What characteristic defines passive sensors?

Passive sensors are defined by their ability to collect data by detecting energy that is naturally available in the environment, primarily through reflected sunlight or thermal radiation emitted from objects. Unlike active sensors, which generate their own signal to measure characteristics of the target, passive sensors rely on external sources of energy to gather information.

The operation of passive sensors revolves around the reflection or emission of energy from the Earth’s surface or atmosphere. For example, a passive optical sensor captures sunlight reflected off the surface of the Earth, which allows it to create images or gather data based on the characteristics of that reflected energy. This characteristic allows passive sensors to be used in various applications, including remote sensing for environmental monitoring, land-use planning, and even in meteorology.

While passive sensors can operate under various lighting conditions, including daylight or low-light environments, they do not emit their own energy like active sensors do. Therefore, it is the reliance on external radiation, particularly reflected energy from other sources, that fundamentally distinguishes passive sensors from their active counterparts.