Car navigation

The earth with the GPS-satellites turning around it. The number of visible satellites is always changing. That is given here in blue for 45° North latitude at any moment during the earth's rotation.

Car navigation allows an automobile driver to see where he or she is on a map at any given time. Automobile navigation can rely on GPS (Global Positioning System), or it can be DVD- or data-based. GPS-bsed navigation uses radio signals from special satellites that send their position and the time of transmission. The GPS receiver catches the signal and calculates the time. The GPS can then calculate the distance from the satellite with the well-known radio signal speed.

How many satellites?

When driving a car with such a GPS receiver, you may receive a satellite which is above for instance Dover in the south and another one which is above Perth in the north. Then you know precisely how far you are from Dover and how far from Perth. With that two possible positions of your car at the flat land are precisely determined.

To know which one of both is the right place, one needs a third satellite, for instance above Land's End in the west. With that you can also reckon with the heigth, you can also drive in the mountains with it. With three satellites in fact there is still a third possible point, but that is twice as high as the satellites above the earth's surface and does not need to be reckoned with in car travel.

But we still need a fourth satellite, because the GPS receiver needs to know the time very precisely. In between it knows your car position, so that it can calculate the distance from that satellite. Then it knows the time of reception, because that is the time of transmission plus the travel time of the signal.

The GPS receiver

A GPS receiver (TomTom One)

At any moment the GPS receiver searches for the strongest satellites, calculates the position and the driving direction and then shows it all on a small screen as a three-dimensional map. With that it also shows your driving speed and how far it is to the next junction.

You even may add the address where you want to go. Then it delivers you nicely within a few meters in front of the house. One can even be shown where there are traffic jams, speed cameras and traffic lights!

When you deviate from the indicated route, the receiver instantly begins to calculate with the new data. That often takes some minutes, because all that approaching calculations have to be carried out again.

Maps

To show where you are with the calculated position, the GPS receiver contains digital maps. With a good brand of receiver you often can download new ones, in which recent changes in the street system are included.

Other websites

• Media related to Navigation systems at Wikimedia Commons