發表於 2014-2-12 10:49:09
Tiny discrepancies between the GPS receiver's onboard clock and GPS time, which synchronizes the whole global positioning system, mean distances calculated can drift. There are two solutions to this problem. The first would be to use an atomic clock in each receiver costing $100,000. The second is to use some clever mathematical trickery to account for the time-keeping error based on how the signals from three or more satellite signals are detected by the receiver, which essentially allows the receiver to reset its clock.
There is also an intrinsic error source in GPS associated with the way the system works. GPS receivers analyze three signals from satellites in the system and work out how long it has taken each signal to reach them. This allows them to carry out a trilateration calculation to pinpoint the exact location of the receiver. The signals are transmitted by the satellites at a specific rate. Unfortunately, the electronic detector in standard GPS devices is accurate to just 1 percent of a bit time. This is approximately 10 billionths of a second (10 nanoseconds). Given that the GPS microwave signals travel at the speed of light, this equates to an error of about 3 meters. So standard GPS cannot determine position to greater than 3-metre accuracy. More sophisticated GPS receivers used by the military are ten times more accurate to 300 millimeters.