Basics of Remote Sensing – Geography – UGC NET – Notes

Introduction

• Remote sensing can be broadly defined as the collection and interpretation of information about an object, area, or event without being in physical contact with the object. Aircraft and satellites are the common platforms for remote sensing of the earth and its natural resources.
• Aerial photography in the visible portion of the electromagnetic wavelength was the original form of remote sensing but technological developments has enabled the acquisition of information at other wavelengths including near infrared, thermal infrared and microwave. Collection of information over a large numbers of wavelength bands is referred to as multispectral or hyperspectral data.
• The development and deployment of manned and unmanned satellites has enhanced the collection of remotely sensed data and offers an inexpensive way to obtain information over large areas. The capacity of remote sensing to identify and monitor land surfaces and environmental conditions has expanded greatly over the last few years and remotely sensed data will be an essential tool in natural resource management.
• Remote sensing in 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 contact with the object, area, or phenomenon under investigation. It is a technology for sampling electromagnetic radiation to acquire and interpret nonimmediate geospatial data from which to extract information about features, objects and classes on the Earth’s land surface, oceans and atmosphere (and, where applicable, on the exteriors of other bodies in the solar system, or, in the broadest framework, celestial bodies such as stars and galaxies).
• Without direct contact, some means of transferring information through space must be utilized. In other words, remote sensing refers to instrument-based techniques used in the acquisition and measurement of spatially organized (distributed) data/information on some property (ies) (spectral; spatial; physical) of an array of target points(pixels) within the sensed scene that correspond to features, objects and materials, doing this by applying one or more recording devices not in physical, intimate contact with the item(s) under surveillance.
• The remote sensing techniques involve amassing knowledge pertinent to the sensed scene (target) by utilizing electromagnetic radiation, force fields, or acoustic energy by employing cameras, microwave radiometers and scanners, lasers, radio frequency receivers, radar systems, sonar, thermal devices, seismographs, magnetometers, gravimeters, scintillometers and other sensing instruments.
• All these advanced instruments gather different types of data that can be interpreted to derive accurate, large-scale information about the Earth’s surface and atmosphere. Because these data and images are digital, they can easily be quantified and manipulated using computers.
• This makes remote sensing a uniquely versatile tool, since the same data can be analyzed in different ways for different applications. Some of the fields that use remote sensing are agriculture, forestry, geology, archaeology, oceanography, architecture, meteorology, etc.

Concept of Remote Sensing

• When electromagnetic radiation falls upon a surface, some of its energy is absorbed while some is transmitted through the surface and the rest is reflected.
• Surfaces also naturally emit radiation in the form of heat. Photographic films or digital sensors in the satellite or aerial vehicle record the reflected and emitted radiation. Since the intensity and wavelengths of this radiation depend on the nature of surface, each surface is described as possessing a characteristic spectral signature.
• Specific instruments and software’s are used to identify and distinguish between different spectral signatures which will be ultimately useful in mapping the extent of surfaces. Satellite remote sensing is widely used as a tool in many parts of the world for the management of the natural resources and global activities. The remote sensing is divided into two major categories – satellite remote sensing and aerial photography.
• The satellite may be geostationary (which permits continuous sensing of a portion of the Earth) or sun-synchronous with polar orbit (which covers entire Earth at the same equator crossing time. The LANDSAT series satellites have a repeat period ranging from 16-18 days, whereas in IRS satellite, it is 22 days. Sensor is a device used for making observations and uses satellite as platform and observes large areas of the Earth surface.

Components of Remote Sensing

• Remote sensing technology involves the acquisition of information about the Earth’s surface without direct physical contact. This process depends on several interrelated components that work together as a system. The major components of a remote sensing system are described below.

Energy Source or Illumination:

• The energy source provides the electromagnetic radiation (EMR) required to sense objects on the Earth’s surface. Based on the origin of energy, remote sensing systems are classified into passive and active systems.

i) Passive System:

Passive remote sensing systems rely on natural sources of energy, primarily the Sun.

• The Sun emits electromagnetic radiation

• This radiation interacts with Earth’s surface features

• The reflected or emitted energy is detected by sensors

In some cases, the Earth itself acts as a source of energy by emitting radiation, especially in the thermal infrared region.

Examples:

• Optical satellite imagery

• Thermal sensors measuring land surface temperature

ii) Active System:

Active remote sensing systems use artificially generated energy sources.

• The system emits its own electromagnetic signal

• The signal interacts with Earth’s surface

• The reflected energy is recorded by the sensor

Examples:

• Radar

• LiDAR

Active systems are particularly useful under poor illumination conditions, such as at night or during cloudy weather.

Platforms:

Platforms are the vehicles that carry sensors and enable them to observe the Earth’s surface from different heights and perspectives.

Types of Platforms:

• Ground-based platforms: Towers, tripods

• Airborne platforms: Aircraft, helicopters, drones

• Spaceborne platforms: Space shuttles and satellites

The choice of platform depends on factors such as spatial coverage, resolution, cost, and application.

Sensors:

Sensors are devices that detect and measure electromagnetic radiation reflected or emitted from Earth’s surface features.

Examples of Sensors:

• Cameras

• Scanners

• Radiometers

• Radar sensors

Sensors are designed to operate in specific wavelength regions such as visible, infrared, or microwave bands.

Detectors:

Detectors are components of sensors that convert incoming electromagnetic energy into a measurable signal.

• In traditional systems, detectors produce photographic records

• In modern systems, detectors produce digital signals

Detectors play a critical role in determining the sensitivity and accuracy of remote sensing data.

Data Processing:

Data processing involves converting raw sensor data into usable information.

Processing includes:

• Signal correction

• Image enhancement

• Data calibration

• Digital image processing

Processing may be done using photographic methods or digital techniques, depending on the nature of the data.

Institutionalization:

Institutionalization refers to the organizations responsible for the planning, execution, data processing, analysis, and dissemination of remote sensing data.

Examples:

• International organizations

• National space agencies

• Research institutions

• Universities and academic centers

These institutions ensure the effective use of remote sensing technology for scientific, economic, and societal applications.