Introduction

     The science of remote sensing has emerged as one of the most fascinating subjects over the past three decades. Earth observation from space through various remote sensing instruments has provided a vantage means of monitoring land surface dynamics, natural resources management, and the overall state of the environment itself. (Joseph, 2005)

     Remote sensing is defined, for our purposes, as the measurement of object properties on the earth’s surface using data acquired from aircraft and satellites. It is therefore an attempt to measure something at a distance, rather than in situ. While remote-sensing data can consist of discrete, point measurement or a profile along a flight path, we are most interested here in measurements over a two dimensional spatial grid, i.e. images. Remote sensing systems, particularly those deployed on satellites, provide a repetitive and consistent view of the earth that is invaluable to monitoring the earth system and the effect of human activities on the earth. (Schowengerdt, 2006)

Definition of Remote Sensing 

     Remote means away from or at a distance, sensing means detecting a property or characteristics. Thus the term Remote Sensing refers examination, measurement and analysis of an object without being in contact with it.        

     Remote Sensing is the science and art of acquiring information about the earth surface without actually being in contact with it. This is done by sensing and recording reflected or emitted energy and processing, analyzing and applying that information.

     There are many possible definitions about what Remote Sensing actually is. One of the most acceptable definitions of Remote Sensing is “Remote Sensing is broadly defined as collecting and interpretation information about a target without being in physical contact with the object”. Air crafts and satellites are the common platforms for remote sensing observation.

     According to United Nation (as part of the general assembly resolutions A/RES/41/65, 95th Plenary meeting, 3 December, 1986), “The term Remote Sensing means the sensing of the Earths surface from space by making use of the properties of electromagnetic wave emitted, reflected or diffracted by the sensed objects, for the purpose of improving natural resource management, land use and the protection of the environment.”  

“Remote Sensing is the acquiring of data about an object without touching it.” Such a definition is short, simple, general and memorable.

     According to CanadaCenter for Remote Sensing, “Remote sensing is group of techniques for collecting image or other forms of data about an object from measurements made at a distance from the object and the processing and analysis of the data.”

Components of Remote Sensing

     In much of remote sensing, the process involves an interaction between incident radiation and the targets of interest. This is exemplified by the use of imaging systems where the following seven elements are involved:

1. Energy Source or Illumination (A) - The first requirement for remote sensing is to have an energy source which illuminates or provides electromagnetic energy to the target of interest.

2. Radiation and the Atmosphere (B)– as the energy travels from its source to the Target, it will come in contact with and interact with the atmosphere it passes through. This interaction may take place a second time as the energy travels from the target to the sensor.

3. Interaction with the Target (C)- once the energy makes its way to the target through the atmosphere, it interacts with the target depending on the properties of both the target and the radiation

4. Recording of Energy by the Sensor (D) - after the energy has been scattered by, or

Emitted from the target, we require a sensor (remote - not in contact with the target) to collect and record the electromagnetic radiation.

5. Transmission, Reception, and Processing (E) - the energy recorded by the sensor has

to be transmitted, often in electronic form, to a receiving and processing station where the data are processed into an image (hardcopy and/or digital).

6. Interpretation and Analysis (F) - the processed image is interpreted, visually and/or

Digitally or electronically, to extract information about the target which was illuminated.

7. Application (G) - the final element of the remote sensing process is achieved when we apply the information we have been able to extract from the imagery about the target in order to better understand it, reveal some new information, or assist in solving a particular problem.  

Principles of Remote Sensing

     Remote sensing has been defined in many ways. It can be thought of as including traditional aerial photography, geophysical measurements such as surveys of the earth’s gravity and magnetic fields and even seismic sonar surveys. However, in a modern context, the term remote sensing usually implies digital measurements of electromagnetic energy often for wavelengths that are not visible to the human eye. The basic principles of Remote Sensing may be listed as below:

  1. Electromagnetic energy has been classified by wavelength and arranged to form the electromagnetic spectrum.
  2. As electromagnetic energy interacts with the atmosphere and the surface of the Earth, the most important concept to remember is the conservation of energy (i.e., the total energy is constant).
  3. As electromagnetic waves travel, they encounter objects (discontinuities in velocity) that reflect some energy like a mirror and transmit some energy after changing the travel path.
  4. The distance (d) an electromagnetic wave travels in a certain time (t) depends on the velocity of the material (v) through which the wave is traveling; d = vt.
  5. The velocity (c), frequency (f), and wavelength (l) of an electromagnetic wave are related by the equation: c = fl.
  6. The analogy of a rock dropped into a pond can be drawn as an example to define wave front.
  7. It is quite appropriate to look at the amplitude of an electromagnetic wave and think of it as a measure of the energy in that wave.
  8. Electromagnetic waves lose energy (amplitude) as they travel because of several phenomena.

