GPS Finds Your Location by Measuring Time

GPS satellites do not look down, recognize a phone and report its position. A receiver compares the arrival timing of signals from several satellites, estimates its distance from each one and finds the three-dimensional point consistent with all those distances. The fundamental measuring tool is precise time, not a viewing angle.

Every GPS satellite continuously broadcasts information including when a signal was sent and where the satellite is in its orbit. The receiver compares transmission and reception times, then multiplies travel time by the speed of light. Because a tiny timing error becomes a large distance error, satellite atomic clocks and ground-control corrections to clocks and orbits are essential.

One measured distance places the receiver somewhere on a sphere centered on that satellite. A second sphere narrows the possibilities to a circle, and a third usually reduces them to a small number of intersections. Finding position from distances to known points is more accurately called trilateration than triangulation.

A practical receiver's clock is far less accurate than the atomic clocks in orbit. There are therefore four unknowns: three spatial coordinates and the receiver's clock offset. Signals from at least four satellites allow the calculation to solve for position and the common timing error together. Additional satellites can improve geometry, reliability and error estimation.

The point on a screen is imperfect because signals do not travel only along an unobstructed vacuum path. The ionosphere and troposphere affect propagation, while reflections from buildings or the ground create multipath signals that appear to have traveled farther. Accuracy also worsens when satellites cluster in one direction or trees, structures and indoor walls block the sky.

A GPS receiver does not need to transmit its position back to the satellites for the basic calculation. It passively listens to broadcast signals. A smartphone may nevertheless combine cellular networks, Wi-Fi, barometric pressure and motion sensors to start faster or work better in cities. A map application's blue dot can therefore represent fused positioning, not GPS alone.

The claim that “three satellites are enough” belongs to a simplified diagram with a perfect clock or reduced dimensions. Operational GPS uses a fourth satellite to estimate the inexpensive receiver's clock error, then applies error models and extra observations. A pocket location fix is essentially a distance puzzle solved from four or more synchronized timetables arriving from space.