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  • GPS satellites are constantly bathing the world in their electromagnetic signally glow,

  • but how EXACTLY do these satellites find you?

  • Saying a satellite finds YOU is very Enemy of the State, very NBC TV government drama;

  • but GPS satellites don't actually track you, they're simply broadcasting a signal that

  • you pick up. What would be a more accurate way to look at it is, GPS doesn't find you,

  • but you find it.

  • The Global Positioning System is network of around 30 satellites, a receiver, and a SUPER

  • accurate clock; all pulled together with math. The system was created in the 1980s for the

  • military, but they opened it up for civilian use, and it has since changed how we find

  • directions, look for pizza, and connect with each other. But how it works, requires a TON

  • of incredible science.

  • Firstly, we have to know what time it is. I know that seems strange, but if you don't

  • have a SUPER ACCURATE measure of time, you can't do GPS! The most accurate measure of

  • time humans have, is the constant, predictable vibration of an atom. The U.S. Naval Observatory

  • in Washington, D.C., is the official United States time. They mark time, by measuring

  • billionths of a second using 9,192,631,770 electron vibrations of a Cesium-133 atom.

  • This atomic clock is accurate to nanosecondswhich is super important.

  • Secondly we have to have a satellite network which ALSO know the time. As of October 19,

  • 2015, there were GPS 31 satellites with three atomic clocks on board, as well as transmitters

  • to send microwave signals out at the planet below. The system is flying at 11,000 kilometers

  • per hour, about 20000 kilometers above our heads (7000mph/12500mi) and is maintained

  • by the U.S. Air Force. Those signals? Those are what are telling you where you are on

  • the planet!

  • Okay, so. GPS signals are sent at exact intervals. Sayevery few seconds. Embedded in microwave

  • signal is the ID of the satellite, the health of the satellite, the location of all the

  • satellites in the system, and the precise date and time. VERY precise. Because Tens

  • of thousands of kilometers away on the ground, the signals will be picked up by your smartphone

  • once you click, let the pizza app know your location.

  • Smartphones and GPS units have to have very precise time, because microwave signals travel

  • at the speed of light, roughly 300,000 kilometers per second, so in a fraction of a second,

  • the signal travels from each satellite to you! This is why we need atomic clocks, because

  • 20-30 nanoseconds matter! But we'll come back to this in a sec. The receiver can tell where

  • each satellite is, by the difference in time lag. Using a mathematical process called trilateration

  • the GPS unit can determine your exact location. Basically, a sphere is drawn indicating the

  • time lag from each satellite and the overlapping point is your current location! After ALL

  • THAT, the pizza app knows exactly where you are.

  • Before you get confused, there's NOT an atomic clock in your smartphone, but as long as the

  • smartphone clock is pretty good, it can will regular updates from the Observatory clock

  • and stay on track! The more satellites it can "see" the more accurate the location.

  • If you pick up three satellites, your GPS can determine your 2D location (latitude and

  • longitude), but with four it can tell your altitude too! As you move, the time lag from

  • the satellite to the antenna of each signal will change, and by constantly updating the

  • trilateration math, the location dot on your map moves.

  • Even after all this work just to get a pizza delivered, there's a ton more complications.

  • Satellites need replacement, and only have an operational life of 7-15 years. So they

  • have to keep launching new ones. We're on our fifth generation now, and they're getting

  • a lot better, but they're not perfect. The satellite signals can bounce off an ionized

  • layer of the atmosphere called ionosphere -- AM radio bounces off it from the ground

  • too. It's not friendly to signals going through it. But on top of THAT GPS satellites are

  • traveling FAST and far away, meaning Einstein's General & Special Relativity comes into play.

  • Even if the atomic clocks are super accurate, the ones on the satellites are ticking faster

  • than the Naval Observatory clock, because gravity is stronger on Earth and they're moving

  • so fast! SO! The GPS has to mathematically calculate and adjust for the roughly 45,000

  • nanoseconds per day of difference in time; since being off by 20-30 nanoseconds can make

  • the system pretty much worthless. This a big deal. It's frickin' crazy.

  • Thanks to the space program, Einstein's relativistic physics, atomic clocks, and smart phones,

  • you my friend, can get a pizza delivered to your exact location -- give or take a few

  • feet. What do you want? We're not perfect. Yet. The next generation should get us within

  • inches.

  • And thank you also to the US Air Force for sponsoring this message, and making sure the

  • pizza guy knows where to come. Every day American Airmen go above and beyond to break barriers

  • both professionally and personally. The United States Air Force is powered by Airmen, fueled

  • by innovation. Crazily enough, your BRAIN has GPS too, after

  • a fashion. A set of cells creates a map of the stuff around youNo, no, I'll let this

  • super handsome guy with a great t-shirt collection explain it.

GPS satellites are constantly bathing the world in their electromagnetic signally glow,

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衛星があなたの正確な位置を追跡する方法 (How Satellites Track Your Exact Location)

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    richardwang に公開 2021 年 01 月 14 日
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