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A GPS Review of FactsThe Satellites A network of more than 50 satellites covers the globe to enable the GPS to work. To find the position of a GPS signal, at least three satellites have to be able to receive the signal. They then compare notes and triangulate the desired position. A GPS unit radios the time as correctly as its time piece can tell. The satellites that receive the signal compare the time sent to their own very accurate atomic clocks and figure the amount of time it took to receive the signal, they compare their different amounts of time along with their positions and are able to nearly pinpoint the location where the signal originated. They can then transmit the longitude and latitude - if three satellites received the signal - and altitude - if four satellites received it - to the sending unit. This is a simplified GPS review. Possible Problems From the GPS review above, you can guess that the accuracy of the time piece in the sending unit can have an impact on the accuracy of the positioning information. Clock errors are not the greatest source of error, though. This part of the GPS review will explain the others. The most frequent source of errors is atmospheric. When the signal passes through the ionosphere to reach the satellites, it can run into conditions that speed it up or slow it down, affecting the time calculations. The chances for problems increase as the angle of the signal increases to reach a satellite. Fortunately, there is a way to correct this problem for the military, other government agencies and surveyors whose positioning signals are used for very important reasons. Your GPS review so far makes it evident that inaccuracies will occur if the signal is bounced around by skyscrapers, mountains, or other terrain. This will cause distortions similar to the atmospheric type and are also avoidable by systems more sophisticated than the consumer type. The next type of problem occurs because the satellite updates its position about every twelve minutes. If a signal reaches the satellite near its update time, the satellite won't know its correct position and make its calculation based on bad information. For most uses, these problems don't amount to much. |
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