Which navigation system does not require external information to function?

The Inertial Navigation System (INS) is a standalone navigation method that does not rely on external information or signals. It operates by using internal gyroscopes and accelerometers to calculate the aircraft's position based on its last known location. The system continuously updates the current position by measuring the motion of the aircraft from that reference point, making it independent of external sources.

In contrast, the VOR (VHF Omnidirectional Range) and ILS (Instrument Landing System) both depend on ground-based stations for guidance and navigation, receiving external signals to determine aircraft position and approach paths. GPS (Global Positioning System) relies on satellites for location information, and therefore also requires external input to function correctly. This distinction illustrates why INS is the only navigation system listed that operates independently of external information sources.