GPS.外文翻译全球定位系统的介绍.doc
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1、资 料 及 译 文 译文题目:Introduction to the Global Positioning System 全球定位系统的介绍 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
2、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 DODs investment in GPS is immense. Billions and billions of dollars have been invested in creating this technol
3、ogy 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
4、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 DODs massive
5、 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 accurat
6、e 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 err
7、or, 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 leve
8、l 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 LocationIn a nutshell, GPS is based on satellite ranging - calculating the distances between the receiver and the
9、 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 the
10、se 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 w
11、ould 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 e
12、xactly 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 locat
13、ion.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
14、 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 thi
15、s 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
16、 Locations of GPS Satellites are DeterminedGPS 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 orb
17、its, and thus the locations of the satellites, are known in advance. Todays 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 sate
18、llite 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 atmosphe
19、ric 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
20、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 SatellitesGPS determines distance between a GPS satellite and a GPS r
21、eceiver 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 rece
22、iver 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 signals travel time can be determined.In order to do this, the satellites and the recei
23、vers 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 a
24、nd 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 positi
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