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Here’s one of the major advances of recent science, global positioning system. Since 1993, anyone on Earth can, using a small receiver, locate himself on the map. In any weather, at any time of day, with enough precision to follow dense city streets (at least since the limitation on civilian precision were lifted in the year 2000). Many people underestimate this, but think about it, navigation was vital to people ever since the first hunter-gatherer left home intending to return. Long before anyone needed a computer, long before anyone needed a refrigerator, people had to navigate. Landmarks had to be memorized and recorded on maps, intricate celestial navigation techniques were created to cross the seas and the oceans, which have no landmarks. The invention of the radio brought about radars, and the ability to see the land beyond the horizon, but still, GPS, once it became cheap and available to any regular person, absolutely revolutionized travel. This alone is worth all the billions spent on space exploration programs. And it’s not just private auto and hand held navigation, it’s air traffic control, geodetic surveying, offshore oil exploration, sea travel and railroad navigation, scientific research, all use GPS today.

So what kind of science is behind this little device? It receives radio signals from space, from the 24 navigation satellites orbiting Earth in near perfect circular 12-hour orbits. Each satellite maintains precise cesium atomic clock, synchronized with all satellites in the system, and the GPS receiver is able to compare the timestamp transmitted by the satellite with the current time, learning how long it took the wave to travel (and it travels with the speed of light). From the time it took the signals from several satellites to reach the receiver, it calculates the distance from each satellite, and since they constantly report their position in space about the Earth, the received can calculate its own position.

The kicker? GPS relies in its calculations on the dilation of time caused by the motion of the satellites and on the effect the Earth’s space-time curvature has on the flow of time. It does not just prove Einstein’s theory of relativity, it relies on it for daily operation, and millions of people rely on it every day in their cars and airplanes. Because the satellites are farther away from Earth than the ground clocks, they tick faster by about 45,900 ns/day, but it is partly canceled because their clocks tick slower by about 7,200 ns/day due to their speed. With the speed of light 1 foot per nanosecond, this drift would throw all receivers off by miles! Fortunately, it was calculated, thanks to Einstein, and pre-programmed into the atomic clocks run on the satellites. There are other small effects, such as the 23 ns/day drift due to 1% eccentricity of the orbit taking the satellite closer and farther from earth every few hours, which are also accounted for, and the overall precision of the entire 24 satellite system is always kept at error level less than 200 ns. The GPS also relies on the fact that the speed of light is constant at any time of day, in any direction, since that is the number it multiplies by time to obtain the distance from each satellite, and it shows, in practice, that it is indeed constant, down to at least +/- 12 m/s.

There is plenty more science behind GPS: the computers and electronics that run both the receivers and the satellites, radio engineering, the entire immense array of space science which brought those satellites up in orbit — chemistry of fuels and materials, physics and mathematics of space travel, geodesics that creates the maps the receivers show, but such tangible use of relativity is what I find most amazing.