Plain English Meaning
A radio signal that travels straight through the air from the transmitter to the receiver, including any part of it that bounces off the ground along the way. It does not bend around the Earth -- it goes in a straight line, so the transmitter and receiver generally need to be within sight of each other.
Full Definition
The component of a radio ground wave that travels in a direct line from the transmitting antenna to the receiving antenna, together with the portion that reflects off the surface of the Earth. The space wave is one of the two parts of the ground wave, the other being the surface wave that follows the curvature of the Earth.
Context Anchor
Seen in radio communication and navigation discussions, especially when explaining why VHF signals are mostly limited by distance, altitude, terrain, and the curve of the earth.
Derivation
Called a 'space' wave because it travels through open space (the atmosphere) in a straight line, as opposed to the surface wave which clings to and follows the Earth's surface.
Why Pilots Care
It explains why VHF communications and navigation signals have a limited range that increases with altitude.
Grounding Statement
If the transmitting antenna and the aircraft are too low or blocked by terrain, the space wave may not reach the aircraft clearly.
Intuition Check
“Space” does not mean outer space here. It means the open air path between the transmitter and receiver.
Example Sentence 1
VHF communication radios depend on the space wave, which is why range increases with altitude.
Example Sentence 2
At our cruising altitude the space wave from the distant VOR provided reliable navigation until we passed the radio horizon.