The microwave technology is based on the so-called Doppler effect. The radar sensor emits (via its emitter TX) continuously microwaves with a specified constant frequency within a predefined field. Any object (including persons) within the detection field will reflect back the microwave to the sensor; the receiver RX will “catch” this reflected wave.
If the object is still , the emitted and the reflected waves will have the same frequency, i.e. they will have the same wavelength d; the sensor will not go into detection.
However, if there is a movement (of an object or a person) in the detection field, the reflected microwave will have a different frequency, i.e. a different wavelength d, than the emitted one; the sensor will sense this difference.
Depending on the fact that the moving object is approaching the sensor or departing from the sensor, the reflected wave will have – in comparison to the emitted one – respectively a smaller wavelength or a bigger wavelength. Thus, this technology allows distinguishing between an approaching and a departing movement; this capacity is called “direction sensing”.
Based on this capacity of “direction sensing”, two different kind of radar sensors can be distinguished:
Unidirectional radar sensors : this kind of sensors filters one of the two moving directions and goes only into detection in case of movement along the other direction.
Bidirectional radar sensors : this kind of sensors is sensitive to both directions and goes into detection in case of approaching and departing objects.
Furthermore, the radar technology offers the possibility to reject cross traffic (i.e. traffic parallel to the sensor) as well as to differentiate between pedestrians and vehicles.