What is Remote Sensing? How it works?

Generally to learn about our environment we use our senses of seeing, touching, smelling and hearing. These senses help to sense an object from close proximity. In the sameway if the resources of the earth have to be surveyed or sensed for small areas, ground survey was much in use. Ground survey of resources can cover only small areas at a time. It takes many months to make a comprehensive surveyof all resources in a district. So ground survey was difficult to cover large areas simultaneously.

In order to cover larger areas, surveys fromelevationwas undertaken. The first attempt was made by attaching a camera to a balloon which flew over Paris in 1858. After the first world war, aeroplanes carried cameras to take pictures of the ground below. Systematic air photographs were taken from low flying aircraft so as to eliminate the effect of clouds. Thus air photographs are taken for relief, drainage land use pattern of the earth and were used to prepare maps showing relief, drainage, vegetation etc.

Now-a-dayswith the improvement of science and technology, especially after the development of remote sensing technology,we are able to collect data on any part of the earth within short span of time without visiting the places personally. Without trodding into the thick equatorial forest of Congo basin region, wading through snow bound Antarctica, and footing on sand filled Thar desert, we are able to collect even minute details about these regions without any physical contacts.

What is Remote Sensing?

“Remote sensingis a technique which provides up-to-date information about an area such as relief features, climate, vegetation, mineral, marine resources etc. with the help of “Sensors”, without having any direct physical contact with the objects of the area.” Remote sensing is simply the observation of an object from a distance.

Sensor is a device used in the Remote sensing, which senses the objects without having direct physical contact just like operating a Television or an Air Conditioner with the help of a remote control unit. Prior to remote sensing technology, various forms of aerial photography have been used to create maps of the earth’s surface since the eighteenth century.

Satellite remote sensing can be traced back to themid-1940s and the launch of various rockets from White Sands in New Mexico and which contained cameras on board. The first satellite sensor dedicated to the multi-spectral remote sensing of earth’s surface was launched in 1972. Initially it was as Earth Resources TechnologySatellites (ERTS). This series was renamed ‘Landsat” in 1975 and is still in operation today.

Process involved in Remote Sensing

The picture given here explains the process involved in remote sensing. Let us see the process step by step.

Energy source (A) – The first requirement for remote sensing is to have an energy source, which provides electromagnetic energy to the object of our study. The sun provides electromagnetic energy in the form of light waves. This energy consists of electromagnetic waves, which travel in a sinusoidal motion. All waves travel in the sames peed, but the wavelengths may vary. This resulting range of wavelengths gives rise to the electromagnetic spectrum. Wavelengths are measured in micro metres. (1micrometre = 10-6metres.)

The basic principle underlying remote sensing is the measurements of electromagnetic energy reflected or emitted by various features on the earth’s surface as well as in the atmosphere. At one end of the spectrum are radiowaves and microwaves that have very long wave lengths. At the shorter end of the spectrumare x-rays and gamma rays and in the middle is the narrow range of optical wave lengths visible to human eyes.Various sections of the spectrum are of use in remote sensing. These are primarily atmospheric windows – bands of wave lengths where the radiation passes easily through the atmosphere to the surface of the earth. The most commonly used are visible and infrared wavelengths.

• Visible portion, consisting of blue, green and red : 0.4 µmto 0.7 µm.
• Infrared portion, consisting of near, short-wave and thermal : 0.7 µm to 14µm.

A satellite sensing system will be designed to image a certain portion of the spectrum, i.e. to record the amount of radiation reflected or emitted at various wave lengths. In order to collect a sufficient amount of energy to provide a reliable measurement, satellite sensors will record the reflectance over an interval of wavelengths. These are referred to as channels or wavebands. The Advanced VeryHighResolutionRadiometer (AVHRR) for example images at one visible channel, one near infrared channel, two short-wave infrared channels and two thermal infrared channels. It is therefore a multi spectral sensor, as it images over more than one waveband of the electromagnetic spectrum. So the sources of Electromagnetic radiation (EMR) are

• Natural source
• Man made source

Thematerial sources of EMR is the sun and the emitted and reflected energy from the earth’s Surface. The artificial source of EMR is Radar Systems, which operates in the microwaves band of the electromagnetic spectrum.

