What distinguishes passive sensors from active sensors?

Passive sensors are distinguished by their reliance on external light sources to function, primarily capturing reflected sunlight or emitted thermal radiation from the Earth's surface. This means they do not need their own power source to produce the signal they detect. Instead, they utilize natural energy sources, which allows them to operate in a variety of environmental conditions as long as adequate illumination is present.

In contrast, active sensors generate their own energy to illuminate the target area and measure the reflected signals. Therefore, the lack of a need for an external power source is a fundamental characteristic that sets passive sensors apart from active sensors.

The context of why other options are less accurate is that while passive sensors can operate at nighttime when detecting thermal emissions, their capabilities are dependent on environmental conditions rather than solely day or night use. Additionally, the claim that passive sensors cannot measure wind speed is incorrect because the ability to measure wind speed can relate to the broader context of the data collected, whereas the operating altitude does not specifically differentiate between passive and active sensors directly.