Why not put far-IR sensors on satellites?
Thermal far-IR scanners have the disadvantage of rapidly moving parts, making them mechanically complex relative to other remote sensing systems. Such parts don’t last long in space, making scanners impractical for use aboard satellites. Also, there is a tradeoff between temperature sensitivity and pixel size. A temperature sensitivity of 2 degrees Centigrade would require a field pixel as large as a baseball diamond. Furthermore, if one tried to increase temperature sensitivity the pixel size would get even larger. In an aircraft at several thousand feet a pixel size of two meters – six-and-one-half feet – is compatible with a temperature sensitivity of several tenths of a degree Centigrade. Thus, airborne scanners are appropriate for crop management of individual fields while a satellite based thermal sensor is not. Finally, there is no practical way to cool sensors aboard a commercial agricultural satellite to liquid nitrogen temperatures.
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