Sun rays andAtmosphere (B) – The sun rays passing through the atmosphere interact with it and get into contact with the earth’s object. These rays reflect back to the sensor, through the atmosphere again, because Electromagnetic energy passing through the atmosphere is distorted or scattered.

Sunrays and object on the earth (C) – After getting into contact with the object on the earth, the rays interact with the object. The interaction depends upon the properties of the objects or features such as tone, size, shape, texture form and color. So the amount and characteristics of radiations emitted or reflected fromthe earth’s surface is dependent upon the characteristics of the objects on the earth surface.

Recording of the energy by the sensor (D) – The reflected, emitted and scattered radiations carry information about the objects and areas on the earth surface and it varies from object to object. The reflected and emitted long wave radiations recorded by the sensors are called special signatures.

The reflected sun rays from the object are recorded by the sensor. They carry information about the objects and their properties which vary from object to object according to their physical properties. For example the information about a mountain may differ from that of an ocean.

Satellite sensors record the intensity of electromagnetic radiation (sunlight) reflected fromthe earth at different wavelengths. Energy that is not reflected by an object is absorbed. Each object has its own unique  spectrum. Remote sensing relies on the fact that particular features of the landscape such as bush, crop, salt-affected land and water reflect light differently in different wavelengths. Grass looks green, because it reflects green light and absorbs other visible wavelengths. This canbe seen as a peak in the green band in the reflectance spectrum for green grass above.  The spectrum also shows that grass reflects even more strongly in the infrared part of the spectrum. While this cannot be detected by the human eye, it can be detected by an infrared sensor.

Instruments mounted on satellites detect and record the energy that has been reflected. The detectors are sensitive to particular ranges of wavelengths, called bands. The satellite systems are characterised by the bands at which they measure the reflected energy. The Land sat satellite, which provides the data used in this project, has bands at the blue, green and red wavelengths in the visible part of the spectrum and at three bands in the near mid infrared part of the spectrum and one band in the thermal infrared part of the spectrum. The satellite detectors measure the intensity of the reflected energy and record it as a number between 0 and 255.

Transmission,Reception and processing (E) - The recorded information by sensor are transmitted to processing station where the data is processed into image. The term image is used for any pictorial representation of data.Two types of images are photographs and digital image.

Interpretation andAnalysis (F) – These imageries are interpreted by the ground stations in visual, digital and electronic forms. Here, Data collection is in the form of Satellite imageries, fromwhich the details regarding objects can be obtained based on their spectoral reflectance properties and variations in the photo elements like tone, texture, shape, size, pattern etc.

Processing of the data – The processing of the data is carried out using instruments like photographic enlarger. Classification of data, otherwise called as grouping of data is based on their spectoral response once the grouping is done. There is a need to determine the accuracyof the classification. The remote sensing data has to be supplemented by ground truth.

Application (G) – The extracted information by the ground station are used for better understanding of the objects on the earth’s surface. It may reveal new information and solutions to the problem of particular area.

Mapping of the processed information – The grouped or classified data has to be transferred on to a plain sheet for effective communication. With the help of computers, cartographers, the map makers draw maps. Computer aided design programmes (CAD) are of much use in mapping.

Advantages of Remote Sensing

• Remote sensing detect features which are not visible to the human eye,  such as a dense forest, Antarctic region and in accessible areas.
• It provides up to date and continuous information about an area, such as  the changing pattern of wealth, land use etc.
• It helps the planners for formulating polices and programmes to acheive the holistic functioning of environment, because of its speedy, accurate and up-to-date information.
• It caters the information needed by the agriculturists to identify the areas affected by pests, crop disease,water logging,wasteland etc.
• It spots the areas of natural disasters such as Tsunami, drought prone, flood affected and cyclone hit areas. It is highly useful for detecting damage, estimating the loss, for providing relief, rehabilitation and, helps in reconstruction.
• The most important utility of Remote sensing is into the science of cartography. It enables the cartographers to prepare thematic maps like geological maps, soil maps, population maps etc with greater accuracy and speed.