A Landsat image straight off the satellite does not look like a picture of the Earth. It is a set of staggered strips, the colors offset from one another, the detectors visibly out of step. Everything that makes Landsat data trustworthy — every pixel in the right place, every brightness value scientifically correct — is added on the ground by the U.S. Geological Survey's Calibration and Validation (Cal/Val) team at the Earth Resources Observation and Science Center.
A raw (Level 0) Landsat 9 composite of bands 6, 5 and 4, showing the staggered sensor chip assemblies and misaligned bands. Credit: USGS.
Two sensors
| Operational Land Imager (OLI) | Thermal Infrared Sensor (TIRS) | |
|---|---|---|
| Sees | visible, near-infrared and shortwave infrared light | thermal infrared |
| Focal plane | 14 staggered sensor chip assemblies | 3 staggered sensor chip assemblies (A, B, C) |
| Spectral bands | 9 | 2 |
| Resolution | 30 m (15 m for the panchromatic band) | 100 m |
Both image a swath 185 km wide.
What throws the pixels off
Sensor artifacts, viewing and sun angles, the atmosphere, terrain, stray light, spectral variation and the spacecraft's attitude — its orientation in space — all affect the final image. The Cal/Val team works in three steps, for both position and brightness:
| Geometric (where pixels land) | Radiometric (how bright they are) | |
|---|---|---|
| Characterization | assesses how the sensor and spacecraft shift pixel positions | assesses how the sensor affects pixel brightness |
| Calibration | corrects the shifts so pixels fall in the right geographic place | removes sensor effects and ties pixel values to an accepted reference |
| Validation | independently measures and reports the final location accuracy | independently measures and reports the final brightness accuracy |
From raw to finished
| Level | What it is |
|---|---|
| Level 0 | raw, uncorrected digital numbers, with odd and even detectors, chip assemblies and bands all misaligned |
| Level 1R | brightness corrected into calibrated, scaled values — but the geometric offsets are still there |
| Level 1 Terrain Precision (L1TP) | the overlapping edge imagery removed, the data geolocated onto a map grid and registered to the Landsat ground control library, allowing for terrain — radiometrically and geometrically precise, and registered to one another |
The same kind of scene as a finished Level 1 Terrain Precision composite of bands 6, 5 and 4. Credit: USGS.
The team monitors every active Landsat mission continuously, adjusts calibration as needed, and publishes the results in quarterly USGS Open-File Reports — the work behind Landsat's reputation as a "gold standard" product.
Sources
Based on "Landsat 8–9 geometric and radiometric calibration and characterization," a USGS fact sheet from the Earth Resources Observation and Science (EROS) Cal/Val Center of Excellence, a work of the United States government in the public domain.
ライセンス： CC0 1.0（パブリックドメイン） · 出典 pubs.usgs.gov
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