Solar Illumination and Sensor Viewing Angle Coefficient Files

Landsat Sensor Viewing Angle Bands Known Issues

In April 2020, the USGS identified an issue with the Landsat 4-7 Angles Creation Tool which is provided to help users generate solar illumination and sensor viewing angle bands from the angle coefficient file (*_ANG.txt) provided with Collection 1 Landsat 4-7 Level-1 products.

Return to Solar Illumination and Sensor Viewing Angle Coefficient Files page
 

Under certain circumstances, there can be discontinuities in the sensor zenith and azimuth bands generated by these tools. The more widely used solar angle bands which are used to perform per-pixel solar corrections for Top-of-Atmosphere (TOA) reflectance conversion are impacted, though it is less visible.

Collection 1 Landsat 8 data products are not impacted by this issue. This has been corrected in Landsat Collection 2 and will not impact the angle bands which are bundled as part of the Collection 2 Level-1 products. 

The Landsat Angles Creation Tools have been updated to correct for this issue. Users of these tools, particularly those who rely upon the sensor viewing angles, should download and use the updated versions of these tools.

Angle calculation relies on a pixel's line and sample location within the final processed Level-1 data (L1TP, L1GT, LTGS), as well as the line and sample location in the original radiometrically-processed Level-1 data (L1R). Two issues related to the L1R location cause some angle calculations to be incorrect in Landsat 7 ETM+ and Landsat 4-5 TM scenes (Landsat 8 data are not affected):

  • When calculating the L1R location, if the scan direction is undefined due to being in the fill area, a default L1R location of “0,0” is used.  This conflict between L1R and the final Level-1 locations causes incorrect angles to be calculated in areas outside of the valid image data bounds. This causes discontinuities in the angles near the edge of the actual image data.
  • When the coefficients used to calculate the L1R sample location are read incorrectly from the angle coefficient file, it causes the sensor viewing angles to be incorrect for all Landsat 7 ETM+ and Landsat 4-5 TM scenes. Sensor azimuth angles are misregistered by up to 0.7 degree and sensor zenith angles are misregistered by up to 0.07 degree.

The error is present in all Landsat 4-7 scenes but is most apparent in scenes processed to the Albers projection with minimal tilt, where the L1R sample location should be strongly related to the final-processed Level-1 data sample location.(Albers scenes are used as input into U.S. Landsat Analysis Ready Data .)

Combined, these issues cause visible discontinuities of angles to appear within the image bounds for Albers scenes with minimal tilt, as illustrated in the examples below:

Landsat 4 TM Band 5 Visible Discontinuity of Angles

Example Landsat 4 TM image (Path 16 Row 41, Acquired June 29, 1992) illustrates (from left to right): An overview of Band 5 with fill pixels in cyan; A closeup of the lower left corner of Band 5; and, the same location in the Sensor Zenith Angle Band.  The red square illustrates where the discontinuity appears in the angle band, and its location well within the bounds of the Band 5 image.

 

Landsat 7 ETM+ Band 5 Visible Discontinuity of Angles

Example Landsat 7 ETM+ image (Path 16 Row 41, Acquired August 31, 2009) illustrates (from left to right): An overview of Band 5 with fill pixels in cyan; A closeup of the lower left corner of Band 5; and, the same location in the Sensor Zenith Angle Band.  The red square illustrates where the discontinuity appears in the angle band, and its location well within the bounds of the Band 5 image.

The handling of the digital elevation model (DEM) has a bug where pixels at elevation “0” are being set to a default height of “4000” instead of using the actual “0” elevation. This causes invalid angles that appear as speckles through the angle bands, as illustrated in the examples below:

Landsat 5 TM Sensor Azimuth angle pixels

Example images illustrate Sensor Azimuth angles at line 4722, sample 2738 of Landsat 5 TM (Path 16 Row 40, Acquired July 16, 1995) in UTM projection. The display is stretched for the digital number range of 220-240, illustrating one speckled pixel with a value 131 in the “before” image on the left.