FAA Ground School Practice Test

How many Global Positioning System (GPS) satellites are required to yield a three-dimensional position solution?

1. 3

2. 4

3. 5

4. 6

The correct answer is: 4

To determine a three-dimensional position solution using the Global Positioning System (GPS), at least four satellites are necessary. Each satellite provides a one-dimensional measurement of the distance from the satellite to the GPS receiver based on the time delay of the transmitted signal. When you receive signals from three satellites, you can calculate a two-dimensional position solution, which typically includes latitude and longitude. However, this does not account for altitude. To accurately compute the altitude and thus achieve a complete three-dimensional position—latitude, longitude, and elevation—a fourth satellite is required. The fourth satellite helps to solve for time discrepancies in the receiver’s clock, which can introduce errors if not properly accounted for. Essentially, the fourth satellite allows for a more precise fix on the position in all three dimensions. This requirement is why four satellites are essential for obtaining a three-dimensional position solution in GPS systems.