It is essential for us to understand the properties and characteristics of the elements of the earth around us. While some of these elements can be sensed directly by naked eye, some others are at far off distances and cannot be sensed, directly by us. For example, how could we get to assess the natural resources hidden beneath the soil, bottom of the sea and beyond the mountains? The increasing population and fast depleting of natural resources strengthen the significance of such knowledge.
The science developed by humankind is useful in the appraisal of resources and to finding better means of utilisation of resources of the earth. As local area data are not available to the extent demanded, we need the remotely sensed data. Hence, the collection of statistical information through remote sensing techniques becomes inevitable. In the past, the information about the resources were collected from the maps and the toposheets. Later airborne photographs and the satellite images have come into wide use in resources appraisal.
The term 'Remote Sensing' is widely used by the space scientists. The terms came into use only in 1960. Remote sensing can be defined as the act of observation of earth's features without having direct contact with the objects by sensors like the cameras to collect information and to interpret the information later.
Various platforms are being used for the collection of remote sensing data. The satellites fitted with sensors, cameras fitted in aircrafts, baloons and tall buildings are the most commonly used platforms in remote sensing activities. In general, remote sensing can be divided into two types: Aerial Remote Sensing and Satellite Remote Sensing.
Aerial remote sensing was carried out at first with balloons in 1858. With the discovery of aeroplanes in 1902, remote sensing was carried out through photographing from airplanes, right from 1909. Aerial photographs were very much in use during the First and Second World Wars. After the First World War, aerial photographs and their interpretations were much widely used for constructive purposes in forests appraisal and environmental management research.
Generally, the aerial photographs are made in order to analyse the prospects for developing resources and resolving problems in the context of a district or a river basin or an urban centre. When detailed information is needed, the aircraft flies at low altitude to get large scale aerial photographs. When special studies are conducted for town planning, crop planning and conservation, 1:20,000 or 1:10,000 scale aerial photographs are made.
To study the land uses, soil types, forests and water resources of a larger area like a district, the aircrafts fly at higher altitudes to take photographs to a scale of 1:50,000 or 1:63,360. Based on our requirements, black and white (panchromatic) photo films, colour films or any other special films can be used.
Based on the nature of remote sensing by satellites, it may be broadly classified into three types:
a) Geo - Stationary satellite
b) Sun - Synchronous satellite
c) Spy - Satellite
Geo Stationary-Satellite: Geo stationary satellites are launched to collect information about a larger area of the earth surface at a regular time interval. These satellites are placed in space at an average altitude of 36,000 km above the equator. These satellites advance in the same direction and speed as the rotation of the earth. Hence, these satellites observe the same portion of the earth at all times. For example, the INSAT satellite which is placed above the Indian subcontinent collects data and monitors the weather conditions of India. We see such imageries as those received from INSAT while the Doordarshan (Indian Television) telecasts the news bulletins. The announcement of weather forecasting is based on the information collected from such imageries regarding land, water and cloud cover and also information collected from the meteorological stations. Since these satellite imageries show large portions of the earth's surface, minor elements of the earth cannot be seen.
Sun - Synchronous Satellite: These satellites are placed at an altitude of about 600 to 900 km in space. They move North South above the earth and collect information for use on the ground. These satellites take just about 50 minutes to traverse from north pole to south pole within. Since these satellites collect information with the help of sunlight and pass the equator at a specific day time, they are called sun synchronous satellites.
It was the USA that launched the first satellite in 1972. It was a sun synchronous satellite and was later known as LANDSAT. Later 6 satellites were launched, in a series and in collaboration with the European countries. The French Government launched SPOT 1 satellite in the year 1986. Later, SPOT 2 and SPOT 3 were also launched. As a parallel venture, the Indian Government launched IRS 1A in 1988 and later IRS 1B. Likewise, other satellites such as ERS (European Remote Sensing Satellite) and JRS (Japanese Remote Sensing Satellite) series currently revolve round the earth. Images from such satellites can give accurate information just as the topographic maps. These imageries give important information on land uses, water resources, roads and settlements.
