What is required for 3D position calculation in GPS?

To calculate a three-dimensional position using GPS technology, a minimum of four satellites is necessary. This requirement arises because determining a position in three-dimensional space (latitude, longitude, and altitude) involves solving a set of equations based on the time it takes for satellite signals to reach the receiver.

One satellite can only provide information about your position along a sphere defined by its distance from that satellite, while two satellites add a second sphere, allowing for two possible intersection points. A third satellite narrows this down to one point on a sphere. Without a fourth satellite, determining the exact altitude becomes impossible due to the lack of reference for the receiver's clock, which may not be perfectly synchronized with GPS satellite clocks. Thus, the information provided by this additional satellite is crucial for achieving an accurate 3D position calculation and correcting for time discrepancies.

In summary, the correct answer emphasizes the necessity of four satellites to obtain a reliable three-dimensional position in GPS navigation.