The modern, civil earth observation satellite era began with the launch of the first Landsat satellites in the 1970s. In contrast to earlier military satellite missions such as those in the classified “Corona” program, the Landsat satellites were designed to support civilian applications ranging from natural resources management to urban and regional planning.
These two images cover a portion of Green Bay and western Lake Michigan. The city of Green Bay, Wisconsin, is located at the lower left, while Sturgeon Bay on the Door peninsula is at the center right.
The top image was taken on June 25, 1979, from the Landsat 2 satellite. The original scanner instrument on Landsats 1, 2, and 3 was not sensitive to the full range of wavelengths of visible light, so this image is shown as a “false-color” composite, using wavelengths of green, red, and near-infrared light. In this representation, features that reflect highly in the near-infrared part of the spectrum (such as healthy vegetation) appear in red hues. Each pixel represents a square on the ground about 76 meters on a side (250 by 250 feet). Even from an altitude of 570 miles, features such as roads, fields, and coastal landforms can be seen.
The bottom image is a nearly true-color Landsat 7 scene from September 8, 2000. Landsat 7 has more spectral channels, making it sensitive to more wavelengths of light, and five times greater spatial resolution. Each pixel in the image covers a 15-by-15-meter (50-by-50-foot) area on the ground. Among the features visible in this image are algal blooms and submerged sandbars in Green Bay, various types of wetland plant communities, and the expansion of urban and suburban areas since the 1979 image was taken.