Remote sensing may be defined as an art and science of collecting information about change to normal objects, area or phenomena from a distance without physical contact with it. In fact, eyesight, smelling, and hearing are also some of the forms of remote sensing. However, in surveying we use the term remote sensing for collecting information about objects ion the earth from aircraft and satellite stations using electromagnetic energy. When electromagnetic energy is made to fall on the object, it is partly
3. transmitted and
Different objects have different properties of absorbing, scattering, transmitting and reflecting the electromagnetic energy. Hence, these properties can be used to identify objects. The reflected light is mixed with emissions from the earth.
In remote sensing from satellites the electromagnetic waves are sent to earth surface. Depending upon the property of objects on Earth, the electromagnetic waves of different intensity and wavelengths are absorbed, scattered transmitted and reflected. The reflected waves in the bandwidth of infrared, thermal infrared and microwaves are picked up by sensors mounted on the satellite. Since each feature on the earth has different reflection property, it is possible to identify the features on the earth from satellite. The user has to identify the object and determine its extent from the earth by studying satellite data. This is called processing of data. For quantifying the objects, computers are used.
In remote sensing from aircrafts, the reflected sunray is picked up by a camera and the features are hence studied.
Remote sensing observation platforms:
First remote sensing platform used was cameras on balloons. The aircraft was extensively used as platforms later on. Nowadays, satellites are being used as platforms. Since satellite platforms are convenient and economical in the long run, they have replaced all other platforms. There are two types of satellites used for remote sensing:
2. near earth
Geostationary satellites rotate around the earth at the same speed as that of earth. Hence they appear stationary when observed from earth. These satellites are at an altitude of about 3600km from surface of the earth above the point on equator. As they are geostationary, they can be used for remote sensing a particular area on the earth. For example, Indian satellites are useful for remote sensing areas in or around India.
Near earth satellites have speed different from that of earth. Hence they appear as if they are rotating about earth with some relative velocity. The path may close up on itself if the orbital parameters are suitably chosen. In such cases, the satellite revisits a given location at a regular interval of time. By adjusting the speed of these satellites, they can be set to revisit a given place at the desired time intervals.
The near earth satellites trace a path in a sun synchronous orbit defines by its fixed inclination angle from the earth's north south axis. These satellites orbit at a distance of few hundred to a few thousand kilometers. Hence in biological and environmental studies, data are collected at specific intervals. This is useful for visible and infra red observation. IRS studies have this kind of orbit.
Types of remote sensing:
Based on the source of electromagnetic energy used, remote sensing is classified into passive remote sensing and active remote sensing.
Passive remote sensing:
All stars and planets emit electromagnetic energy. However for the earth, the energy emitted by the sun is more predominant than others and is used for remote sensing. Remote sensing using the sun is called passive remote sensing.
Active remote sensing:
In active remote sensing, artificially produced electromagnetic waves are used. The microwaves are produced by a klystron or a travelling wave tube. Radio detection and ranging is a commonly used technique to transmit electromagnetic wave to remote targets. The reflected signal is received by the sensors. The time delay and the level of reflected signal provides information about the distance of the target and its surface reflectivity. This gives precise information about the target. Since the microwaves are not influenced by atmosphere, they may be transferred back to the earth stations for the further analysis and studies.
Advantages of remote sensing:
The major advantages of remote sensing over other methods of ground investigation are:
1. Accessibility: some areas may not be accessible for ground survey where as by remote- sensing, all regions on the earth can be accessed.
2. Time saving: Remote sensing can produce very reliable information about land use, natural hazards, etc in a very short time. This is not possible by land survey.
3. Multi disciplinary: Remote sensing is used by workers in different departments like civil engineers, geologists, forest and revenue department etc.
Hence, though the initial cost is more, overall benefit to cost ratio is higher and better.
Applications of remote sensing:
1. Resource exploration:
Geologists use remote sensing to study information about the formation of sedimentary rocks and identify deposits of various minerals, detect oil fields and identify under water resources. It is also used to identify potential fishing zone, coral reef mapping and to find other ocean wealth.
2. Environmental study:
Remote sensing can be used to study cloud patterns and to predict rains. Water discharge from various industries can be studied for their dispersion and harmful effects, if any on the living animals. Oil spillage and oil slicks in the ocean can be studied for their dispersion and for their possible harmful effects. Soil erosion and sediment transportation are also studied by remote sensing.
3. Land use:
By remote sensing mapping of a large area of land is possible in a short time. The changes taking place in the areas of the forest, agriculture, residential and industrial areas can be assessed regularly and monitored .It is eve possible to find out areas of different crops.
4. Site investigation:
Remote sensing is used extensively in site investigations for dams, reservoir, bridge, pipeline etc. It is also used in locating ground water supplies to towns and industries and for locating construction materials like sand and other gravel for the new projects.
5. Archaeological investigations:
Archaeological pattern of prehistoric land use may be recognized in remote sensing. Many structures in if old era are now buried underground and not known. But due to the change in moisture content and other characteristics of the buried objects and upper new layer, remote sensors are able to recognize the buried structures of archaeological importance.
6. Natural hazard study:
Using remote sensing the following hazards can be assessed and managed:
5. hurricanes and cyclones
The natural hazards can be predicted to some extent and their damage can be minimized.