Which navigation aid uses space waves?

• A. VOR, ILS
• B. DME
• C. TACAN
• D. ADF

Answer: (A)

Space waves are radio signals that travel through the air (line-of-sight) from the transmitter to the aircraft, rather than following along the Earth's surface as ground waves do. VOR and ILS operate using such space-wave signals. VOR provides directional information by emitting signals in a pattern that the aircraft’s receiver interprets to determine bearing to or from the station. ILS uses localizer (VHF) and glideslope (UHF) components that transmit through space to guide the aircraft to the runway, again relying on signals that travel through the air.

In contrast, ADF (NDB) uses low- and medium-frequency signals that primarily propagate as ground waves, hugging the surface. DME and TACAN involve microwave signals for distance measurements to a station; while these use space waves, their main role is distance rather than the bearing guidance emphasized by VOR/ILS. Thus, the nav aids that distinctly rely on space-wave propagation for bearing and approach guidance are VOR and ILS.