17 Remote Sensing Technology

P. Jaisridhar

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1.        INTRODUCTION

 

1.1. What is Remote Sensing?

 

Remote sensing means sensing the objects that are far away from the reach of our senses. It is a tool to identify the characteristics of an object without having any direct contact over it.

 

In other words, it can be defined as, science of acquiring information about earth’s surface viz., land cover, oceans & coastal, agriculture, forestry, geographical mapping etc, through sensing, energy emission and processing, analysing and recording reflected images.

 

1.2. How does it work?

 

Remote sensing process involves interaction between radiation emitted from the object and the target of focus. This can be explained by the use of imaging system that follows seven elements. However, it should be noted at this point that remote sensing not only uses imaging system but also uses non-imaging sensors and sensing of emitted energy.

 

2. ELEMENTS OF REMOTE SENSING PROCESS

 

The seven elements involved in the remote sensing process are,

a)      Energy source

b)      Radiation and the Atmosphere

c)      Interaction with the target

d)     Recording the energy by sensors

e)      Transmission, Reception and Processing

f)       Interpretation and Analysis

g)      Application

 

a.    Energy Source: is the first and the foremost requirement in remote sensing. The main energy source here is the Sun which provides electromagnetic energy to the target of focus. Target we mean here is the earth’s surface.

 

b.   Radiation and the Atmosphere: is a twice occurring process in remote sensing. First time, this process occurs when the energy travels from its source to the target through atmosphere. Second time, this process occurs when the energy is travelled from the target to the sensors.

  1. Interaction with the target: always depends on its radiation emission when the energy travels from the source to the target through atmosphere.
  2. Recording of energy by the sensors: Tools that we use for sensing remote objects involves sensors which has the capability of collecting and recording the electromagnetic radiation emitted from the target.
  3. Transmission, Reception and Processing: Once the energy is recorded by the sensors, it is transmitted to the processing station in electronic form. The processing station then receives the data and transform them in a picture form.
  4. Interpretation and Analysis: Once the image is processed, it is converted into digital mode to analyze its illuminated parts and study the information contained in it.
  5. Application: is the final element in the remote sensing process. The success of the remote sensing process lies on its effective application towards any problem solving incidence.

3. TYPES OF REMOTE SENSING

 

Remote sensing technology had been popular since several decades and is still ruling. Its applications in various fields are highly useful for study and research purpose. Unlike other techniques, remote sensing involves additional practical works and digital image processing knowledge for handling and data interpretation. Remote sensing is one of the primary data sources in Geographic Information System analysis. There are two components in Remote Sensing viz., Platform and Sensors. The former is a vehicle that carries sensors. Eg Satellite, aircraft, balloons etc. While the latter is a device that receives electromagnetic radiation and converts it into a signal that can be recorded and displayed as either numerical data or an image.

 

Remote sensing is broadly classified into two types

 

a) Passive Remote Sensing – It means sensing the energy that are radiated from the terrain naturally.

b) Active Remote Sensing – A good example for an active remote sensing is a Radar. When the electromagnetic radiation is provided with our own energy source to illuminate the terrain it is called active remote sensing.

 

4. DATA COLLECTION IN REMOTE SENSING

 

There are two types of data collection in remote sensing.

 

a) Multistage Remote sensing data collection – It is further divided into three types viz., Satellite based remote sensing, Aerial surveying and Ground based remote sensing (GPRS) and Low Altitude Remote Sensing. Each stage contains its own advantage and disadvantages.

 

i. Satellite based remote sensing – helps in establishing a stable platform by obtaining more accurate data with less geometric error. Fixed spatial resolution and more time consumption are some of the disadvantage in this method.

 

ii.  Aerial Surveying – causes high geometric error and requires sophisticated geometric correction model. They can be used only for a specific purpose and involves high cost. Aerial surveying is differentiated from satellite based remote sensing by variable spatial resolution and non-constraint of time.

 

iii.  Ground based remote sensing (GPRS) – is mainly used for scientific experiment purpose (eg.) study of blind spots on earth, soil compositions, traces of water bodies etc.

  1. b) Stereo Pair Remote Sensing Data Collection – It is done when two images of the same area are acquired on different days and from different orbits, one taken from East and the other taken from West. To achieve this, there must be significant difference in the inclination angles. Stereo pair images uses photogrammetric technology to generate Digital Elevation Model (DEM) as shown below.

India 5. RESOLUTIONS IN REMOTE SENSING There are four types of resolutions in Remote Sensing

  1. a) Spatial Resolution – provides visible image by detecting even the smallest possible objects on earth.

