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1、英文翻译Introduction to the Global Positioning System 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), fo
2、r 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 technology for military uses. However, over the past several years, GPS has proven to be a useful tool in non-military mapping appl
3、ications 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 meter
4、s. 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 undertaking. As GPS units are becoming smaller and less expensive, there are an expanding number of applications for GPS. I
5、n 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 too
6、l 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 databa
7、se 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 o
8、ur resources.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 position
9、s 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
10、 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 a
11、lone, 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 mile
12、s. 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
13、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
14、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
15、 of three or more known points (GPS satellites)? B) What is the distance between the known points and the position of the GPS receiver?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
16、 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. 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 particul
