GPS is a network of satellites used to determine location on Earth. It includes 24 satellites that circle Earth and constantly send out radio signals. GPS receivers determine their location by measuring the time it takes for signals to arrive from at least four satellites, using a method called trilateration. GPS is used for navigation in various modes of transportation and by hikers. Obstacles can affect GPS signals, but corrections are constantly broadcast to make it more accurate. Other countries have their own GPS systems, such as Russia's GLONASS.