Reading leads to more self-tutoring: the tutor reveals what he just learned: radar is an acronym.
RADAR: RAdio Detection And Ranging
In this context, ranging apparently means determining the detected object’s range, or distance away.
Source:
Mish, Frederick C. (editor). The Merriam-Webster Dictionary. Springfield: Merriam-Webster, 2004.
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.