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TIRS telescope
Diagram of the TIRS telescope and focal plane with the elements contributing to the spectral response labeled.
[Source: USGS Landsat Project] Toward the end of April, reprocessing efforts will begin for Landsat 8 OLI/TIRS scenes acquired from January 1 to March 31, along with data acquired during April, to create nominal Level-1 products containing valid TIRS data.  Additionally, all future Landsat 8 scenes will contain valid TIRS data—however, newly processed data will use preliminary estimated position information from the TIRS scene select mirror (SSM). (See this Landsat Update article for the background on the SSM encoder current anomaly, and below for more details on this effort.)
To compensate for not having TIRS SSM encoder information to indicate where the TIRS sensor is pointing, a new algorithm has been developed to provide estimates for the TIRS SSM encoder position. A new parameter in the Landsat 8 metadata (MTL.txt) file will provide the result of the algorithm: “TIRS_SSM_MODEL.” This parameter will contain one of three labels:

  • Preliminary. The majority of new acquisitions will be processed and labeled as Preliminary because estimated SSM position information will be used to process the data initially. These scenes will have higher uncertainty in the OLI/TIRS band alignment and will likely result in OLI/TIRS band misregistration.  When the TIRS_SSM_MODEL parameter is “Preliminary,” the “IMAGE_QUALITY_TIRS” parameter in the MTL file will be set to a lower value to provide an additional indication that the TIRS quality is not normal.
    After 12 to 16 days (coinciding with both the full moon and midway between full moons of each month), the data will be reprocessed using the refined final estimated SSM position. After the data is reprocessed, it will be re-labeled “Final.”
  • Final.  Scenes that display this label contain TIRS data that have been updated with SSM encoder telemetry information and TIRS LOS model parameters.  These data should align properly with the OLI bands, and the quality will closely approximate the “Actual” encoder values. The registration of the TIRS bands in scenes with the “Final” TIRS_SSM_MODEL should be as good as the “Actual” scenes.
  • Actual.  Scenes displaying this label contain TIRS data processed using actual measured SSM positions determined from the SSM encoder telemetry. Level-1 data products labeled “Actual” will contain the best TIRS quality available, resulting in TIRS bands aligning properly with OLI bands.

TIRS data with the metadata parameter TIRS_SSM_MODEL of “Final” or “Actual” are geometrically accurate enough for normal use; however, the TIRS Stray Light anomalyhas not been addressed so these data are not precise enough for use in a split-window algorithm.
Users are encouraged to provide any feedback or comments about the newly processed TIRS data using the [USGS] Contact Us web page.

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