When an object is properly positioned between the sensor and the window (See Figures 3b and 3c), the ultrasonic pulse from the sensor will reflect repeatedly between the sensor and the object, thus creating a series of secondary echoes.

A coincidental secondary echo condition exists when the distance traveled by the primary echo from an object at or within the window limits is a multiple of (most likely twice) the distance traveled by a secondary echo from the foreground object. As shown in Figure 3c, object #3 is properly positioned halfway between the sensor and the inner window limit and is mistakenly detected as if it were object #1. Exercise caution when considering a SUPERPROX application to avoid this condition.