Which of the following is NOT an externally supported component of the radio navigation system?

The inertial reference system (IRS) is not an externally supported component of the radio navigation system. Unlike TACAN, VOR, and GPS, which receive signals from external sources or stations to determine position and navigation information, the IRS operates independently. It uses internal sensors and algorithms to calculate the aircraft's position, velocity, and orientation based on its own motion. This internal calculation does not rely on external signals or stations, which differentiates it from the other systems mentioned, making it an independent navigation tool.

In contrast, TACAN (Tactical Air Navigation) and VOR (VHF Omnidirectional Range) are ground-based navigation aids that provide bearing and distance information, while GPS (Global Positioning System) utilizes satellite signals to determine the aircraft’s precise location. These systems rely on external signals, which is key in understanding why they are considered externally supported navigation components.