Landsat TM and ETM+ Shutter Intrusion Indicated in New Metadata Field
Over the life of the Landsat 4-5 TM and Landsat 7 ETM+ instruments, the mechanical bumpers used to reverse the mirror direction while scanning have been observed to wear down. This “bumper wear” has led to a gradual increase in the length of time required to turn the mirror around as it scans from the east-to-west and the west-to-east directions.
Return to Collection 2 Level-1 Data Overview
In the baseline Scan Angle Monitor (SAM) operating mode this led to a loss of synchronization between the scan mirror and other instrument components including the scan line corrector and the calibration shutter. This further lead to the inability of the instrument to keep the calibration shutter out of the optical path during normal image acquisitions.
These shutter incursions are the source of the so-called "caterpillar tracks" anomaly. During Landsat Collection 2 processing, a new field in the <SENSOR_ANOMALIES> group within the metadata file (MTL.txt) indicates the presence of shutter intrusion in the scene, displaying either “SHUTTER_INSTRUSION” or “NONE”, depending on whether or not intrusion was detected within the imagery.
Below are examples of Landsat TM imagery for which the shutter appears within the imagery and will be identified as having shutter intrusion within the associated MTL.txt file.
There will be cases where the shutter intrusion is within the image file but along on the outer scene edges only, for which the flag will pick up that shutter intrusion has occurred. In these cases, the SENSOR_ANOMALIES flag will be set to “NONE”, suggesting that shutter intrusion did not occur within the scene. The images below illustrate where the intrusion would not be flagged.
