What is it necessary to observe a minimum of four gps satellites to define position?

Answer 1

The GPS receiver measures the distance from it to a GPS satellite. Then it measures the distance to another GPS satellite. The location is somewhere on a line made up of the millions of plaes that these two distances cross each other. A third satellite distance is required to find out where your GPS receiver is, somewhere on that line.

This process is called "Trilateration" (Three distances). (Not triangulation).

If you want to know how high you are above sea level, you need a distance from a fourth satellite.

Answer 2

To determine a position using GPS, at least four satellites are needed because each satellite provides distance information in the form of a radius from the satellite to the receiver. With four satellites, the receiver can calculate its latitude, longitude, altitude, and the exact time. Three satellites are needed to determine a 2D position on the Earth's surface, while the fourth satellite is necessary to account for the receiver's clock error.