How GPS works
GPS is a global navigation system. Its satellites broadcast timing signals and orbital information that a receiver uses to calculate coordinates.
Orbits and signals
GPS spacecraft occupy medium Earth orbits. Several orbital planes allow receivers in most places to obtain signals from multiple satellites at once. These orbits are higher than low-orbit communications systems, so each spacecraft covers a larger area.
A receiver measures signal travel time. It normally needs at least four satellites to calculate three position coordinates and correct its own clock.
What Above Us shows
The GPS mode displays spacecraft in the current active catalog and their calculated orbits. Geometric visibility on the map does not mean a particular phone receives a signal or that the application knows the accuracy of its navigation fix.