Landsat is the name of a series of Earth observation satellites launched and operated by the U.S. government. The program began in 1972, and continues today with two ailing satellites: Landsat 5 and Landsat 7. This record of over thirty years will hopefully continue when the Landsat Data Continuity Mission (LDCM) secures Landsat 8. The Landsat program has been a major success for NASA and the science community.
Imagery from the Landsat satellites has been of great importance to the development of land cover science, and Earth science in general. Three major sensors have been used on the six successful vehicles: the Multi-Spectral Scanner (MSS) on Landsats 1-4, the Thematic Mapper (TM) on Landsats 4-5, and the Enhanced Thematic Mapper Plus (ETM+) on Landsat 7. The high resolution of these sensors, especially TM and ETM+ (30m) is well suited for determining land cover, vegetation type and health, and geologic characteristics. The World Reference System (WRS) represents the acquisition strategy of Landsat sensors, capturing imagery at regular intervals and constant locations. This allows for ready comparison of images over time, which is excellent for land cover change assessment. Landsat imagery has been well documented, and is one of the most reliable and important Earth observation instruments.
Many further resources are available. To help determine which resource may be most helpful, ascertain which Landsat sensor (often related to date of acquisition) is appropriate. Also consider there are many levels of processing and file formats available for Landsat imagery. The USGS is responsible for at least the initial levels of processing for most Landsat imagery. The GLCF has Landsat products available at many levels of processing and in many file formats, as they are provided to the GLCF. A standardized copy of each scene, however, should be available in GeoTIFF format, in UTM coordinates, and in WGS84 datum. The Landsat GeoCover collection is available in its entirety at the GLCF, including coverage of the Earth in three separate epochs (1975, 1990, 2000). All Landsat imagery at the GLCF is available for free.