Which type of navigation system cannot operate without any external signals or inputs?

An Inertial Navigation System (INS) is designed to operate independently of external signals or inputs, relying solely on the internal sensors to calculate the aircraft's position, orientation, and velocity over time. It does this by measuring the acceleration and rotation of the aircraft and integrating these measurements to determine changes in position. This internal process allows the INS to function in environments where external navigation aids, such as GPS or radio signals, are unavailable or unreliable, making it unique in its capability to navigate without external assistance.

In contrast, systems like the Global Positioning System (GPS) and the Flight Management System (FMS) rely heavily on external signals. GPS requires signals from satellites to determine a precise position, while FMS often uses data from various sources, including GPS and other navigation aids, to manage flight paths. Ground Control Approach (GCA) also depends on radar and radio communications from ground control to provide guidance to aircraft, highlighting its reliance on external inputs. Therefore, the Inertial Navigation System stands out as the only navigation method that does not depend on external signals, highlighting its importance in situations where such signals might not be present.