全球定位系統(tǒng)的介紹外文翻譯-通信工程-文庫吧
2025-04-17 16:13 本頁面
【正文】 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。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
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