Some example scenes that would be in high demand include science, agriculture or commerce applications or where change is likely occuring, such as the spring green-up. Landsat-7 also attempts to avoid cloudy areas. With these types of constraints, Landsat-7 will actually take about 3 months to acquire pictures of the entire earth. Landsat images are obtained through a process called remote sensing. Remote sensing is the process of gathering data from a distance.
Different objects emit different types of electromagnetic radiation.
The TM sensor detects the electromagnetic energy from the objects and records this information in 7 different wavelengths in the form of an image. Landsat-7 takes images of the earth every 16 days. Landsat-7 can operate in extremely cold temperature. Landsat-7 follows instruction from computers on Earth. Landsat-7 can operate for many years.