30m x 30m Land Sat Image                             67cm x 67cm Bird’s eye view

  1. b) Spectral Resolution – discriminates wavelength interval in an electromagnetic Here, the low resolution yields wide bandwidth and high resolution yields small bandwidths.
  1. c) Temporal Resolution – is the time taken by a satellite sensors to complete one single rotation around the earth’s surface. Unlike geostationary satellite, which revolves in the same direction as earth spins, satellite sensors having temporal resolution will have sufficient time between each rotations. eg. Landsat ETM satellite which takes 16 days for its next visit to the same spot. RADARSAT1 takes 24 days between each rotation.
  1. d) Radiometric Resolution – captures images that emits bright light. This can be perceived based on the reflection of energy emitted from the object that is captured. Higher the resolution, better will be the reflection

6. PRINCIPLES OF REMOTE SENSING

 

Remote Sensing is a multi-disciplinary science that involves combination of many fields’ viz., photography, optics, spectroscopy, electronics and communication, satellites and observatory etc. Each field uses its own tool to capture and record the information through images or electromagnetic signals of different objects. Each object may not necessarily emit the same amount of energy. The energy emission differs from one object to another based on the property of the object being captured. Certain objects emits electromagnetic energy through atomic and molecular oscillation at temperature above absolute zero. Likewise, every single object of nature has a unique pattern of reflection distribution based on its shape, size, physical and chemical properties. These characteristics of object only helps them to be distinguished from one another using remote sensing technique.

 

In today’s world, the contribution of Remote sensing technology is incomparable. It has grown to such an extent that the modern tools used to sense remote objects providesdigital image used for marking natural resources and observing the activities of plant and animal species in the forest, land cover, water bodies etc. In future, there are possibilities of this technology to explore unbelievable objects on space and earth through next generation satellites that are capable of providing much high resolution images for multipurpose application.

 

6.1. How did it started?

 

Well, it’s time to board our time machine and explore when and where this technology started and who revolutionized it. In the journey of search, our first stop would be way back to the year 1839 which marked the beginning of the practice of photography. Later in 1859, Gaspard Tournachon used a parachute to capture first aerial photograph of a hamlet in Paris.

 

In 1873, the Theory of Electromagnetic Spectrum was proposed by James Clerk Maxwell. This was the first and the wide accepted theories in electromagnetic spectrum.

 

In 1909, images from Aircraft created a revolution showing much more objects on earth from the aerial point of view. These images popularized the term Bird’s eye view.

 

In 1935, the first Transmitter Antenna (now called RADAR) was developed with experiments by Heinrich Hertz. The possibilities of making one such object was suggested by James Clerk Maxwell’s seminal work of electromagnetism. The term RADAR was popular in the name of transmitter antenna before it was officially named by the United States Signal Corps in 1939. The first ever experiment of Radar was made in a small ship by a German inventor named Christian Hulsmeyer who used them in a small ship as detection to avoid collision due to fog. Since 1934-35 this invention has allowed eight nations viz., United Kingdom, Germany, USA, USSR, Japan, Netherlands, France and Italy to develop independent Radar in a great secrecy system type. These nations later shared their information to commonwealth countries like Australia, Canada, New Zealand and South Africa.

 

In 1959, the first space photograph of earth was captured by Explorer-6, an American Satellite. This satellite was designed to study radiations emitted from different astronomical objects, radioactive signals, cosmic rays from space etc. Using this satellite, a total of 827 hours of analog and 23 hours of digital data was obtained. This satellite’s orbit was decayed on July 1, 1961.

 

Other important markers in the History or Remote Sensing are tabulated below, Table 1: History of Remote Sensing

7. APPLICATIONS OF REMOTE SENSING:

 

Remote sensing technologies have been used in wide range of application. Each application has specific demands, spectral and spatial resolution. These application are captured through optical sensors and projected as a LANDSAT image or as a SPOT image. In a foggy environment, the targets won’t be visible. During this time the optical sensors cannot be used to view the targets. Example: Polar region (Artic & Antarctic). The experience inadequate solar illumination for months together. In such cases, Radar provides reliable data.

The sensors in Radar provides own illumination with long wavelength that can capture the objects even if it is hidden by heavy fog or extreme weather condition. The applications of remote sensing in this paper do not touch wide area of research but emphasis on objects associated with Earth surface. Some applications are explained in Table 2 as follows.

 

Table 2: Applications of Remote Sensing

These  application  are  actually  representative  and  not  comprehensive.  Let’s  take agriculture and forestry for example as a topics of discussion and see how remote sensing technology is applied in the field of agriculture and forestry.

 

7.1. Agriculture

 

We know that agriculture is the backbone for most of the countries around the worldfeeding millions of population every day. It plays a significant role in every nation. Foragriculture to be practiced in a cost effective manner in large scale farms, farmer needs to be informed efficiently about crop health, extent of infestation, stress damage, yield potential and soil condition in order to adopt viable strategy for farming operations. This is clearly possible only with the help of remote sensing tools like satellite and airborne images.

 

Remote sensing provides information to assess crop type, area covered by a particular crop and also the health of vegetation. Agriculture application of remote sensing involves multispectral sensors because once the crop health changes, the spectral reflection of a field will also change. It helps in classifying crop types for assessing what type of crop is grown in which area and when.

