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GPS

Global Positioning System

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What is GPS?How does it works?

GPS stands for Global Positioning System.

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What is GPS?

• GPS stands for Global Positioning System

• 27 Earth-orbiting satellites.

• There are always at least four satellites "visible" in the sky.

• A GPS receiver locates four or more of these satellites, figures out the distance to each, and uses the information to deduce its own location.

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Trilateralization in 2-D

• What if you were totally lost and a friendly local tells you that you are 625 miles from Boise Idaho. You could be anywhere on a circle around Boise that has a radius of 625 miles, like this:

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Trilateralization in 2-D

• You ask somebody else where you are, and she says, "You are 690 miles from Minneapolis, Minnesota." If you combine this information with the Boise information, you have two circles that intersect. You now know that you must be at one of these two intersection points.

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Trilateralization in 2-D

• If a third person tells you that you are 615 miles from Tucson, Arizona, you can eliminate one of the possibilities, because the third circle will only intersect with one of these points. You now know exactly where you are -- Denver, Colorado.

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Trilateralization in 3-D• Trilateralization in 3-D is pretty much the same as in 2D

but instead of circles, you are working with spheres. Because of the added dimension you need four spheres (the Earth acts as one).

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Crucial Information

So, in order to figure out where you are, the GPS receiver has to know two things:

1 The location of at least three satellites above you.

2 The distance between you and each of those satellites.

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But how does the machine that you hold in your hand figure all of this out?

• Electromagnetic Energy!• Radio waves are electromagnetic energy, so are X-rays.• Electromagnetic waves travel at the fastest speed possible,

the speed of light (about 186,000 miles per second, 300,000 km per second).

• Radio waves lie at the bottom end of the spectrum.

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There and back again

• The distance from you to a satellite is calculated by the time that it takes for a radio wave to travel from the GPS unit to the satellite and then back to the GPS unit. D = V*t

• How long do you think that delay would be?

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Let’s do the calculation!

• If the satellites orbit 19,000 km from the surface of the earth, and the speed of light is 300,000 km/sec

• t = d/V = 2 * (19,000 km) / 300,000 km s-1

• t = 0.127 s

• So these clocks have to be REALLY accurate, actually they are nuclear clocks.

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GPS Error Correction

• GPS signal errors are caused by– atmospheric conditions– timing errors– orbit errors

• Signal errors cause loss of accuracy

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Differential Correction

• Stand alone GPS is accurate to within 10 meters• Differential correction can improve normal GPS to

give you accuracies down to (potentially) below 1 meter

• Achieved by using a stationery base station GPS, subscription to commercial satellite broadcast service, or free service

• WAAS is the free differential correction developed by FAA

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Differential Correction

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GPS: Basic Uses• Locating

– determining a basic position

• Navigating– getting from one known point to another

• Tracking– monitoring the movement of people/things

• Mapping– creating maps of places/objects/fields etc.

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So who uses GPS?

• Transportation / Asset tracking– Trucks, Trains, Taxis, Ships, Parolees, etc.

• Guidance– Planes, ships, cars, farmers

• Emergency management– 911 locator, Roadside assistance (ONSTAR)

• Outdoors people– fishermen, hunters, etc.

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How would you use GPS?

• Camping• Hiking• Hunting• Where you left your car?

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The End

Anna Cathey

Thanks: Calvin Perry, Tasha Wells

www.howstuffworks.com (this site rocks)