A Location Built from Time
1A phone displays a dot on a map, but no satellite is watching that dot move. GPS, the Global Positioning System, works through radio signals sent by satellites orbiting Earth. Each signal includes information about the satellite’s position and when the signal was sent. A GPS receiver, such as the one in a phone, listens to those broadcasts. It does not need to send a message back to the satellites to calculate its location.
2The receiver compares a signal’s sending time with its arrival time. Radio signals travel at the speed of light, so travel time can be used to estimate distance. Even a tiny timing error can produce a large distance error. One satellite’s signal cannot identify a unique location: many places could be the same distance from that satellite. Measurements from additional satellites narrow the possibilities.
3The receiver’s clock creates another challenge. Satellite clocks are extremely precise, while a phone’s clock is much less precise. A receiver generally needs signals from at least four satellites to calculate its three-dimensional position and correct its clock error. The receiver combines these measurements rather than choosing the strongest signal as its answer. Receiving more signals can help, although signal quality and the satellites’ arrangement in the sky also matter.
4A clear view of the sky helps signals reach a receiver directly. Beside tall buildings, some signals may be blocked. Others may bounce off walls before reaching the device, taking a longer path. The receiver can interpret that extra travel time as extra distance. These effects help explain why a location dot may drift or jump even when someone is standing still. A receiver indoors may struggle to obtain a usable position at all.
5Finding coordinates is different from drawing a map or planning a route. Mapping software uses coordinates to place a dot on stored map information. It also uses information about streets and paths to suggest directions. A road missing from the map cannot be added merely by receiving better satellite signals. Likewise, a correct location does not guarantee that the software knows about a recent road closure.
6GPS reception itself does not require a cellular data connection. A device with suitable downloaded maps can use GPS while offline. Without downloaded information, an app may be unable to display a detailed map or obtain current traffic reports. Conversely, having a cellular connection does not remove buildings that block satellite signals. Understanding these separate jobs helps a user investigate whether a problem involves reception, map information, or an online service.