ABSTRACT
Imagine it’s foggy and you can’t move forward, but you have a bag of tennis balls and you want know if there’s anything ahead of you. You can try listening to see if you can hear any echoes (passive sensing) or you can throw a tennis a ball and note whether it bounces off a nearby object and returns. Now imagine a whole bag of tennis balls thrown at a constant rate of speed. With a few calculations based on when the balls return, you could determine the distance to an object. By throwing a large number of balls in an ever-widening arc, you could even get some idea of the size and shape of the object. This is the general concept behind radio detection and radio ranging.
