The radiant energy that is measured by an aerial or satellite sensor is influenced by the radiation source, interaction of the energy with surface materials, and the passage of the energy through the atmosphere. In addition, the illumination geometry (source position, surface slope, slope direction, and shadowing) can also affect the brightness of the upwelling energy. Together these effects produce a composite "signal" that varies spatially and with the time of day or season. To produce an image that we can interpret, the remote-sensing system must first detect and measure this energy.
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