全球定位系統的介紹外文翻譯-通信工程(文件)
【正文】 tells you that X,Y,Z is an oak tree, or a spot in a stream with a pH level of . 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 puters 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 pute 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 landbased radar systems. Therefore, the orbits, and thus the locations of the satellites, are known in advance. Today39。ＧＰＳ是美國國防部（ＤＯＤ）因為其在軍事定位裝置方面巨大的應用而開發(fā)的。 GPS 衛(wèi)星的軌道足夠高以避免以土地為基礎的系統相關的問題，還可以在世界上任何地方提供每天 24 小時準確的定位。由于 GPS單位正變得更小，更便宜， GPS的應用正在不斷增加。 很多人認為， GPS 已經在地理信息系統（ GIS）領域發(fā)揮最大的應用。 GPS告訴我們，“在哪里”， GIS告訴我們“是什么”。 GPS 用這 3 個或更多的已知的參考和測量距離然后“三角測量”出額外的位置。只有這些信息，你很難找到我，因為在這球形的邊界上有無數的位置點。 添加更多的范圍將進一步減少可能的地點。其中一個可能是沒有意義的，并因此可以被消除。國防部可以非常準確地預測衛(wèi)星的路徑與時間的關系。你的星歷像“巴士時刻表”，提醒你每顆衛(wèi)星在一個特定的時間點在什么地方。任何偏差（由于自然的大氣現象，如重力），被稱為星歷誤差。無線電波以光速
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