Remote Sensing System

     With the general background treatise on remote sensing we have made so far, it would now be easier to make an analysis of the different stages in remote sensing. They are

  1. Origin of electromagnetic energy (sun, transmitter carried by the sensor).
  2. Transmission of energy from the source to the surface of the earth and its interaction with the intervening atmosphere.
  3. Interaction of energy with the earth’s surface (reflection/absorption/transmission) or self-emission.
  4. Transmission of the reflected/emitted energy to the remote sensor placed on a suitable platform, through the intervening atmosphere.
  5. Detection of the energy by the sensor, converting it into photographic image or electrical output.
  6. Transmission/recording of the sensor output.
  7. Pre-processing of the data and generation of the data products.
  8.  Collection of ground truth and other collateral information.
  9. Data analysis and interpretation.
  10. Integration of interpreted images with other data towards deriving management strategies for various themes or other applications.

Applications of Remote Sensing

Some of the important applications of Remote Sensing technology are: 

  1. Environmental assessment and monitoring (urban growth, hazardous waste).
  2. Global change detection and monitoring (atmospheric ozone depletion, deforestation, global warming).
  3. Agriculture (crop condition, yield prediction, soil erosion).
  4. Nonrenewable resource exploration (minerals, oil, natural gas).
  5. Renewable natural resources (wetlands, soils, forests, oceans).
  6. Meteorology (atmosphere dynamics, weather prediction).
  7. Mapping (topography, land use. Civil engineering).
  8. Military surveillance and reconnaissance (strategic policy, tactical assessment).
  9. News media (illustrations, analysis).

     To meet the needs of different data users, there are many remote sensing systems, offering a wide range of spatial, spectral and temporal parameters. Some users may require frequent, repetitive coverage with relatively low spatial resolution (meteorology).

      Others may desire the highest possible spatial resolution with repeat coverage only infrequently (mapping); while some users need both high spatial resolution and frequent coverage, plus rapid image delivery (military surveillance).Remote sensing data can be used to initialize and validate large computer models, such as Global Climate Models (GCMs), that attempt to simulate and predict the earths environment. 

The basic advantages of remote sensing are listed below:

  1. Relatively cheap and rapid method of acquiring up-to-date information over a large geographical area.
  2. It is the only practical way to obtain data from inaccessible regions, e.g. Antarctica, Amazonia.
  3. At small scales, regional phenomena which are invisible from the ground are clearly visible (e.g., beyond the mens visibility); for example, faults and other geological structures.
  4. Cheap and rapid method of constructing base maps in the absence of detailed land surveys.
  5. Easy to manipulate with the computer and combine with other geographic coverage’s in the GIS.             

     Remote Sensing is the gathering of information concerning the earth’s surface that do not involve contact with the surface or object under study. The techniques include- aerial photograph, multi-spectral and infrared imagery and radar. With the help of remote sensing, we can able to get accurate information  about the earth’s surface including its components like- forest, landscape, water resources, oceans etc. This information helps the researchers to their research activities about the earth’s components concerning its sustainable management and conservation and so on.

     In order for a sensor to collect and record energy reflected or emitted from a target or surface, it must reside on a stable platformremoved from the target or surface being observed. Platforms for remote sensors may be situated on the ground, on an aircraft or balloon (or some other platform within the Earth's atmosphere), or on a spacecraft or satellite outside of the Earth's atmosphere. Ground-based sensors are often used to record detailed information about the surface which is compared with information collected from aircraft or satellite sensors. In some cases, this can be used to better characterize the target which is being imaged by these other sensors, making it possible to better understand the information in the imagery.