Definition
A radar system located on the ground that detects and tracks aircraft by transmitting radio energy and analyzing the returning echoes. Air traffic controllers use ground radar to monitor aircraft positions, altitudes (when combined with transponder data), and movement within their assigned airspace.
Plain English
A radar set up on the ground that watches aircraft in the sky, used by controllers to see where you are and what you're doing.
Context Anchor
Seen in discussions of encoding altimeters, where aircraft altitude information is sent for display on air traffic control radar equipment.
Derivation
Radar comes from RAdio Detection And Ranging — a system that uses radio waves to find objects and measure how far away they are. Adding 'ground' simply specifies that the radar equipment sits on the ground rather than in an aircraft.
Why Pilots Care
Ground radar allows controllers to verify an aircraft's reported altitude and maintain safe separation.
Intuition Check
Ground radar does not mean radar used to scan the surface beneath the airplane. Here it means radar equipment located on the ground and used to observe aircraft.
Example Sentence 1
Once airborne and clear of the local terrain, the controller picked us up on ground radar and assigned a heading direct to the next fix.
Example Sentence 2
During the approach, ground radar confirmed the airplane's position and height above the airport.