Our ancestors had to go to pretty extreme measures to keep from getting lost. They erected monumental landmarks, laboriously
drafted detailed maps and learned to read the stars in the night sky.
Things are much, much easier today. For less than $100, you can get a pocket-sized gadget that will tell you exactly where you are on Earth at any moment as long as you have a GPS receiver and a clear view of the sky.
When people talk about "a GPS," they usually mean a GPS receiver. The Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting
satellites . The U.S. military developed this
Satellite network as a military navigation system, but soon opened it up to everybody else.
Each of these solar-powered satellites circles the globe at about 12,000 miles (19,300 km), making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites "visible" in the sky.
A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration.
Imagine you are somewhere in the states you are TOTALLY loss.
Let say you find a friendly local and he tells you "You are 625 miles from Boise, Idaho." This is a nice fact, but it is not particularly useful by itself. You could be anywhere on a circle around Boise that has a radius of 625 miles.
You ask somebody else where you are, and she says, "You are 690 miles from Minneapolis, Minnesota." Now you're getting somewhere..
You now know that you must be at one of these two intersection points of the overlapping radius circles, if you are 625 miles from Boise and 690 miles from Minneapolis.
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.
This same concept works in three-dimensional (trilateration ) space, as well, but you're dealing with spheres instead of circles.
Fundamentally, three-dimensional trilateration isn't much different from two-dimensional. If you know you are 10 miles from satellite A in the sky, you could be anywhere on the surface of a huge, imaginary sphere with a 10-mile radius.
If you also know you are 15 miles from satellite B, you can overlap the first sphere with another, larger sphere. The spheres intersect in a perfect circle. If you know the distance to a third satellite, you get a third sphere, which intersects with this circle at two points.
The Earth itself can act as a fourth sphere -- only one of the two possible points will actually be on the surface of the planet, so you can eliminate the one in space. Receivers generally look to four or more satellites, to improve accuracy and provide precise altitude information.
In order to make this simple calculation, then, the GPS receiver has to know two things:
· The location of at least three satellites above you
· The distance between you and each of those satellites
The GPS receiver figures both of these things out by analyzing high-frequency radio signals from the GPS satellites.
Radio waves are electromagnetic energy, which means they travel at the speed of light. The receiver can figure out how far the signal has traveled by timing how long it took the signal to arrive.
The length of the delay is equal to the signal's travel time. The receiver multiplies this time by the speed of light to determine how far the signal traveled. Assuming the signal traveled in a straight line, this is the distance from receiver to satellite.