Stray light from far out-of-field has affected the Landsat 8 Thermal Infrared Sensor (TIRS) absolute calibration since launch. An attempt to correct for it was made in January 2014 (see calibration notice from January 29, 2014) but work continued to develop a real model of the stray light effect in the telescope. An algorithm was developed and implemented into the processing system in February 2017.
The out-of-field source of the stray light was determined based on scans of the moon and tests of a spare telescope in the lab. In general, light was impinging on the detectors from a ring about 13 degrees outside of the field of view. Each TIRS detector is affected differently so a stray light source model was generated for every TIRS detector. On-orbit, the source of the stray light from all locations is not known, but the stray light correction algorithm uses the radiance at the edge of the image as an approximation for what might be out of view (Figure 1).
Vicarious calibration data over large water bodies were used to assess the stray light corrected image data. Initial results indicate significant improvement in the absolute accuracy of both TIRS bands. Errors were reduced from 2.1K @ 300K with no correction to 0.3K with the stray light correction for Band 10 and from 4.4K to 0.19K for Band 11. Variability of these errors is reduced as well, from 0.87K to 0.52K at 300K for Band 10 and from 1.67K to 0.91K at 300K for Band 11.
Additional work is underway to assess whether this correction is adequate for use with the split-window atmospheric correction technique. Until that work is complete, it is not recommended that Band 11 be used for the split-window technique.
Figure 1. Illustration of source of stray light for a single TIRS detector. The source of the stray light is within the TIRS interval to the north and south of the detector, but to the east the stray light originates from outside the sensor’s field-of-view (also to the west, though this particular detector is not affected by stray light to the west). For the locations that fall within the FOV, the TIRS scene radiance is used as the stray light radiance. For the locations that fall outside the FOV, the radiances at the edge of the scene are used as a surrogate for the actual source. This assumes that the radiances at the scene edges are roughly correlated to the radiance field outside the FOV. (Gerace and Montanaro, 2017)
Reference: Aaron Gerace, Matthew Montanaro, Derivation and validation of the stray light correction algorithm for the thermal infrared sensor onboard Landsat 8, Remote Sensing of Environment, Volume 191, 15 March 2017, Pages 246-257, ISSN 0034-4257, http://dx.doi.org/10.1016/j.rse.2017.01.029.