What Is Landsat?

Landsat is a joint NASA and United States Geological Survey program of Earth-observing satellites that has imaged the land surface continuously since 1972, producing the longest uninterrupted space-based record of terrestrial change. The first spacecraft, launched in July 1972 and initially named the Earth Resources Technology Satellite, established the pattern that later missions preserved: a sun-synchronous polar orbit, a moderate spatial resolution suited to fields and forest stands rather than individual buildings, and a repeat cycle short enough to follow a growing season. NASA builds and launches the observatories; the USGS operates them after commissioning and manages the archive at its Earth Resources Observation and Science Center.

Moderate resolution is the defining design choice. At 30 meters per pixel across most spectral bands, a single Landsat scene covers roughly 185 kilometers on a side, which is coarse enough to map a watershed in one pass and fine enough to distinguish agricultural fields, clearcuts, burn scars, and urban expansion. The Worldwide Reference System grid of paths and rows gives every scene a stable identifier, so imagery collected decades apart can be stacked and compared pixel by pixel.

Instruments and Spectral Coverage

Sensor design has evolved across the series while preserving band continuity. Early missions carried the Multispectral Scanner and later the Thematic Mapper, both whiskbroom scanners using an oscillating mirror. Landsat 8 introduced pushbroom instruments with long detector arrays, pairing the Operational Land Imager for visible, near-infrared, and shortwave-infrared bands with the Thermal Infrared Sensor. Landsat 9, launched in September 2021, carries OLI-2 and TIRS-2 as near-identical rebuilds of the Landsat 8 instruments, a deliberate choice that reduced development risk and guaranteed radiometric consistency with the existing record. OLI-2 delivers 30-meter multispectral bands and a 15-meter panchromatic band with improved radiometric precision over its predecessor, while the thermal bands are acquired at 100 meters and resampled.

The Open Archive and Data Products

Landsat data became free of charge in 2008, and that policy change transformed the program from a source of individual scenes into a foundation for continental and global time-series analysis. Users now work mostly with the Collection 2 products, which apply consistent geometric and radiometric calibration across the entire archive so that scenes from the 1980s and the 2020s can be compared without reprocessing. Level-2 science products deliver surface reflectance and surface temperature rather than raw sensor counts, removing atmospheric effects so that derived indices such as the normalized difference vegetation index are comparable across dates and locations. With two satellites in orbit and their ground tracks offset, the effective revisit interval is eight days.

Continuity and Calibration

The scientific value of the program rests on cross-sensor continuity. Each new instrument is characterized against its predecessor during an underfly, a maneuver in which the incoming satellite passes directly beneath the operating one so that both image the same ground under the same illumination within seconds. Onboard solar diffusers, blackbody references, and periodic lunar views track detector drift over the mission lifetime. This calibration discipline is what allows analysts to attribute a change in reflectance to the landscape rather than to the instrument, and it underpins operational products such as national land cover maps and global forest change datasets.

Applications

Landsat has applications in a range of fields, including:

• Agricultural monitoring, crop type mapping, and irrigation water use estimation
• Forest inventory, deforestation tracking, and post-fire burn severity mapping
• Urban growth and impervious surface analysis
• Surface water extent, glacier retreat, and snow cover studies
• Geologic mapping and mineral exploration
• Disaster response, including flood extent and volcanic activity assessment