After the Sun has created electromagnetic radiation, the portion of it that has made its way to the top of the Earth’s atmosphere must pass through the atmosphere, be reflected by the Earth’s surface, pass through the atmosphere again on its way back to space, and then arrive at the sensor to be recorded.
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When the radiation field passes through empty space, nothing happens, but when it interacts with the Earth’s atmosphere and surface, several things happen. Because the measured radiation contains information about the Earth’s environment as a result of these interactions, it’s critical to examine exactly what happens in these interactions and how it affects the radiation field.
The radiation field that has passed through the atmosphere without being absorbed or scattered back into space has now reached the Earth’s surface. Each individual photon can now only do one of two things for any wavelength relevant to remote sensing: it can be absorbed by the Earth’s surface or it can be reflected back toward space.
The reflectance of a surface is the chance of reflection rather than absorption, and it is determined by the material on the surface as well as the wavelength of the incoming radiation.
Each surface material has its own’signature,’ which determines how much radiation is reflected at each wavelength.
Water, for example, reflects a little amount of blue and green wavelengths (usually around 5%–10% depending on turbidity), less of red wavelengths, and nearly none of infrared wavelengths.
Except for specific wavelengths that are successfully absorbed by liquid water in the leaves, vegetation reflected roughly half of all incoming infrared energy.
Before any space-based sensor can measure the component of the radiation field that is reflected by the Earth’s surface, it must naturally make its way back up through the atmosphere, with all of the refraction, scattering, and absorption that entails.
While there are numerous relative advantages and disadvantages to airborne vs. space-borne sensors, one notable advantage is the ability of airborne sensors to measure the reflected EMR field before it needs to pass through the atmosphere a second time.
