Electromagnetic Radiation:
The source of remote sensing comes from electromagnetic radiation. This can either be from a natural source, for example the sun, or from an artificial source, for example a satellite.
The energy produced for remote sensing is dependent on the wavelengths and frequencies of this electromagnetic radiation. Short wavelengths have more energy and therefore provide a better signal for remote sensing than long wavelengths.
The Electromagnetic Spectrum:
The electromagnetic spectrum has been divided into intervals of different wavelengths (Graham, 1999). The best range of wavelengths for remote sensing are the visible region, the infrared region and the microwave region; these have wavelengths ranging from 100nm to 10cm. The visible light region consists of wavelengths from 0.4 to 0.7 um (Graham, 1999). The infrared region then expands from 0.7 to 100 um (Graham, 1999). Microwave radiation has longer wavelengths from 1mm to 1m (Graham, 1999).
Visible:
The visible region can be detected by the eye, and can be split up into different colours. The blue region has wavelengths from 400-500nm, green is from 500 to 600 nm, and red 600 to 700nm. Reflectance and absorbency of each of these wavelengths is different in different properties, and can explain the colouring of objects.
Infrared:
This can be broken down to near-infrared (NIR) at 700 to 1300nm, shortwave-infrared (SWIR) at 1300 to 3000nm, mid-infrared (MIR) at 3000-8000nm, and thermal infrared (TIR) and far-infared (FIR) at 8000 to 1cm. It has a longer wavelength than visible light and so can’t be seen without the use of sensors.
Microwave:
This can either be passive microwave remote sensing (at 0.15 to 30cm wavelength) or active microwave remote sensing (over 30cm wavelength). Passive microwave sensor detect natural sources of energy, whereas active microwave sensors detect artificial sources of energy. Active has more energy available than passive, and produces an image of higher spatial resolution.
I found learning about the different regions of the spectrum interesting, as I didn’t previously know how wavelength could affect energy, and the implications this would have on remote sensing. Learning this will be helpful in the understanding of how certain objects will have variance in reflectance of these wavelengths, and how this effects monitoring them using remote sensing.
References:
Graham, S. (1999) Remote Sensing/ NASA Earth Observatory. Available at: http://earthobservatory.nasa.gov/Features/RemoteSensing/remote.php
NASA (2016) Climate Science Investigations. Available at: http://www.ces.fau.edu/nasa/module-2/radiation-sun.php
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