GPS

GPS the global positioning system is actually a constellation of 27 Earth-orbiting satellites(24 in operation and three extras in case one fails).each of these 3000-4000 pound solar-powered satellites circles the globe at about 12000 miles,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.

Trilateration tells us that the location of a point can be defined by the distances of that point to 3 other points.That means you can find out your location on the Earth if you know:

1 The locations of three satellites above you.

2 the distance between you and each of those 3 satellites.

To know where the satellites actually are isn't particularly difficlut because the satellites travel in very predictable orbits.Besides,the Department of Defense constantly monitor their exact positions and transmits any adjustments to all GPS receivers as part of the satellites's signals.

The GPS receiver figures out distances by analyzing high-frequency,low-power radio signals from the GPS satellites.