How Does A Gps Operate

When people discuss "a GPS," they typically mean a GPS receiver. The Global Positioning System (GPS) is in reality a constellation of twenty-seven Earth-orbiting satellites (twenty-four in operation and three extras just in case one fails). The United States military developed and implemented this satellite network as a military piloting system, but before long opened it up to everyone else.

Each one of these three thousand- to four thousand-pound solar-powered satellites circulates the globe at around twelve thousand miles (19300 km), making two full rotations daily. The orbits are arranged so that during any time, anyplace on Earth, there are at minimum four satellites "visible" in the sky.

A GPS receiver's task is to locate four or more of these satellites, work out the distance to each one, and apply this information to deduct its own position. This procedure is based on a simple mathematical rule known as trilateration. Trilateration in three-dimensional space could be a bit tricky, so we will begin with an explanation of simple two-dimensional trilateration.

Suppose you're somewhere in the U.S. and you are completely lost -- for some reason, you have perfectly no hint where you are. You see a friendly local and ask, "Where in the world am I?" He replies, "You’re six hundred and twenty-five miles from Boise, Idaho." This is a good, true fact; however it is not particularly useful by itself. You could be anywhere on a circle around Boise which has a radius of six hundred and twenty-five miles.

You ask another local where you are, and she replies, "you're six hundred and ninety miles from Minneapolis, Minnesota.” Now you are getting somewhere. When you combine this information with the Boise information, you've two circles which intersect. You therefore know that you must be at one of these two points of intersection, if you're six hundred and twenty-five miles from Boise and six hundred and ninety miles from Minneapolis.

When a third local tells you that you are six hundred and fifteen miles from Tucson, Arizona, you will be able to eliminate one of the possibilities, since the third circle will only intersect with one of these points. You straightaway know precisely where you are – Denver, Colorado.

This similar concept works in three-dimensional space, likewise, but you are dealing with spheres rather than circles. Basically, three-dimensional trilateration Is not much different from two-dimensional trilateration, only it is a bit trickier to visualize. Try to visualize the radii from the earlier examples going off in all directions. So rather than a series of circles, you obtain a series of spheres.