GPS: From Sputnik's Beep to Einstein's Clocks in Your Pocket

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The Global Positioning System began with a Soviet satellite and two curious physicists. In 1957, William Guier and George Weiffenbach of the Johns Hopkins Applied Physics Lab tuned in to Sputnik 1 and noticed its radio pitch shifting as it crossed the sky. Using the Doppler effect and the lab's UNIVAC computer, they mapped its orbit. Deputy director Frank McClure then asked the inverse question: if you know exactly where a satellite is, can it tell you where you are on Earth?

This episode follows that question through the Navy's Transit system, Cold War submarine targeting, and the 1973 merger of rival military programs into GPS. It credits Roger Easton, Ivan Getting, and Bradford Parkinson, along with Gladys West, whose geodetic model mapped Earth's lumpy gravity, and Elizabeth Essex-Cohen, who studied how the ionosphere bends radio signals. It also explains trilateration, relativity, and how a classified military tool ended up guiding everyday life.

Transit, launched by the Navy in 1960, used only five satellites and gave a ship a location fix about once an hour.
Satellite clocks run 38 microseconds fast each day because of relativity, and a single microsecond of error shifts a position by about 300 meters.
After Korean Air Lines Flight 007 strayed into Soviet airspace and was shot down in 1983, killing 269 people, Ronald Reagan pledged GPS for civilian use.
During the Gulf War, troops short on military receivers asked families to mail store-bought boating and hunting GPS units, forcing the military to switch off selective availability.
In 2000, Bill Clinton ordered selective availability permanently disabled, and civilian accuracy jumped from about 100 meters to about five.

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