What is an Impulse Radar or GPR Test?

What do these numbers mean?  Rebar or other embedded material in a wall reflect back a portion of the radar wave traveling through the wall.  Once the radar is calibrated to the structure, the distance from the surface of the wall to the embedded object of flaw can be measured on the image of the radar wave reflecting from within the wall.  Standard recommended depths for rebar in concrete exposed to the elements lead to the color code system above with blue being greater than recommended depth, green being at recommended depth, and red being significantly less than the recommended depth.