GPS Receivers: Determining Location by Trilateration
The signals transmitted by the GPS satellites travel freely in the space and in the atmosphere and reach our devices, which are called GPS receivers. The receiver, upon receiving information from the satellites computes the distance to each satellite. This method is called trilateration.
Let’s stop at this point and think about the trilateration issue: if you consider the satellite as a point which transmits the data to all directions, the transmission takes a spherical shape. If you slice the spherical shape, where one point is the satellite and the other is the Earth, you will have a circular shape. If you intersect more than one circle, then you will have an area. If you continue intersecting more circles, then the area becomes very small.
In the image on the left, you see two circles. Imagine the circles as the signals from the GPS satellites. In this setup, you are basically somewhere in the yellow area.
In the image on the right, there are three circles. Note that all the circles’ radii are different. But the addition of the third circle significantly reduced the size of the area, which is the black area.
Combine these with the trilateration (or maybe quadlateration, pentalateration or more) and you will end up with smaller areas. This is the case with the Global Positioning System: the more satellites you see, the more precise is your location.