Private Pilot Private Exam

How many GPS satellites are required to yield a three-dimensional position (latitude, longitude, and altitude)?

1. 2

2. 3

3. 4

4. 5

The correct answer is: 4

To determine a three-dimensional position—latitude, longitude, and altitude—at least four GPS satellites are required. This is because each satellite provides a range measurement, which allows the GPS receiver to calculate its distance from that satellite. With data from one satellite, a user can determine a position on a sphere surrounding that satellite, providing two possible locations (above and below the sphere). With two satellites, the intersection of the two spheres results in a circle, but does not provide altitude. Adding a third satellite helps narrow down the position to two points, as it creates another sphere, but still leaves uncertainty in altitude. Only when a fourth satellite is included can the GPS receiver accurately calculate the user's exact position in three dimensions. The fourth satellite allows for correction of any timing errors in the satellites' signals, which is crucial for accuracy. Thus, to achieve a precise three-dimensional geographical position, four satellites must be in view of the GPS receiver.