October 2, 2019 - Acquisitions of Landsat 8 Data in Lower Truncation Mode
Between September 12, 2019 and December 22, 2019, a number of Landsat 8 Operational Land Imager (OLI) scenes are being acquired in lower truncation mode.
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Selected ascending (nighttime) scenes of paths 24, 25, 26, and 27, along with selected descending (daytime) scenes of Path 40, 41, 42 and 43, are affected. The dates and affected paths are shown in the image below.
NOTE: Specific rows for each path will be added to the 2019 information on Landsat 8 Lower Truncation when the exact acquisition schedules are received, which is typically 2-3 days prior to acquisition.
The detectors of Landsat 8 OLI can register 14 bits of data. However, only the upper or lower 12 bits are transmitted from the spacecraft to the ground. During normal operations, the upper 12 bits are transmitted to the USGS ground station to maximize the dynamic range. On occasion, USGS and NASA calibration and validation engineers may submit a special request to acquire some path/rows in the lower truncation mode meaning that the spacecraft only transmits data to the ground station from the instruments lower 12 bits.
During lower truncation mode acquisitions, regions of high brightness within a scene exceed the lower 12-bit range of 4096 (digital number values per detector), causing the count to begin again using the 13th and 14th bits, which are beyond the lower 12-bit range reported. This causes the numeric values to "roll over" and start counting from zero again. Pixels affected by ‘roll over’ do not correctly reflect the brightness of the Earth’s surface and will visually take on an apparently random value (See example image below).
These roll over values are not differentiated from valid values elsewhere in the image. More so, the cloud information in the Landsat 8 OLI Quality Assessment Band is unreliable.
NOTE: Data acquired in lower truncation mode are available for download from EarthExplorer, and while most cloud free acquisitions will not be affected by the lower truncation mode setting, users should use caution if using these images for operational activities. The metadata field SENSOR_MODE indicates "TRUNCATION_OLI = "LOWER" for these scenes.
Please contact USGS EROS Customer Services with any questions regarding this announcement.