Step 4: Assign Colors

Schematic depicting satellite channel responses to key atmospheric and surface features

To assign the spectral channels to the right primary colors, we need to know how each channel responds to key atmospheric and surface features. From this generalized schematic, we can see that the relative amount of reflected radiation in the three solar channels varies depending on the features observed. The relative degree of reflectivity is about equal over some features, such as the ocean. But there are sharp differences over others, such as ice clouds. We exploit these differences when we match a channel with one of the three RGB colors.

Reflectances of different land/atmospheric features in the VIS0.6, VIS0.8, and NearIR 1.6 channels, each is placed in one of the 3 RGB color guns (red, green, blue)

Here is a summary of the relative reflectivities in our hypothetical landscape.

  • Bare land, especially when dry, is strongly reflective in the 1.6 µm near-infrared channel
  • Vegetated surfaces are strongly reflective in the 0.8 µm visible channel
  • Water phase clouds have about the same reflectivity in all three channels
  • For ice phase clouds and snow cover, reflectivity is strong for 0.6 µm Vis and 0.8 µm Vis channels, but weak for 1.6 µm NIR channel since ice crystals reflect poorly at that wavelength
  • Ocean water is poorly reflective in all three channels