The Global Positioning System (GPS) is a satellite-based radionavigation system owned by the United States government. It provides geolocation and time information to a GPS receiver (in phones, watches, bike chips, . . . ) anywhere on the Earth where there is an unobstructed line of sight to several GPS satellites.

Each satellite sends its exact position relative to the Earth and the distance to a device. The device knows the radius of a sphere with the satellite in the centre. The device collects the info from several satellites and calculates its own position on the Earth.

From the mathematical point of view, what is the least number of satellites in direct view that are needed to define the exact position of a device on the Earth?