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How Does GPS Work?

What exactly is GPS and how does it work? How does your smartphone or tablet use GPS? Tech Talker gives you the rundown on the Global Positioning System and how your smartphone or tablet always knows where you are.

By
Eric Escobar,
January 22, 2015
Episode #157

Page 1 of 2

Hey everyone! This week, I’ll be covering how GPS works and how your smartphone or tablet uses GPS and other methods to find you.

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What Is GPS?

GPS, or the Global Positioning System, was created by the United States military in 1995 for troops to locate themselves and targets, no mattter the conditions on the battlefield.

Flash forward 20 years and almost everyone has access to GPS. It's hard to believe that only a couple of decades ago you needed a map and a compass to get somewhere! Now, both of those are neatly coupled on one portable device.

How Does GPS Work?

The simple answer: satellites. The process, however, is far from simple.

It's pretty amazing how these satellites actually determine your position. Each of them contains super accurate atomic clocks and emits a radio frequency that is received by whatever device you’re using. This radio frequency contains the satellite's time and position, and because light moves at one speed, your device can determine how far away it is from each satellite by measuring the time it takes to receive a signal.

Now, in order to lock onto your location, your device needs to be able to “see” at least 4 satellites. Now I say "see," but I'm using that term loosely because each of these satellites orbits about 12,000 miles above the earth. Basically, there has to be at least 4 satellites above you whose signal is not blocked.

Most people think that you only need 3 satellites to triangulate your position (hence the term "tri-angulate"). However, with GPS you need at least 4 satellites: 3 for position coordinates and 1 to perform error correction. Error correction is needed to fix any distortions that might occur from the radio signal that is being sent to your device. Your device then takes this information and performs a ton of complicated math to triangulate your position anywhere on the planet.

What’s even more amazing is that these satellites are traveling so fast, they actually embody Einstein’s theory of relativity in two ways:

1.     The faster an object moves, compared to another object (say the Earth), the slower time appears for that object.

2.    The more gravity is felt by an object, the slower time appears for that object.

Essentially, the net of these two effects causes the clocks onboard each satellite to tick 38 microseconds faster than the same clock on Earth. So if you counted 24 hours on Earth, the satellite would actually experience 24 hours plus 38 microseconds. That’s 38 millionths of a second!

Which means that the clock onboard the satellite actually experiences time faster than we do. This comes back to Einstein and his theory of relativity in which he postulates that time is “relative” based on the gravity you're experiencing and how fast you're moving.

While 38 millionth of a second doesn’t seem like a lot, if this wasn’t accounted for in each satellite, your device would be off by about 6 miles after just one day.

So let’s do a little recap here. In order for your phone to locate you using GPS:

  • It needs to lock onto at least 4 satellites, circling 12,000 miles above Earth.

  • It needs to receive a radio frequency signal from each of these satellites.

  • These signals need to be corrected for time in two different ways.

Then, if that weren’t enough, your device must perform fairly complex math in order to give you directions to the nearest Starbucks!

 

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