What is Remote sensing ?
Remote sensing refers to recording/observing/understanding (sensing) the movements of objects or events at distant (remote) locations. In remote sensing, the sensor is not in direct contact with objects or events. Information needs a physical carrier to travel from the objects/events to the sensor through an interference.
Electromagnetic radiation is commonly used as an information carrier in remote sensing. The output of a remote sensing system is typically viewed as an image representing the scene. Image analysis and interpretation are essential for extracting useful information from an image. The human visual system is an example of a remote sensing system in this general sense.
Types of remote sensing
• Satellite remote sensing
These remote sensing satellites are equipped with many sensors, they face the earth. They appear to be constantly moving around the Earth in predictable orbits.
In satellite remote sensing of the Earth, sensors observe from a layer of the atmosphere separating the sensors from the Earth's surface. Therefore, it is essential to understand the effect of the atmosphere on the electromagnetic radiation traveling from Earth to the sensor through the atmosphere. Atmospheric components cause wavelength dependent absorption and scattering of radiation. These effects reduce the quality of the images. Some atmospheric effects can be corrected before the images are subjected to further analysis and interpretation.
• Optical and Infrared Remote Sensing
In optical remote sensing, optical sensors detect solar radiation reflected and scattered from Earth, and create images like the ones above from a camera in space. Visible and infrared light is reflected in different ways in different materials such as water, soil, vegetation, buildings and roads.
They have different colors and brightness when viewed under the sun. Interpretation of optical images requires knowledge of the spectral reflection signatures of the various materials (natural or man-made) covering the Earth's surface.
There are also infrared sensors that measure the thermal infrared radiation emitted from the Earth, from which the surface temperature of the land or sea is obtained.
• Microwave remote sensing
There are some remote sensing satellites that carry passive or active microwave sensors. Active sensors emit pulses of microwave radiation to illuminate the areas to be imaged.
Images of the Earth's surface are created by measuring microwave energy scattered from land or sea to the sensor.
These satellites have their own "flashlights" emitting microwaves to illuminate their targets. Thus images can be acquired day and night. Microwaves have an added advantage because they can penetrate clouds. Images can be acquired even when there are clouds covering the Earth's surface.
Brief History of Remote Sensing
Balloonist G. Tournachon (aka Nadar) photographed Paris with his balloon in 1858. Messenger pigeons, kites, rockets and unmanned balloons were also used for the early images.
Systematic aerial photography was developed for military surveillance and reconnaissance purposes occurring in World War I and with the use of modified aircraft such as the P-51, P-38, RB-66 and F-4C until mountaineering during the Cold War. was reached. Or specially designed platforms such as the U2/TR-1, SR-71, A-5 and OV-1 series were designed in both overhead and stand-off collection. Later imaging technologies included infrared, conventional, Doppler and synthetic aperture radar.
The development of artificial satellites in the late 20th century allowed remote sensing to progress on a global scale until the end of the Cold War.
Infiltration on various Earth observation and weather satellites such as Landsat, Nimbus and recent missions such as Radarat and UARS provided global measurements of a variety of data for civilian, research and military purposes. Space probes to other planets have also provided the opportunity to study remote sensing in outer space, the synthetic aperture radar on the Magellan spacecraft provided detailed topographic maps of Venus and the instruments on SOHO studied the Sun and the solar wind. allowed to do.
Recent developments include the development of image processing of satellite imagery in the 1960s and 1970s. Several research groups in Silicon Valley, including NASA Ames Research Center, GTE and ESL Inc., have developed Fourier transform techniques leading to the first significant enhancement of imagery data. The first commercial satellite (Ikonos) was launched in 1999.
Limitations of Remote Sensing
• Remote sensing has not completely replaced ground based survey methods, mainly due to its limitations, which are as follows:
• Cost of data collection and data procurement.
• Problems with data analysis and interpretation.
• Problems with all-weather capability because some sensors cannot see through the cloud.
Remote Sensing Applications
• Cameras on satellites and aircraft take images of large areas on the Earth's surface, allowing us to see much more than we can see standing on the ground.
• Sonar systems on ships can be used to view images of the ocean floor without having to travel under the sea.
• Cameras on satellites can be used to make images of temperature changes in the oceans.
• Large wildfires can be mapped from space, allowing rangers to see a much larger area from the ground.
• Track clouds to help track or forecast the weather, and help spot dust storms.
• Track the growth of a city and changes in agricultural land or forests over many years or even decades.
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