 

a)Crop type mapping: Provides information on the status of crop production, crop rotation, grain supplies, crop stress and damage due to natural calamities etc. Through Visible Infra-Red Technology, the chlorophyll content of plants can be assessed easily. Data captured through multi sensors provide more information than a Mono sensor. Radar is an excellent observatory tool that provides information on plant structure and moisture and can even penetrate through water vapor with its long wavelength and its active sensing capabilities.

 

b) Crop Monitoring & Damage Assessment: All crops do not grow evenly across the field. Some grow taller and some show stunted growth. Basically, crop growth is an important factor that determines yield. These growth differences may be due to lack of soil nutrients, infestation of weeds, diseases, pest, stress etc. Through remote sensing data, a farmer can be made aware to identify areas within the fields expressing deficiencies so that, he can be recommended to apply correct dosage of fertilizer or apply growth sprays, pesticides

 

Source: Natural Resource Canada   or  herbicides.  This  approach  not  only improves  productivity  but  also  reduces farm  input  cost  and  environmental  impacts.  Recent  advances  in  information  and communication technology allows farmers to observe images of their fields and make timely decision on managing the crops. The above image taken from the Canada Centre for Remote Sensing shows a field exhibiting healthy vegetation with thick patch of chlorophyll content. It is an excellent example to show the difference between irrigated land and non-irrigated land. Here, the irrigated land in the image appear to be bright green in real color simulated image whereas, darker area are dry with minimal vegetation.

 

7.2. Forestry

 

Forest play an important role in balancing Earth’s temperature and exchange of gases. They form a major source of connection between three spheres viz., atmosphere, geosphere and hydrosphere. Tropical rainforest which houses diversified species are more capable of adapting to changing environmental condition. They provides habitat for numerous animal and are important source of medicinal ingredients. Depletion of forest is the main issue concerning forest management. This happens mainly due to two factor viz., natural causes and manmade causes. Natural causes of forest depletion are due to natural fires and infestation of foreign particles. Manmade causes are due to clear-cutting, burning, land conversion and demand for commercial exploitation of forest trees for timber and wood. Commercial forestry is an important industry throughout the world where the basic principal is to crop the forest and re-harvest again. However, it is pretty sure that these companies are aware about the conservation of native species in the forest and sustainable forest practices. Only thing they need to follow is to be economical and efficient in extracting wood from the forest. Forestry application of remote sensing include reconnaissance mapping, commercial forestry and environmental monitoring.

 

a)Reconnaissance mapping: This technique is applied to monitor forest depletion, forest cover updating, crop cover update and measuring biophysical properties of forest stands.

b) Commercial forestry: Inventory and mapping application in commercial forestry is helpful in collecting detailed information about harvest, vegetation density, availability of timber, forest fire areas, regeneration assessment, invention of new species and forest etc.

c)  Environmental monitoring: Assessing forest cover, watershed areas, coastal areas, and forest health through remote sensing may provide information to the authorities concerned with the conservation of natural resources. Information like deforestation, species inventory, forest health and vigor would be a good example in this context.

  1. CONCLUSION:

Thus remote sensing forms a valuable tool in mapping and monitoring of biodiversity and provides valuable information to quantify spatial patterns, biophysical patterns, ecological process that determine species richness and anthropogenic factors causing loss and for predicating response of species to global changes. From balloon photography to advanced aerial photography remote sensing technology have grown and applied in vast areas. The characteristics of earth’s radiation and its chemical composition in different regions are very useful for identifying and characterizing earth and atmosphere features.

 

you can view video on Remote Sensing Technology

REFERENCES

 

1) Samarakoon Lal. Introduction to Remote Sensing. Article available at http://www.isprs.org/caravan/documents/Lao_Basic_RS.pdf

2) A Canada Centre for Remote Sensing Tutorial. Fundamentals of Remote Sensing.Natural Resources Canada. Article available at http://www.ldeo.columbia.edu/res/fac/rsvlab/fundamentals_e.pdf

3) KoKo Lwin. 2008. Fundamental of Remote Sensing and its application in GIS. Division of Spatial InformationSciences,UniversityofTsukuba.Articleavailable http://giswin.geo.tsukuba.ac.jp/sis/tutorial/koko/remotesensing/FundamentalRemoteS pdf

4) http://www.wamis.org/agm/pubs/agm8/Paper-2.pdf

5) https://www.scribd.com/document/306953868/Remote-Sensing-pdf

6) Sahoo, P. M. Use of Remote Sensing for Generation of Agricultural Statistics. Journal of Indian Society of AgriculturalStatistics.276-284.Articleavailableat http://www.iasri.res.in/ebook/TEFCPI_sampling/USE%20OF%20REMOTE%20SEN SING%20FOR%20GENERATION%20OF%20AGRICULTURAL%20STATISTICS .pdf

7) Field Techniques Manual: GIS, GPS and Remote Sensing. Article available online https://www.rgs.org/NR/rdonlyres/D5D046D8-5582-41D0-BEF8-D15A148C5D32/0/Chapter1GISGPSRemoteSensingandFieldwork.pdf