Though humans cannot see light outside the visible spectrum, satellites are able to detect wavelengths into the ultraviolet and infrared. The Landsat 7 satellite uses an instrument that collects seven images at once, with each image showing a specific section of the electromagnetic spectrum, called a band. Each image highlights a different part of the electromagnetic spectrum.
The satellites original images are all acquired in black and white, so color must be assigned via computer to the black and white images. The three primary colors of light are red, green and blue, and each color is given a different band/image. Once the three images are combined, you will have what is called a “false color image." A common band combination shows green, healthy vegetation as bright red, which is useful in forestry and agricultural applications. Landsat images are also used to gather geological and hydrological data along with other kinds of environmental monitoring. In a helpful explanation, the folks at Landsat offer this catchy formula: "One common way that primary colors are assigned to bands can be easily remembered using the mnemonic:
"RGB = NRG (Red, Green, Blue = Near Infrared, Red, Green...)Red = Near IR (ETM+ band 4)Green = Red (ETM+ band 3)Blue = Green (ETM+ band 2)"
There. That should be easy to remember. My suggestion: try singing it.
All images courtesy USGS National Center for EROS and NASA Landsat Project Science Office