Which statement best defines a space wave?

• A. They bounce off the ionosphere.
• B. They travel along the ground.
• C. They pass through the ionosphere.
• D. They are not used in navigation.

Answer: (C)

Space waves are electromagnetic waves that travel through space and the atmosphere with a direct line of sight between transmitter and receiver, typically passing through the ionosphere rather than being reflected. This makes them the usual mode for line-of-sight navigation and communication systems, such as VHF/UHF, where signals reach receivers without bouncing off the ionosphere.

The ionosphere can reflect lower-frequency waves back to Earth (skywave), which is why bouncing off the ionosphere describes that other propagation mode. Signals that travel along the ground are ground or surface waves, which behave differently from space waves. And in navigation, space waves are indeed used for many systems, so saying they’re not used would be incorrect.