Spy Satellites: To collect secret information such as the movement of army troops and locations of atomic power stations, the spy satellites are put to use, with sophisticated gadgetry. Manufacturing of these satellites are expensive and complex. Besides, these satellites operate only for a limited time period. Also, only a few countries in the world have such satellites. Israel is said to be one of those countries specialising in this remote sensing.
1. Sun as a Source of Energy: Sun is the prime source of energy to the world. It is the resource for all activities of the life forms.
2. Emission of Sun's Energy : Solar energy is emitted into the space. The emission is in the form of various electromagnetic waves. It consists of gamma rays to radio waves (short wave length). This band of rays is called the 'electromagnetic radiation' (EMR).
3. Interaction of Solar Energy with Atmospheric Elements: When the solar energy passes through the atmosphere, many elements await to meet the energy. A portion of the 'electromagnetic radiation' is absorbed by carbon di oxide, ozone, moisture and dust and reflected back. So, the balance of electromagnetic radiation reaches the earth's surface as sunlight.
4. Interaction of Sunlight with Terrestrial Features: Electromagnetic waves in sunlight have different wave lengths. A number of bands can be identified based on the wave lengths. These bands of radiation fall on the objects of the earth and get reflected, differently by different objects. The reflectance varies according to the wave length. Through such reflectance, various wave lengths help in remote sensing to identify various elements over the earth. Thus, the spectral reflectances from the earth, in fact earth objects, are of many thousand types.
5. Terrestrial Radiation of the Earth's Element: The solar energy, along with the energy already stored in the elements on the earth, are radiated back into the atmoshpere. Any object, with a temperature of about 0o K (273o C) will emit energy. Thus, all objects over the earth have temperatures above 0o K and therefore emit energy at varying levels.
6. Collection of Information/Data: The energy thus reflected and emitted by the earth's features are recorded by cameras and sensors fitted onto the various platforms. The cameras record the energy in films and the sensors convert the energy into electrical signals and send them to the earth's receiving stations.
7. Data Acquisition by the Earth Station: The electrical pulses from the remote sensors are converted into 'digital' numbers. Each point or picture element gets different (pixel) digital numbers. Thus, a satellite image is composed different digital values or pixels.
8. Remotely Sensed Data Supply: Satellite images and the aerial photographs can be obtained from three organisations or places in India as indicated below:
National Remote Sensing Agency, Hyderabad
Indian Air Force, New Delhi
Air Survey Company, Calcutta.
Remote sensing has been and is being used very widely, to obtain information accurately, with speed and ease, about the vast stretch of land features and water bodies of the earth. The important fields in which remotely-sensed data are used are given below for an appreciation of the technique of remote sensing.
1. Geology. To identify rock types, earth lineaments (fault lines) and landslides, remote sensing is being used.
2. Meteorological Research. To study various components of meteorology such as cloud cover, intensities and variations in weather elements, global climate change and so on.
3. Oceanography. To study various elements of oceanography such as surface temperatures, ocean currents, sea erosion, wave patterns and marine resources.
4. Water and Land Resources. Remote sensing is highly useful in identifying water bodies such as lakes, ponds and rivers, their aerial extent and quality, snowmelt, runoff, surface flow, irrigation, land suitability, soil type and moisture capacity of soil.
5. Land Use, Agriculture and Forestry. Remote sensing is useful also in the field of urban and regional land use studies. Urban development, crop identification and estimation, crop diseases detection, forest cover mapping and deforestation are other uses of remote sensing.
6. Hazard Control. To measure the natural and man made hazards like storms, landslides, floods and pollution and to control such hazards.
7. Land Survey and Cartography. Aerial photagraphs are widely used for updating the existing maps and to measure heights. Both aerial and satellite data products are used to renew the existing transportation routes and planning.