【正文】 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 衛(wèi)星的軌道足夠高以避免以土地為基礎的系統(tǒng)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。 很多人認為， GPS 已經(jīng)在地理信息系統(tǒng)（ GIS）領域發(fā)揮最大的應用。 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。 添加更多的范圍將進一步減少可能的地點。國防部可以非常準確地預測衛(wèi)星的路徑與時間的關系。任何偏差（由于自然的大氣現(xiàn)象，如重力），被稱為星歷誤差。從衛(wèi)星接收的代碼可以跟接收器產(chǎn)生的代碼進行比較。 GPS 信號從衛(wèi)星到接收機的速度非?？?，所以，如果兩個時鐘是不同步的，即使只有一小部分，所確定的位置數(shù)據(jù)也會大大失真。在確定 GPS 定位上，四個方程表示從四個不同衛(wèi)星的信號。星歷誤差的數(shù)據(jù)可能不完全模擬真實的的衛(wèi)星運動或時鐘漂移的準確率。星座的幾何形狀被幾個因素評價，所有這些都歸入精度稀釋的類別中，或稱為 DOP。其他的 GPS接收器可能有能力使用在視野中的所有的衛(wèi)星，從而盡可能的最大限度地減少了 DOP。您的計算出的位置可能有所不同，這取決于使用的用于測量的衛(wèi)星。星歷數(shù)據(jù)的差距可以推出 15 米的位置誤差，時鐘漂移差距引起 米的位置誤差，測量噪聲可以導致 010 米的誤差。然而，仍然有誤差。然而，衛(wèi)星與衛(wèi)星之間的時鐘速率總是會有輕微的變 化。這個時間間隔為代碼的旅行時間。 通過使用星歷表的信息誤差數(shù)據(jù)，可以很精確地在一個給定的時間確定 GPS衛(wèi)星的位置。因此，軌道，衛(wèi)星的位置，都是預先已知的。再增加一個范圍，你可以找出我的確切位置。首先，我告訴你，我離你的房子正好是 10 英里遠。 GIS 從根本上說是地球的一個描述的數(shù)據(jù)庫（或地球的特定部分）。當使 用差分技術時，用戶可以得到精度為 5米以下的定位。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 pared with the code generated by the receiver. By paring 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