 Detecting modulation depends on the angle of the source versus the target; if the source is too far off-center with a turbine or other moving surface, the modulation may not be evident because the moving part of the engine is shielded by the engine mounting.Modulation increases, however, when the source is at right angles to the axis of rotation of the moving element of the target.For fully exposed moving elements, (e.g., propeller blades or helicopter rotors), modulation is a function of the radar beam being off-center to the center of the moving element.