What type of beacon is characterized as a nondirectional beacon operating in low to medium frequencies?

A nondirectional beacon (NDB) is a type of radio beacon that emits signals in all directions, allowing aircraft to determine their direction relative to the beacon. Operating in low to medium frequencies, NDBs are primarily used for navigation and are particularly advantageous for enroute navigation and non-precision approaches.

NDBs serve as a reliable navigational aid because they can provide continuous guidance to pilots, regardless of their position relative to the beacon. Since they operate on low to medium frequencies, they can effectively cover longer distances and penetrate various geographic features such as mountains or urban environments.

The other options represent different systems integral to air navigation. A localiser is part of the Instrument Landing System (ILS) and provides guidance specifically for lateral alignment with the runway during an approach. The glideslope, also part of ILS, provides information on the vertical descent angle necessary for a safe landing. ILS, which is a comprehensive landing system, includes both the localiser and glideslope for precision landings but does not operate in the same frequency range as NDBs. Thus, the characteristics of the NDB distinctly differentiate it from these more specialized systems utilized typically during approaches and landings.