Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): A remote sensing system that possesses only a sensor and depends on an external source to irradiate the target to be sensed is called an active remote sensing system.
Reason (R): Synthetic Aperture Radar uses microwave band to estimate soil moisture.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Active Remote Sensing = Own energy source (e.g., Radar, LiDAR).
Passive Remote Sensing = External energy source (e.g., Sunlight/thermal emission).
Microwaves are highly sensitive to the soil water dielectric constant, making radar excellent for estimating soil moisture.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Remote sensing systems are classified as active or passive based on the energy source used to illuminate the target.
Active systems generate their own electromagnetic energy, whereas passive systems rely on natural energy sources like solar radiation.

Step 2: Detailed Explanation:
Let us analyze both statements separately:
- Assertion (A): A remote sensing system that relies on an external, natural source of energy (primarily the Sun) to illuminate the target is defined as a passive remote sensing system.
Examples include standard optical cameras and multispectral scanners.
An active remote sensing system, on the other hand, possesses its own energy source (such as a transmitter) to emit radiation toward the target and then measures the backscattered signal.
Examples of active systems include Radar and LiDAR.
Therefore, Assertion (A) is incorrect because it incorrectly defines an active remote sensing system.
- Reason (R): Synthetic Aperture Radar (SAR) is an active sensor that transmits microwave signals to the Earth's surface.
Because water has a very high dielectric constant compared to dry soil, the backscattered microwave signal is highly sensitive to the water content in the soil.
As a result, SAR is widely used in the microwave band to estimate soil moisture levels across different terrains.
Therefore, Reason (R) is correct.
Since the Assertion is false and the Reason is true, option (D) is the correct choice.

Step 2: Final Answer:
Assertion (A) is false but Reason (R) is true.