Design Document for MULLI

MULLI

AUTHOR: Tim Baltzer

Functional Summary:

Reverse the effects of illumination correction applied during calibration

Comments:

- The name MULLI comes from ILLUMination correction, in reverse
  order.
- This module will be called by procedure REATMO, used to process
  existing composites to apply atmospheric correction with water
  vapor taken into effect.

Background:

While developing the software to reprocess existing composites to apply atmospheric correction with water vapor, it was realized that some of these composites were created with illumination correction applied during calibration. This illumination correction needs to be 'backed out' prior to applying atmospheric correction to the existing composites. To complicate matters, the solar zenith angle used to apply illumination correction during calibration was not the solar zenith angle at the pixel being processed, but the solar zenith angle of the nadir pixel (see AVHRRCAL). This nadir solar zenith angle is not easily resurrected (since the image is now in composite space), but must be resurrected to accurately 'back-out' the effects of illumination correction. A separate routine must be written to accomplish this, a follow-on requirement from the effort to develop software to reprocess existing composites.

Requirements:

  • The input images must all be the same size and in the same projection and space.
  • The input images must have the correct projection and datum code fields specified in the DDR.
  • This module will be used to process any composite that has had illumination correction applied during calibration.
  • Must be able to handle mask values (output the same unaltered mask value).
  • Must be able to handle undefined regions, output same value (an undefined area within the Goodes projection).
  • Composites with up to 300 input acquisition images need to be accommodated.
  • PARAMETERS:

    IN
    Input image. IN must specify two LAS image bands which may be of any data type. Band one must contain AVHRR channel 1 data and band two must contain AVHRR channel 2 data. The values contained within IN must represent AVHRR data calibrated with illumination correction applied, such as that produced by AVHRRCAL.

    DATEBAND
    Date band. Image containing the date band from the compositing process. This image must be the same size and in the same projection space as IN.

    DATELOG
    Date log. The log file associated with the DATEBAND image which specifies the scenes used to generate the composite. The 'short' version (see '1992p19.log' for an example), and the 'long' version (see 'us_232245.log' for an example) are supported.

    SCALE
    Scale method. Scaling method used to store IN and OUT. The data type indicated in the image's DDR will be used to determine the scaling factor and offset value to use when scaling/unscaling the data. Valid values are 'GLOBAL' and 'US'. NOTE: Since Global 1KM composites are typically calibrated without applying illumination correction, SCALE should rarely be specified as 'GLOBAL'.

    OUT
    Output image containing calibrated AVHRR channels 1 and 2 WITHOUT the effects of illumination correction. The output image is the same size, data type, and in the same projection as the input image specified by IN.

    REQUIRED ALGORITHM:

    The effect of illumination correction on calibration coefficients
    for optical bands can be expressed as follows:
    
    C_with_illum = C_without_illum * d * d / cos z
    
    where    C_with_illum = the new Calibration coefficient with illumination
    	                correction
          C_without_illum = the old Calibration coefficient without illumination
    	                correction
    	            d = distance between the sun and the earth
    	            z = solar zenith angle
    
    Consequently, to 'undo' the effects of illumination correction,
    the following equation needs to be applied:
    
    C_without_illum = C_with_illum * cos z / (d * d)
    
    Refer to the 'EQUATION.NOTES' file (in the AVHRRCAL source code directory)
    for more information.  Also refer to the NOAA POD Section 3.3 for more
    information.
    

    Scope/Limitations:

    - will not support windowing of the composite
    

    Overall design:

    Algorithm:

    Testing criteria:

    - Test with a US composite with illumination correction applied, and a Global
      1KM composite with illumination correction applied (providing one can be
      found!, this is not typically done).
    - Calibrate an AVHRR acquisition with illumination correction applied and
      without illumination correction applied.  Run MULLI on the calibrated image
      where illumination correction was applied.  The resulting output image
      from MULLI should be very close if not exactly the same as the image
      calibrated without illumination correction applied.
    - Run the above test with the images also reprojected into composite space
      instead of still being in satellite space.