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Old 12-04-13, 05:09 PM
  #4  
prathmann
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GPS receivers have gotten more sensitive, but there are still some inherent limitations in the startup times. When the GPS has been turned off for a few hours or more it needs to get the ephemeris data from the satellite that specifies exactly where it is in its orbit. It's not much data but the link is extremely low bandwidth (50 bps) and therefore it takes 18 seconds to send the complete message and it repeats every 30 seconds. If the signal is interrupted during that time (say you're moving and a tall building gets in the way) then the receiver has to wait for the next repetition until it finally gets the full message. So moving around while the receiver is first acquiring a signal can greatly increase the time to get the necessary data. And it needs to be receiving signals from at least 3 satellites (preferably 4 or more) to determine its position. So delays in areas with lots of tall buildings or other obstructions are not unusual. Note that GPS receivers combined with cellphones can do better since the data message with the orbit information can be sent over the cellular network.
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