What characterizes active remote sensing?

Active remote sensing is characterized by the system generating its own signal to probe the environment, illuminating the target, and then measuring the reflected energy that comes back. This process allows for a direct measurement of the characteristics of the target, as the emitted signal can be tailored and controlled, leading to higher precision than passive methods.

In contrast, the first option refers to passive remote sensing methods that rely on detecting natural radiation emitted from objects, such as sunlight reflected from the Earth's surface. The second option also pertains to passive remote sensing, as it mentions reliance on external light sources. The last option describes passive signals, further emphasizing a lack of active engagement in data collection.

Thus, the emphasis in active remote sensing is on the system's own signal to gather information, making it distinct in its ability to gather data under various conditions and times, independent of natural illumination.