How many GPS satellites are needed for an RPA to obtain a 3D fix on its location?

To determine the exact location of an RPA (Remotely Piloted Aircraft) in three-dimensional space, a minimum of four GPS satellites is required. This is because each satellite provides a specific piece of information regarding the RPA's position: one satellite can provide the distance to itself, but it cannot determine a fixed position in three-dimensional space.

By utilizing three satellites, the RPA can ascertain its position along the two-dimensional plane (latitude and longitude), but it would still be uncertain about its altitude. The fourth satellite allows the system to resolve the altitude, thus achieving a true 3D fix. This is essential for ensuring accurate navigation and safety during operation.

The additional satellites enhance accuracy and reliability but are not necessary for the basic 3D positioning requirement. Therefore, four is the correct answer to achieve a comprehensive fix on the RPA's location.