What is a fully self-contained navigation system that requires no external input?

A fully self-contained navigation system that requires no external input is indeed represented by an inertial navigation system (INS). This system operates by utilizing internal sensors, typically gyroscopes and accelerometers, to track the position, orientation, and velocity of an aircraft. Because it relies on data from its own sensors to calculate its location, the inertial navigation system does not need information from external sources such as satellites or ground-based stations, making it autonomous.

In contrast, other navigation systems such as the Global Navigation Satellite System (GNSS) and ground-based navigation systems depend on external signals. GNSS relies on satellites to determine location, while ground-based navigation systems utilize infrastructure such as beacons or transmitters that provide positioning information. Radio navigation systems also depend on external radio signals to ascertain position. The key attribute that makes the inertial navigation system unique is its independence from external inputs, allowing it to function seamlessly in environments where external signals may not be available, such as in dense urban areas or underwater.