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Types of satellite: Principle, Scope and Application
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- Broadly satellite systems can be classified into 3 types based on their distinct design and application.
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Earth Observation And Remote Sensing
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- Earth Observation refers to any activity of acquiring information about the Earth’s surface, sea and atmosphere including temperature, density, chemical composition, humidity, wind speed and direction etc.
- The technology used to acquire this information is called remote sensing.
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Principle Behind Remote Sensing
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- Remote sensing is the process collecting information about an object, without coming in contact with it, from a distance.
- Remote sensing technology uses sensors that can sense reflected or emitted light from an object thereby acquiring its characteristics.
- Illustration 1: Our ability to see
- All the seeing we do in our daily lives is due to remote sensing.
- When light falls on an object it is reflected by the object which is what falls on your eyes. You have sensors in eyes called rods and cones that collect information from the reflected light which enables you to see the object. (its size, shape and colour)
- Note: Rods sense the intensity of light (how bright or dim), cones sense colour of the object.
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Illustration 2: Camera
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- Like your eyes, cameras have sensors that sense size, shape and colour of the object. The sensors in modern day cameras are called CCDs (Charge Coupled Devices)
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Remote sensing: ‘Seeing’ Visible And Invisible Light
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- Further the information that can be acquired from remote sensing depends on which light the sensor is sensing.
- Sensors sensing visible light tells you about size, shape and colour. (Eg: Eye, Camera)
- Sensors able to sense thermal infrared from an object tells you the temperature of the object. Eg: COVID cameras
- Further the light from the object of interest can be
- Reflected light from the object
- Emitted light from the object: Every object that has temperature emits light. (electro-magnetic radiation). Higher the temperature more energetic waves they emit.
- A refined version of the last statement is ‘The higher an object’s temperature, the more intensely the object emits electromagnetic radiation and the shorter the wavelength at which it emits most strongly. ‘
- Eg: Sun emits both visible and invisible light like UV, radio, gamma etc. But most of the light emitted from sun (44%) is visible light.
- Earth on the other hand emits most of its light in the thermal infrared (far-end of infrared).
- Visible light from sun is more energetic than thermal infrared as you can see in the figure. This is because average temperature of surface of sun is about 5800 K.
- Depending on the sensitivity of the sensor and which light (in the electromagnetic spectrum) is detected you may gather different information.
- Sensors used in remote sensing are all about sizes. Sensors correspond to the size of the electromagnetic wave that we want sense.
- Further remote sensing is all about the interaction of light, both visible and invisible, with matter. If the sensor is sensitive to detect this interaction you get to acquire different information.
- Eg: X-ray imaging is also about remote sensing. When you beam a flash of X-ray light onto the body, it passes through most of your body but stopped and reflected by bones which are made of heavy atoms (calcium). This is why you can image bones using X-rays.
- Sensors used to detect clouds in weather satellites interact with water molecules due to their distinct size.
- James Webb telescope has sensors that can detect multiple invisible lights including infrared.
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Spectroscopy: ‘Seeing’ Chemical Composition
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- Atoms of each chemical element absorb, scatter (reflect and refract) light at a unique set of wavelengths characteristic of that element alone.
- By studying the amount of absorption and scattering we can identify the chemical composition of any object. This is called spectroscopy.
- This is how we identify chemical composition of matter in stars and mineral composition of rocks on earth surface.
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Scope of Remote Sensing Satellites
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Sensor in remote sensing system are designed to measure 4 attributes
- Spectral (variation in colour)
- spatial (degree of resolution)
- temporal (changes over time)
- polarisation (a property of reflected light that carries information about the physical and chemical attributes of the reflecting source).
Put together they can help study characteristics, such as the temperature of the air or sea surface, salinity, soil moisture, sea ice, the amount of water in the atmosphere, wind speed and direction.
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Active and Passive Remote Sensing
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- The passive technology uses sensors to detect electromagnetic radiation reflected by the Earth.
- The active technology involves satellite illuminating the Earth using an onboard radio or light source (RADAR, LIDAR etc) and detecting the reflections. In short it involves backscattering. This is similar to what flashlights do in your cameras.
- LIDAR operates in the visible range, RADAR in the radio (microwave) range
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