A carefully designed network of satellites at 20,200 kilometers altitude work together to find your position on earth so that you find the right one. Now a new and even more precise system is ready.

The core of the Global Positioning System, the GPS system, is 31 navigation satellites, orbiting 19300 kilometers above the earth.

Each satellite has both a radio transmitter and an extremely precise atomic clock, which indicates the time with an accuracy of three billion parts of a second.

The orbits of the satellites are planned so that there are at least four pieces above the horizon at all times, no matter where on earth you are.

Each navigation satellite constantly tells you what number it has, where it is and exactly when it sent its message.

It does this with the help of radio signals, and it is these signals that a GPS receiver captures.

### Developed for military use

The GPS receiver also contains a very accurate clock and can, therefore, calculate how far the signals from the satellites have traveled.

When the GPS receiver receives information from several satellites, a computer in the GPS receiver can compare the information and, using a mathematical model, determine the exact position, which is then dotted on a map.

The accuracy is generally 20-75 meters, but with various technical measures, it can be improved, so that the uncertainty drops to only a few centimeters.

Precision is necessary, among other things, when an area is to be mapped.

The GPS system is American and was originally developed for military use, but for many years it has been freely available.

A European and even more precise system called Galileo is scheduled to be ready by 2020.

### The position is determined by several satellites

As a result of the position of the orbits, a GPS receiver always has contact with four satellites.

Each satellite transmits a signal specific to that particular satellite and tells you how far away the receiver is.

Thus, the individual satellite cannot determine the exact position of the receiver – it can, however, four satellites.

1. Satellite A tells that the receiver is at a distance of 21000 kilometers from the satellite. The recipient can thus be anywhere on the surface of the globe.

2. Satellite B shows that the receiver is 22000 kilometers from it. This means that the receiver must necessarily be somewhere on the circle that occurs where the two spheres intersect.

3. Satellite C reveals that the receiver is 20000 kilometers from it. This means that the receiver must be in one of the two points where the three globules intersect.

Which of the two points the receiver is in is revealed by the fourth and last satellite.

The satellites are located in six orbits around the earth

In orbit around the Earth, there are 31 GPS satellites. They are located in six orbits 20200 kilometers above the earth. The speed of the satellites is 3.2 kilometers per second, and two revolutions around the earth take a day.