Basics of Remote Sensing & Electromagnetic Radiation Concepts

 Introduction  What is Remote Sensing  Remote sensing Definitions  Energy source and Electromagnetic Radiation Principles  Energy Interaction with the Atmosphere  Energy Interaction with Earth surface feature  Energy Recorded – At the Sensor  An Ideal Remote Sensing System  Real Remote Sensing Systems Characteristics

 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.

 Remote sensing process involves the process of interaction between incident energy and the target of interest under observation.  It can be summarized as follows: ◦ Illumination or Energy Source ◦ Radiation and its interaction with the atmosphere ◦ Interaction of radiation with the target ◦ Recording of reflected energy by the sensor ◦ Transmission, Reception and Processing of recorded data ◦ Interpretation and Analysis of Data ◦ Application of the technology

1. Energy Source or Illumination: For Remote Sensing the very first requirement is to have an energy source illuminating the target object or a source providing electromagnetic energy on to the target of interest. SUN Source:marine.usgs. gov SONAR LiDAR Source: wy.nrcs.usda.gov RADAR Source: en.wikipedia.o rg

2. Radiation and the Atmosphere: During its travel through the atmosphere from its source to the target, it comes in contact with the gaseous molecules present in the atmosphere and interact with them. This phenomena of interaction may take place a second time as the electromagnetic energy travels back from the target to the sensor. Atmosphere

3. Interaction with the Target : Once the energy from Sun or any other source reaches to the target after travelling through the atmosphere, it interacts with the target.  This interaction depends on the properties of both the target and the radiation. Atmosphere Target

4. Recording of Energy by the Sensor: After interaction with the target the radiation energy gets scattered or reflected back from the target which in turn is recorded by the sensor which is far away from target.  This sensor can also record emitted electromagnetic energy from the target depending on its sensing capability. Atmosphere Sensor

5. Transmission, Reception, and Processing: The energy recorded by the sensor is transmitted in electronic form to the ground where data receiving/processing station do the preliminary processing required to convert the data into image.  This processed data can be a hard copy image or a digital image. Atmosphere

6. Interpretation and Analysis: The processed image is interpreted, visually and/or digitally or electronically, by the user community to extract information about target under observation. Atmosphere Analysis

7. Application: The information collected from interpreted and analyzed image is lastly put to use for better understanding of a situation spatially, reveal some new information or solving a particular problem or to make better informed decision. Atmosphere A pp li c a t i o n

 "Remote" means far away. Thus remote sensing means sensing or observing things and phenomena from a distance.  Of our five senses (i.e. hearing, sight, touch, smell, taste) we use three (i.e. hearing, sight & smell) as remote sensors when we: ◦ watch a football match (sense of sight) ◦ smell freshly baked cake (sense of smell) ◦ hear, train whistle from a distance (sense of hearing)

 In today's world of science, "remote sensing" means observing the Earth with electronically devised sensors from high above the Earth surface.  These sensors are like cameras except that they use not only visible light but also other bands of the electromagnetic spectrum beyond visible range such as infrared, radar and ultraviolet.

 As these sensors are on board so high up, that they can take images of a very large area in a single snap, sometimes even a whole province/state.

 Today, many remote sensing satellites are available, launched by different countries catering to different humanity needs.  India also have different satellites like Metrological, communication, etc. Thousand of images are acquired and sent every day from the satellites to receiving stations on Earth.  In continuation the Earth's entire surface is imaged every week or so.

 Earlier, the energy collected and measured in remote sensing has been only electromagnetic radiation, including visible light and invisible thermal infrared (heat) energy, which is reflected or emitted in varying degrees by all natural and artificial objects.

 The purview of remote sensing has been recently broadened to include sound energy, which is propagated in a media like water in sea.  The instruments used for this special technology are called as remote sensors and include photographic cameras, mechanical scanners, imaging radar systems, and LiDAR, etc.

 Sensors are designed to both collect and record specific types of energy that is reflected back towards them.  Remote sensing devices can be differentiated as active or passive based on the source of illumination.

Optical WavelengthsWavelength (mm) Portion Visible (Reflective) Near Infrared (Reflective) Middle Infrared (Reflective) Far Infrared (Thermal, Emissive )