What are VORs, VORTACs, and DME limited to?

• A. Ground-based signals only
• B. Global coverage
• C. Line of sight
• D. Line of hearing

Answer: (C)

The concept being tested is the line-of-sight nature of these radio navigation aids. VORs, VORTACs, and DMEs are ground-based transmitters that send signals which travel in straight lines. An aircraft must have an unobstructed path to the station for the signals to be received clearly; the Earth’s curvature and antenna heights create a horizon, so once you’re beyond that line of sight, the signals can’t reach you. That’s why their usable coverage is limited to a line-of-sight distance around the station and increases only with altitude, never providing global coverage. They’re not about hearing, and global navigation would require satellites or other non-line-of-sight systems.