A passive sensor is a device that detects and responds to some type of input from the physical environment.
Passive sensor technologies gather target data through the detection of vibrations, light, radiation, heat or other phenomena occurring in the subject’s environment. They contrast with active sensors, which include transmitters that send out a signal, a light wavelength or electrons to be bounced off the target, with data gathered by the sensor upon their reflection.
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Both active and passive sensing technologies are often used to make observations and measurements from a distance or on a scale beyond those observable to the naked eye. Sensors can also be used in harsh environments and places inaccessible to people.
Examples of passive sensor-based technologies include: Photographic, thermal, electric field sensing, chemical, infrared and seismic. However, as can be the case with some sensors, seismic and infrared light sensors exist in both active and passive forms.
Depending on what is being sensed these various sensors might be mounted to a satellite, airplane, boat, submarine UAV drone, or from another convenient point of observation such as a building top. The data gathered by remote sensing is used for everything from cartography to resource exploration to atmospheric and chemical measurements. Remote sensing is also one of the basic enabling technologies for the Internet of Things (IoT), in which almost any imaginable entity can be equipped with a unique identifier and the ability to transfer data over a network autonomously.
Both types of sensors have benefits and drawbacks. Passive sensor technologies can't be detected by observed parties as they only sense what is in the environment rather than relying on a transmitter whose activity might be detected with equipment. Active sensors, however, can sometimes be used when passive sources of observations by sensor are impossible. This could be, for example, when the ambient observed phenomena are not available to a CCD camera during the night. Active sensor technologies like LiDAR or radar can still be used independent of daylight to make maps or track movement as they have their own radiations on which to base their observations.