APG DCR-WEB user manual - Manual (Page 6)

DCR-WEB Rev. A3, 10/08
6
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accuracy.
Distance
The shorter the distance from the sensor to an object, the stronger the
returning echo will be. Therefore, as the distance increases, the object requires
better reflective characteristics to return a sufficient echo.
Size
A large object will have a greater surface area to reflect the signal than a
small one, therefore, a large target will be detected at a greater distance than a
small target. The surface area recognized as the target is generally the portion
closest to the sensor.
Angle
The inclination of the object's surface facing the ultrasonic sensor affects the
reflectivity of the object. The portion perpendicular to the sensor returns the
echo. If the entire surface is at a great enough angle, the signal will be reflected
away from the sensor and no echo will be detected. Generally a target at an
angle greater than 5 degrees off perpendicular will not be detected.