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How does a GPS receiver know what time it is?
Involute
0 Points
The typical explanation of how GPS works has the receiver calculating the time-of-flight of the signals from several GPS satellites, and then determining the spheres around each satellite on which the receiver sits. To calculate he time of flight, it’s assumed the receiver compares the time at the receiver to the time stamp in each signal. For this to work, though, it seems to me that the receiver must know its time as accurately as the satellites know there’s. Since the receiver doesn’t have an atomic clock, however, it can’t. If it just uses the time from one of the satellites, it can’t use that satellite in the solution, increasing the number of satellites required for one. What’s really going on? Thanks.
Comments
http://www.avionicswest.com/Articles/howGPSworks.html
And, while the link provides lots of details, it kind of waves its hands on this particular issue, saying that a fourth satellite lets the receiver determine “the time the satellite sent the signal.” Well, we don’t need a fourth satellite to determine that since it’s provided in the signals from all satellites. If the author meant to say the fourth satellite lets the receiver determine time accurately, he doesn’t explain how this is done.