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Basis of Remote Sensing Image Representation. Remote sensing images are representations of part of the earth’s surface as seen from space. The images may be analog or digital. The overall shape of a satellite image can be either square or rectangular. However, The pixel of the satellite image is always square. In the modern era, all satellite images represent the square area of the ground. Once an image has been acquired by a satellite or a sensor, then the spatial resolution of that image cannot be changed. Pixel is a unit of an image and therefore indivisible. It has an intensity value and a location address in the two-dimensional image. The intensity of a pixel is digitized and recorded as a digital number. The address of a pixel is denoted by its row and column coordinates in the two-dimensional image. Multilayer images are formed by stacking from the same area together. it can also be formed by combining images obtained from different sensors and other subsidiary data. Remote Sensing Platforms. The various Remote Sensing platforms are: Satellites, Airborne, Space shuttles, Ground-based. The ground-based platforms can be: Mobile hydraulic platforms | Portable masts | Towers | Weather surveillance radar. Airborne platforms can be balloon-based or drone-based remote sensing platforms. The types of satellite orbits are Geostationary orbit and polar orbit (Sun-synchronous orbit). Multi-Spectral Scanners and Devices Remote sensing can be divided into two broad categories: Active and passive remote sensing. Active sensors have their own energy source for illumination. They emit pulse or signals towards the earth target that they are focusing on. The reflection radiated from the target is detected and measured by the sensor. They have the ability to obtain measurements anytime, regardless of the time of the day or season. In Passive Remote Sensing, energy does not have to be sent towards the earth. The source of energy is the sun. The passive remote sensors record the emission or reflection of the sun's energy coming from the surface of the earth. The source of energy can also be cosmic background or rain emission.
Satellite remote sensors acquire data using scanning systems, which employ a sensor with a narrow field of view, that sweeps over the terrain to produce an image. The two main modes of scanning to acquire multispectral image data are: Across-track scanning (whiskbroom), Along-track (Pushbroom) scanning. Characteristics of Landsat, SPOT, and Sentinel Sensors. The first Landsat mission started in 1972. Several Landsat missions have been launched since then. The latest Landsat is Landsat 8. Landsat 8 was launched on February 11, 2013. SPOT is an acronym for Systeme Pour I’Observation de la Terre. SPOT 1 was launched on February 22, 1986. SPOT 7, which was launched on June 30, 2014, is the latest of the SPOT series. The European Space Agency is currently developing seven missions under the Sentinel programme. Each Sentinel mission is based on a constellation of two satellites to fulfill and revisit the coverage requirements for each mission.