A Position Built from Timing
1A location dot on a phone’s map looks simple, but producing it requires information arriving from far above Earth. GPS, the Global Positioning System, uses a network of orbiting satellites that broadcast radio signals. Those signals include information about when they were sent and where the satellites were in their orbits. A receiver, such as the one in a phone, uses these messages to calculate its own position. The satellites do not need a message from that receiver to make this calculation possible.
2A receiver begins by estimating how long a satellite’s signal took to reach it. Because radio signals travel at the speed of light, travel time can be converted into an estimate of distance. One distance alone places the receiver somewhere on an imaginary sphere centered on that satellite, rather than at a single known point. Comparing several such distance measurements narrows the possibilities to a common position. This explanation leaves out an important difficulty, however: the receiver’s clock is not perfectly synchronized with the satellites’ clocks.
3Even a tiny clock error can produce a large error in the distance calculation because radio signals move so quickly. Imagine timing a runner with a stopwatch that starts late: the recorded time would not match the actual travel time. The consequences are much greater for a radio signal, so a GPS receiver generally uses measurements from at least four satellites to solve for its position and its clock error together. The additional measurement therefore helps correct the timing problem rather than simply providing another vote for a location.
4Having enough satellite measurements does not guarantee that every measurement is equally useful. A building can block a signal, while another surface can reflect a signal toward the receiver along a longer path. That reflected signal arrives later than a direct signal would, which can make the satellite seem farther away. Receivers can compare available measurements and reject some suspicious ones, but these methods cannot remove every source of error.
5Calculating a position is also different from displaying useful directions. GPS supplies location information; mapping software combines that information with stored road layouts and route instructions. A device may receive satellite signals without an internet connection, although downloading a new map or current traffic information can require a separate connection. Thus, a working location dot and a complete navigation service depend on related but distinct kinds of information.