Definition
The distance from a fixed point on the ground to the airplane's path over the ground, used as the reference for flying ground reference maneuvers such as turns around a point or a rectangular course. It is measured along the surface, not through the air, and stays constant only if the pilot actively corrects for wind.
Plain English
It is how far the airplane is from a chosen spot on the ground, measured along the ground itself. To keep that distance the same all the way around a turn, the pilot has to adjust the bank angle to account for the wind pushing the airplane.
Context Anchor
Used in ground reference maneuvers such as a rectangular course, where the pilot tries to keep the airplane the same distance from field boundaries while turning around corners.
Derivation
Ground-based means the reference point is on the ground (not on the airplane or in the air). Radius comes from Latin for the spoke of a wheel — the straight line from the center of a circle to its edge. Together: the spoke-length from a ground point out to the airplane's track over the ground.
Why Pilots Care
Keeping the ground-based radius steady produces a rectangular ground track of consistent size even when wind is present.
Grounding Statement
Picture the airplane drawing a curved line around a field corner; the ground-based radius is the size of that curve on the ground.
Intuition Check
Do not read “ground-based” as meaning the airplane is close to the ground. Here it means the radius is judged by the airplane’s path over the surface, not only by what the airplane is doing in the air.
Example Sentence 1
The instructor told the student to fly a turn around a point while keeping a constant ground-based radius from the silo.
Example Sentence 2
A sudden crosswind gust changed the ground-based radius until the pilot corrected with a slight change in bank angle.