Understanding Active Remote Sensing

The question asks about a specific type of remote sensing where the system itself produces electromagnetic radiation to illuminate a target and then measures the scattered radiation. Let's break down the concepts of remote sensing.

Remote sensing is the science and art of obtaining information about an object, area, or phenomenon through the analysis of data acquired by a device that is not in physical contact with the object, area, or phenomenon under investigation.

Remote sensing systems are generally categorized based on how they acquire the energy or radiation they detect:

• Passive Remote Sensing: These systems detect naturally available energy. The most common source of this energy is the sun, which illuminates the Earth's surface. Passive sensors measure the solar radiation reflected by the target or the thermal radiation emitted by the target itself. They do not produce their own energy source. Examples include conventional cameras, infrared radiometers, and many satellite sensors like those on Landsat or Sentinel-2 that measure reflected sunlight.
• Active Remote Sensing: These systems provide their own energy source (electromagnetic radiation) to illuminate the target. They transmit a signal towards the target and then measure the energy that is reflected or scattered back towards the sensor. Because they provide their own illumination, active systems can collect data day or night and are not dependent on sunlight. The question specifically mentions the interaction of "electromagnetic radiation produced with a specific wave length to illuminate a target... for studying its scattered radiance." This description perfectly matches the principle of active remote sensing.

Based on this distinction, the process described, where radiation is *produced* to *illuminate* a target and its *scattered radiance* is studied, is characteristic of active remote sensing.

Analyzing the Options

• Neutral remote sensing: This is not a standard term used in the classification of remote sensing systems based on their energy source.
• Remote sensing: This is a broad term for studying Earth from a distance. While the process described is a form of remote sensing, the term itself doesn't specify the method of illumination used.
• Active remote sensing: This correctly describes a system that produces its own electromagnetic radiation, transmits it to a target, and measures the backscattered signal. The description in the question aligns precisely with this definition.
• Passive remote sensing: This describes systems that rely on naturally available energy (like sunlight). The question explicitly states the radiation is "produced," which rules out passive systems.

Therefore, the interaction described is known as active remote sensing.

Revision Table: Passive vs. Active Remote Sensing

Additional Information on Active Remote Sensing

Active remote sensing systems work by emitting electromagnetic waves and measuring the response from the target. The time it takes for the signal to return and the intensity of the returned signal provide information about the target's characteristics.

• Radar (Radio Detection and Ranging): Uses microwave radiation. It is particularly useful for observing through clouds and at night. It can provide information about surface roughness, moisture content, and dielectric properties.
• Lidar (Light Detection and Ranging): Uses laser pulses (typically in the visible or near-infrared spectrum). It measures the time for the light pulse to travel to the target and back, allowing for precise distance measurements. Lidar is excellent for creating detailed 3D models of the terrain and vegetation structure.

These active systems are valuable tools in various applications, including mapping, monitoring deforestation, studying ice caps, and measuring atmospheric properties, because they control the illumination source and can collect data regardless of external light conditions.