Radar: RAdio Detection and Ranging (Microwave Operation) , Imaging and Non-Imaging Sensors (Especially for GATE-Geospatial 2022)

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We have learned about remote sensing using the visible and infrared part of the electromagnetic spectrum. Microwave remote sensing uses electromagnetic waves with wavelengths between 1 cm and 1 m. These relatively longer wavelengths have the advantage that they can penetrate clouds and are independent of atmospheric conditions, like Haze. In microwave remote sensing there are active and passive sensors.

  • Passive sensors operate similarly to thermal sensors by detecting naturally emitted microwave energy. They are used in meteorology, hydrology and oceanography.
  • In active systems, the antenna transmits microwave signals from an antenna to the Earth՚s surface where they are backscattered. The part of the electromagnetic energy that is scattered into the direction of the antenna is detected by the sensor as illustrated in below Figure. Radar is active sensor.
Principle of Active Microwave Remote Sensing

Advantages of Active Sensors

There are several advantages to be gained from the use of active sensors, which have their own energy source:

  • It is possible to acquire data at any time including during the night (like thermal remote sensing) .
  • Since the waves are created actively, the signal characteristics are fully controlled (e. g. , wavelength, polarization, incidence angle, et cetera) and can be adjusted according to the desired application.

Imaging and Non-Imaging Sensors

Active sensors are divided into two groups: imaging and non-imaging sensors.

  • RADAR sensors belong to the group of most used active imaging microwave sensors. The term RADAR is an acronym for RAdio Detection and Ranging. Radio stands for the microwave and range is another term for distance. Radar sensors were originally developed and used by the military. Nowadays, radar sensors are widely used in civil applications too, such as environmental monitoring.
  • Non-imaging microwave instruments include altimeters, which collect distance information (e. g. , sea surface height) , and scatterometer, which acquire information about the object properties (e. g. , wind speed) .

This section focuses on the principles of imaging radar and its applications. The interpretation of radar imagery is less intuitive than that obtained from optical remote sensing. This is because of differences in the physical interaction of the waves with the Earth՚s surface.

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