Landsat, a joint program of the USGS and NASA, has been observing the Earth continuously from 1972 through the present day. Today the Landsat satellites image the entire Earth's surface at a 30-meter resolution about once every two weeks, including multispectral and thermal data. The USGS produces data in 3 categories for each satellite (Tier 1, Tier 2 and RT). Read more about the different tiers.
Landsat Multispectral Scanner (MSS): Landsat 1-5 each carried the MSS sensor, which collected four or five spectral bands of surface reflectance in the visible and near-infrared spectrum at 80m resolution. Read more about the MSS system at NASA Landsat Science.
Landsat 1 MSS Collection 1 DN values, representing scaled, calibrated at-sensor radiance.Dataset availability: 1972-07-23 - 1978-01-07
Landsat 2 MSS Collection 1 DN values, representing scaled, calibrated at-sensor radiance.Dataset availability: 1975-01-22 - 1982-02-26