# How Does GPS Work?

Global Positioning Systems rely on receivers that are now the size of a pin and satellites, 24 to be exact, that act as ?man-made stars? that create a network of constellations to mathematically determine measurements as accurately as to the centimeter.

There are five keys to how a GPS (Global Positioning System) works:

Triangulating
Measuring Distance
Getting Perfect Timing
Satellite Positions
Error Correction

The prefix tri- means three. Therefore, triangulating means measuring the distances from three satellites. Although a fourth satellite or measurement is theoretically necessary to determine an exact location, three satellites can determine two possible locations and one of the two locations, or measurements, is theoretically impossible. Therefore, a fourth measurement is not necessary because of the obscurity of one the two possibilities arrived at by the three satellites.

Satellites move though, right? So how can a distance be measured when there are three objects moving?

The answer is easy ? math and science! The distance is found by measuring how long it takes a signal sent from a satellite to get to the receiver. Basically, the speed of light. So that explains the measuring part ? how about the timing part?

Again, it?s based on mathematical equations used in science and math. A signal, called a Pseudo Random Code, is played from the receiver and the satellite. The signal from the satellite is delayed because it has to travel to the receiver (which is already playing the same code). If you could hear both codes they would be all garbled because they would not be at the same point at the same time ? the receiver would be ahead or faster than the code coming from the satellite. The amount the receiver needs to adjust the signal is how the distance or timing is calculated. You must multiply the time it takes for the code to reach the receiver by the speed of light and that equals distance. Fascinating!

The tricky part (wasn?t it tricky already) is timing. The receivers and the satellites need to be perfectly in sync in order to calculate accurately. Since we?re dealing with satellites and huge differences and astounding speeds of light, any delay in timing creates huge errors. Satellites have atomic clocks on them which produces super accurate timing. Since this isn?t possible with the receiver then a fourth satellite measurement is taken which allows any discrepancies to be corrected. This in turn produces no timing errors.

Because satellites are so high up in space (out of our atmosphere) they run on very predictable orbits. The Department of Defense monitors these orbits and if something is out of whack, they send a signal to the satellite which corrects any erroneous information. Therefore, locating the satellites is the easy part.

Since everything mentioned so far about GPS has variables or built-in errors, these errors need to be eliminated or fixed. Through mathematics variables in the atmosphere, satellite conditions and errors in signals can be adjusted.

In short, the way GPS works is to shoot signals to three satellites orbiting in space. Because of science and mathematical equations, the GPS system is able to calculate anything having to do with distance, timing, or location.

The development of GPS has allowed us to determine a basic position, get from one location to another, monitor the movement of people and things, create maps of the world and bring precise timing to the world.
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