GPS is free because it is a U.S. government program that enhances the abilities of navigation, shipping, air travel, banking, construction, energy production/distribution, and virtually every service vital to global infrastructure. From its public debut until some time in the 90’s the government only allowed “selective availability” which diminished the accuracy of civilian receivers. The most recent and all future generations of GPS satellites have no function to enable selective availability without shutting down the satellite entirely(currently 17 operations gps satellites with no selective availability function). The military still has a more enhanced access, but mainly in the form of signal quality via a second antenna on the satellite (so for military receivers it can appear as 2 separate satellites in the exact same overhead position).
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