Which element is NOT part of radio navigation?

Visual checkpoints are not considered a part of radio navigation because they rely on visual reference points that pilots can see rather than on radio signals or systems to aid in navigation. Radio navigation typically involves the use of electronic signals transmitted from ground-based stations or onboard systems.

Options like ground-based radio aids and onboard navigational instruments rely on radio frequency signals to provide pilots with positional information and guidance. Ground-based radio aids include systems such as VORs (VHF Omnidirectional Range) and NDBs (Non-Directional Beacons), which transmit signals that aircraft can use to determine their location relative to these navigational aids. Onboard navigational instruments utilize various radio signals to process and display relevant information for the pilot, assisting in course tracking and maintaining situational awareness.

While weather radar systems might provide useful information for flight operations, they are not specifically categorized as tools for navigation through radio signals; instead, they are used for detecting and tracking weather phenomena. Therefore, visual checkpoints being based on physical landmarks is why they do not fit within the radio navigation framework.