There are many sensors that are used in space applications. Main space applications of sensors are in space are communication ,remote sensing etc. firstly for successful launch of satellite to working of satellite there are many sensors used for space applications.
Remote sensing is one of the main application in space technology used in satellites and many other air crafts there are basically two remote sensing sensors those are passive sensors and active sensors.
Active sensors have its own source of light or illumination. In particular, it actively sends a pulse and measures the backscatter reflected back to the sensor.
But passive sensors measure reflected sunlight emitted from the sun. When the sun shines, passive sensors measure this energy. ( source : gis geography.com)
Active sensors and passive sensors:
By the definition we may have understand active sensors don’t need any external help or any external energy to detect or sense an object basically we can say that it has its own source of energy or light source.
passive sensors detect or measure the light emitted or reflected by the surroundings.
A simple example of active or passive sensors is a camera when you click a photo with flash on it acts as an active sensor because it only produce the source of light to detect the surroundings to click a photo. While taking a photo without using flash it acts as an passive sensor because it uses it surroundings the camera senses the light from surrounding and sense the object.
The usage of active and passive in remote sensing:
When we use active sensors or passive sensors in the remote sensing the sun or stars will be the biggest source of light or energy.
when active sensors are used in remote sensing it will directly sense the object using its own light source while passive sensors take help of sun light
By the help of active sensors we may not get the full details about the object since the source may not be covering a huge area or source may be not be getting a clear image of the object.
But on the other hand using passive sensors we may get even clear images sun is the huge source of light and it produces electromagnetic spectrum which helps us to see it in true colour.
This passive sensor gives a more clarity photos and in many cases we try to use more passive sensors not only we get high defined images but we dont need to waste a lot of energy.
The basic examples of active sensors used in space are Radar,Lidar,GPS etc. The basic example of passive sensor used in space is IR camera.
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