GPS

Lina and Maria are out in a park in Paris with Maria's dog. - Oh Diva ran away! - No problem. This app in the phone shows where she is. If she runs too far away, I get an alarm. - But how does the app know where the dog is? - In Diva's collar there is a small receiver which receives radio signals from satellites located about twenty thousand kilometers above the ground. The satellites send information about the location on Earth over which they are positioned. With the help of this information, the receiver then calculates its own position.

The receiver sends the information to the app in Maria's phone. - But how does the receiver calculate its position? - The receiver receives a signal from one of the satellites. The signal from this satellite contains the information that it is over Notre Dame at a certain time. If the receiver also happens to be at Notre Dame, it receives this information at the moment when the signal reaches Notre Dame. But if it takes longer .. .. then the receiver is not at Notre Dame.

By measuring the extra time it takes for the signal to reach the receiver, it can figure out that it is .. ..three kilometers from Notre Dame. With that information it is possible to draw a circle on a map. The circle has a radius of three kilometers with Notre Dame in the middle of the circle. Now the receiver knows that it is somewhere on the perimeter of the circle. The receiver also receives information from a second satellite.

This one says it's over the Arc de Triomphe. From the extra time it takes for this signal to reach the receiver, it calculates that it is ... ... two kilometers from the Arc de Triomphe. With this information it is possible to draw another circle on the map. This circle has a radius of two kilometers and its centre point at the Arc de Triomphe.

The two circles meet in two places. The receiver on the dog must be at one of the two points. The receiver is also receiving information from a third satellite. This one says it is positioned over the Eiffel Tower. With the extra time it takes for the signal to reach the receiver, the receiver calculates that it is one kilometer from the Eiffel Tower.

With this information, a third circle can be drawn on the map. This circle has a radius one kilometer with the Eiffel Tower in the middle. Now there is only one point where all three circles cross. And there is the dog! This technique is called trilateration.

It was enough for the receiver to get information from three satellites to know exactly where it is ... ... if the receiver has a clock that is precise enough. The receiver calculates like this: The speed of radio waves, which is three hundred million metres per second, multiplied by the extra time it takes for a signal to reach the receiver. If the receiver's clock is slightly inaccurate, there will be a small error: one, or a few metres. The more accurate the clock, the more accurate the position reading.

But, exact clocks cost a lot of money. The receiver would be too expensive. There is another way. The receiver can use a fourth satellite. With four satellites, the receiver can calculate a position that is very accurate without having an expensive clock built in.

And using even more satellites makes it even more accurate. The technique used by the dog's collar is called the Global Positioning System, shortened to GPS. - What a great thing! Michael should put one on all his rabbits.