An active sensor is a sensing device that requires an external source of power to operate; active sensors contrast with passive sensors, which simply detect and respond to some type of input from the physical environment.
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In the context of remote sensing, an active sensor is a device with a transmitter that sends out a signal, light wavelength or electrons to be bounced off a target, with data gathered by the sensor upon their reflection.
Active and passive sensing technologies are both used in remote sensing 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 that are inaccessible to people.
Active sensors are also widely used in manufacturing and networking environments for example to monitor industrial machines or data center infrastructure so anomalies can be detected and components can be repaired or replaced before they break and shut everything down.
Examples of other active sensor-based technologies include: scanning electron microscopes, LiDAR, radar, GPS, x-ray, sonar, 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, an airplane, a boat or a submarine UAV drone. They might also be mounted to another convenient observation point, 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 essential to the Internet of Things (IoT), in which almost any physical or logical entity can be equipped with a unique identifier and the ability to transfer data over a network autonomously.