PDS_VERSION_ID        = PDS3
RECORD_TYPE           = STREAM

OBJECT                = TEXT
   INTERCHANGE_FORMAT = ASCII
   PUBLICATION_DATE   = 2019-06-01
   NOTE               = "Description for Venus Climate Orbiter (VCO, also
                         known as PLANET-C and AKATSUKI) IR2 data
                         calibration."
END_OBJECT            = TEXT

END


IR2 data calibration
====================

  The header keywords relevant to the flux calibration of IR2 data are as
  follows:

    keyword    description
    --------   ------------------------------------------------------------
    I2_C2F     Type of the function that converts pixel counts to radiances
    I2_C2FK1   Conversion factor 1
    I2_C2FK0   Conversion factor 2
    I2_C2FSA   Solid angle of a pixel
    EXPOSURE   Exposure time

  The first keyword I2_C2F defines the type of function and describes how the
  following 4 parameters are used in it. Due to the remaining chromatic
  aberration of IR2 optics, the focal length varies from one filter to
  another. The value of I2_C2FSA (solid angle of a pixel) is therefore
  different for each filter.

  The image is calibrated with a "L1 to L2" program in the following steps:

    A) The header keyword FILTER ("2.02 um" or alike) is used to determine
       what conversion factors, I2_C2FK0 and I2_C2FK1, and focal length
       (prescribed in the program) are to be used in performing a conversion
       with the form of I2_C2F.

    B) Firstly, the entire image is flattened by dividing by a reference
       flat-field image. The name of the flat-field image is given in I2_FLAT
       ('226um_54sec_clean.fit'). Due to the limitation of pre-flight
       calibration process, only high-frequency pattern in the image is
       corrected by this. However, owing to superb uniformity of Pt Schottky
       layer over Si in the sensor, majority of non flatness can be removed as
       close-up images of Venus day-side demonstrate. During this, the data
       format is changed from 16-bit integer to 32-bit floating point numbers.

    C) Secondly, the pixel numbers (counts) are converted to radiances as
       described in the header keyword I2_C2F.

    D) Third process is to correct for the brightness differences between
       quadrants, peculiar to IR2 (and to IR1, also). Four quadrants in the
       PtSi sensor are treated by separate amplifiers. After arrival at Venus,
       it was found that the brightness level of each quadrant changes
       according to the sensor temperature, I2_T_C1. By analyzing all Venus
       images in which the disk appears in multiple quadrants, the correction
       function has been determined and prescribed in the "L1 to L2" program.
       In the header, relevant keywords are:

           keyword    description
           --------   ---------------------------------------------
           I2_T_C1    IR2 CCD temperature
           I2_TCVER   version of "temperature-dependent correction"
           I2_TCF00   correction factor for lower-left quadrant
           I2_TCF10   correction factor for lower-right quadrant
           I2_TCF01   correction factor for upper-left quadrant
           I2_TCF11   correction factor for upper-right quadrant

       By obtaining the sensor temperature from I2_T_C1, the built-in function
       of which version is I2_TCVER is used to compute factors. The factor
       I2_TCF00, I2_TCF10, I2_TCF01, and I2_TCF11 are applied to each
       quadrant. Pixels in each quadrant is multiplied by the corresponding
       factor.

    E) All of the above steps are reversible. Therefore, the radiance in a
       pixel of a L2b image can be converted back to the raw image count by
       dividing by I2_TCFxx (xx needs to be replaced by corresponding
       quadrant), by reverting I2_C2F, and multiplying by I2_FLAT.

    F) The special values, missing value, saturated pixel offset value, and
       dead pixel value, are substituted into the data or are added to the
       data. For the details of the special values, see 'Data' section in
       'catalog/dataset.cat'.