What does passive remote sensing rely on?

Passive remote sensing relies on natural radiation that is reflected or emitted from the target being observed. This method captures the electromagnetic energy that is naturally available in the environment, such as sunlight reflected off surfaces or thermal radiation emitted by objects. Passive sensors, therefore, only detect and measure this energy without introducing any additional signals.

For instance, satellite imagery that uses visible light to capture images of the Earth relies on sunlight reflecting off surfaces like water bodies, land, and vegetation. Thermal imaging, another form of passive remote sensing, detects the heat emitted by objects, providing information about surface temperatures.

In contrast, other methods mentioned, such as artificial light sources or active signal emissions, do not apply to passive remote sensing. Active sensors, for example, use their own emitted signals, like radar or laser, to measure characteristics of the target, which fundamentally differs from the passive approach. Thus, understanding that passive remote sensing hinges on utilizing existing natural radiation is crucial in comprehending how various remote sensing technologies operate.