Passive versus active sensors — source of energy A passive remote sensing sensor relies on which source of energy for illumination of the target?

Introduction / Context:
Remote sensing systems are broadly classified as passive or active, based on how the target is illuminated. This distinction is fundamental for understanding sensor design, data characteristics, and optimal acquisition times.

Given Data / Assumptions:

• Passive sensors measure naturally available radiation.
• Active sensors transmit their own radiation and measure returns.
• We focus on Earth observation at optical/thermal wavelengths.

Concept / Approach:
Passive optical sensors (e.g., Landsat, Sentinel-2) rely primarily on reflected solar radiation (visible/NIR/SWIR) and emitted thermal radiation (TIR) from the Earth–atmosphere system. Active sensors (e.g., LiDAR, SAR) supply their own energy.

Step-by-Step Reasoning:

Verification / Alternative check:
Night-time optical imaging in reflective bands is impractical for passive sensors due to lack of solar illumination, confirming their dependence on the Sun.

Why Other Options Are Wrong:

• flash light as a source of energy: Not used in orbital imaging; unrealistic scale.
• its own source of energy: Defines active sensors (e.g., radar, LiDAR), not passive.
• None/All of these: Contradict the passive definition.

Common Pitfalls:
Confusing passive thermal emission with “own source”; the sensor measures Earth’s emission—it does not generate it.

Final Answer:
sun as the source of energy