Definition 1 of 2
Definition
A radio signal that travels upward from a transmitter, is reflected (refracted) by the ionosphere back toward Earth, and is received at long distances from its source. Sky wave propagation occurs primarily in the low and medium frequency bands and is most pronounced at night, when ionospheric conditions allow signals to bounce farther.
Plain English
A radio signal that bounces off the upper atmosphere and comes back down somewhere far away, instead of traveling in a straight line along the ground.
Context Anchor
Seen in instrument flying and radio navigation discussions, especially with automatic direction finder (ADF) use and nondirectional beacon (NDB) reception at night.
Derivation
Called a 'sky wave' because the signal's path goes up into the sky before returning to the receiver, in contrast to a 'ground wave' that follows the curve of the Earth.
Why Pilots Care
Sky waves extend the range of certain radio signals at night but can also create interference with navigation aids.
Analogy
It is like hearing an echo in a canyon: the sound may reach you directly, but another copy arrives after bouncing off something else. The delayed copy can make the original harder to trust.
Grounding Statement
At night, a radio beacon signal can reach the airplane by a normal near-Earth path and also by a high path that bends back down from above, causing the receiver to act less steady.
Intuition Check
Do not read sky wave as a weather wave or a visible wave in the sky. Here it means a radio signal path that goes upward and returns from high above Earth.
Example Sentence 1
At night, sky wave interference can cause the ADF needle to wander when tracking a distant NDB.
Example Sentence 2
The pilot switched to a lower frequency to reduce sky wave interference during the approach.