实的的卫星运动或时钟漂移的准确率。测量噪声的信号失真，可以进一步增加的位置误差。星历数据的差距可以推出1-5米的位置误差，时钟漂移差距引起0-1.5米的位置误差，测量噪声可以导致0-10米的误差。

星历误差不应该与选择可用性（SA）混淆，SA是国防部对信号的时间和星历的蓄意篡改。

多个反射面到达GPS接收机天线之间的GPS信号被称为多径。因为它是难以完全纠正，即使在高精确度的GPS单元也有多路径误差，多路径误差的是GPS 用户的一个严重的问题。

第七章：测量GPS精度

如上所讨论的，有几个外部来源引起了GPS位置的误差。虽然上面讨论的误差总是会影响精度，确定定位精度的另一个主要因素是正在接收信号的一组卫星（星座）的排布或几何分布。星座的几何形状被几个因素评价，所有这些都归入精度稀释的类别中，或称为DOP。

DOP是的卫星星座的几何形状的质量的指标。您的计算出的位置可能有所不同，这取决于使用的用于测量的卫星。不同的卫星的几何形状，可以放大或减少上述误差预算中的错误。卫星之间的更大的角度会降低DOP，并提供了一个更好的测量量。较高的DOP表示卫星排列位置不合理和一种低劣的测量结构。

一些GPS接收器可以根据历书分析可用卫星的位置，并选择这些卫星的最佳几何形状，以尽可能DOP使降低。 GPS接收器的另一个重要的特点是可以忽略或消除超过用户定义的限制受DOP影响的GPS读数。其他的GPS接收器可能有能力使用在视野中的所有的卫星，从而尽可能的最大限度地减少了DOP。