Question

Which of the following is/are passive remote sensing technique(s)?

• A. Sonar
• B. Radar
• C. Aerial photos
• D. Landsat images

Hint:

Active sensors send out their own signal and read the echo (sonar, radar); passive sensors only record energy already present, like sunlight (aerial photos, Landsat).

Answer 1

The Correct Option is C, D

The question is really asking which of these four tools generates its own energy and which ones simply record energy that is already there. Sort the four options into these two groups.

1. Sonar: works by sending sound waves into water and timing the echo that bounces off an object. Since it transmits its own signal first, it belongs to the active group.
2. Radar: works the same way but with radio or microwave pulses instead of sound, transmitting a signal and reading the reflection, so it is also active.
3. Aerial photos: a camera on an aircraft only captures sunlight that is already bouncing off the ground, it never sends anything out itself, so it is passive.
4. Landsat images: satellite sensors record sunlight reflected by, and heat given off by, the earth's surface, again without transmitting a signal of their own, so this is passive too.

Sonar and Radar both transmit their own energy, so they fall in the active category and are ruled out. Aerial photos and Landsat images both only receive existing natural energy, so they are the passive techniques.

Let's summarize:

• Active means the sensor sends out its own energy and reads the return (Sonar, Radar).
• Passive means the sensor only records existing energy such as reflected sunlight (Aerial photos, Landsat images).

So the correct options are Aerial photos and Landsat images, that is, 3,4.