What is the primary use of a Non-Directional Beacon (NDB)?

The primary use of a Non-Directional Beacon (NDB) is indeed for air navigation, especially in areas with challenging or uneven terrain. NDBs are radio beacons that emit signals to assist pilots in determining their aircraft's position and maintaining the desired flight path. They are particularly valuable when navigating in regions where visual references may be limited due to mountains, forests, or other obstacles that could hinder ground visibility.

By providing a reliable signal that aircraft can home in on, NDBs enhance navigational accuracy and safety. While NDBs can have secondary applications, such as assisting in landing procedures or providing general navigation aids over water, their core functionality is geared towards supporting pilots in maintaining safe navigation in less than ideal flying conditions. This makes them crucial tools in the broader spectrum of aviation navigation aids, especially where ground features do not provide sufficient visual guidance.