What type of navigation systems do not require an external radio signal?

Inertial navigation systems rely solely on internal sensors to track the position, orientation, and velocity of an aircraft. These systems use accelerometers and gyroscopes to calculate the aircraft's current position by integrating the accelerations and rotations that have occurred since a known starting point. As a result, they do not depend on any external radio signals or references, making them entirely self-contained.

In contrast, satellite navigation systems leverage signals from satellites to determine location, while ground-based navigation systems and VOR systems both rely on signals transmitted from land-based stations. Therefore, the characteristic of not requiring an external radio signal distinctly applies to inertial navigation systems, highlighting their autonomous nature in providing navigation information.