This is the 10467th most frequent German word.
radar
Radar: A system for detecting objects and determining their distance, speed, and position using radio waves.
Here, 'Radar' refers to the physical device used for detecting objects in the airspace.
Das Flugzeug wurde auf dem Radar entdeckt.
The airplane was detected on the radar.
In this idiomatic usage, 'Radar' represents general notice or awareness, implying not being noticed.
Seine Pläne bleiben unter dem Radar.
His plans remain under the radar.
In this case, 'Radar' refers to a new technology or equipment for detection purposes.