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How does a GPS receiver know what time it is?

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

  • Tim 1480 Points
    It is a bit of a chicken and egg question, but it would be more appropriate to say that the GPS satellites are broadcasting their current time and current location-- that is what the receiver hears from each satellite. So the receiver knows exactly where each satellite was at a specific point in time.
  • Tim 1480 Points
    This has been my go to description for more technical info:
    http://www.avionicswest.com/Articles/howGPSworks.html
  • Thanks, Tim, but neither response clarifies things for me. Your first response explains how the receiver determines where the spheres are in space, but not how it determines their size (it still needs time-of-flight based on an accurate knowledge of the time).

    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.
  • Tim 1480 Points
    Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets. In a nutshell, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy. In other words, there is only one value for the "current time" that the receiver can use. The correct time value will cause all of the signals that the receiver is receiving to align at a single point in space. That time value is the time value held by the atomic clocks in all of the satellites. So the receiver sets its clock to that time value, and it then has the same time value that all the atomic clocks in all of the satellites have. The GPS receiver gets atomic clock accuracy "for free."
    https://electronics.howstuffworks.com/gadgets/travel/gps3.htm
  • That does a much better job. Thanks. Still a little light on details for me, but at least I have a general idea of what’s going on.
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