A facilities manager is selecting motion detectors for a warehouse aisle. The specification calls for a sensor that actively transmits high-frequency radio waves and triggers an alarm when the reflected signal returns with a Doppler shift produced by movement in its field of view. Which type of sensor meets this requirement?
Microwave sensors continuously emit microwave or millimeter-wave radio energy and monitor the returned signal. Movement in the protected area changes the frequency or phase of the reflection (a Doppler shift), allowing the device to detect motion even in total darkness. Infrared sensors are passive and look for heat changes, ultrasonic sensors emit sound waves rather than radio energy, and pressure sensors only respond when weight is applied directly to them.
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