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How GPS Works
How Your Brain's GPS Works
The Global Positioning System (GPS) is network of satellites and an extremely accurate clock that can provide the exact location and weather to anyone who has a receiver anywhere on Earth. The system was originally created by and for the U.S. Department of Defense, but they since opened it up for civilian use, and it has since changed how we get places and find each other. If you have a smart phone, you probably use GPS every day, but the science behind how it all works is fascinating and complex. The extremely accurate clock used is the atomic clock at the U.S. Naval Observatory in Washington, D.C., which is accurate to the nanosecond and crucial for getting the system to work. Second, it needs a satellite network which also knows the time. Currently there are 31 satellites with three atomic clocks on board. Each of these satellites are sending out microwave signals towards Earth while orbiting 7,000 miles per hour (around 11,000 km/h) at 12,500 miles (20,000 km) above our heads. these microwave signals are sent out at regular intervals and contain information about their location and the time they were sent. This is why needing to know the exact time is so important: GPS receivers triangulate your location by measuring the lag from each satellite signal. If it can pick up three satellites, it can determine your longitude and latitude. If it can pick up four satellites, it can tell your altitude, too.
While keeping 31 orbiting satellites in sync might seem like a complicated proposition, keep in mind that these satellites can break down and need to be swapped out every few years (we're currently on our fifth generation of satellites). Plus they're moving so fast that scientists need to account for Einstein's General and Special Relativity in order for everything to work. Atomic clocks are super accurate, but due to time dilation predicted by Einstein's Relativity, the ones on the satellites are ticking faster than the Naval Observatory clock (because gravity is stronger on Earth and the satellites are moving so quickly). Therefore, the GPS has to calculate and adjust for the roughly 45,000 nanosecond difference per day between the satellites and atomic clock on the ground. Since the system can only work if everything is completely accurate, if they didn't account for even this tiny difference of 20-30 nanoseconds per day, the system would quickly fail.
How GPS Receivers Work (How Stuff Works)
"Fundamentally, three-dimensional trilateration isn't much different from two-dimensional trilateration, but it's a little trickier to visualize. Imagine the radii from the previous examples going off in all directions. So instead of a series of circles, you get a series of spheres."
Real-World Relativity: The GPS Navigation System (Ohio State)
"The current GPS configuration consists of a network of 24 satellites in high orbits around the Earth. Each satellite in the GPS constellation orbits at an altitude of about 20,000 km from the ground, and has an orbital speed of about 14,000 km/hour (the orbital period is roughly 12 hours - contrary to popular belief, GPS satellites are not in geosynchronous or geostationary orbits)."