uvviscal Documentation

uvviscal - Radiometric correction of Clementine UV/VIS camera images



INTRODUCTION
------------


UVVISCAL performs radiometric corrections to images acquired by the
Clementine spacecraft UVVIS camera. UVVISCAL performs dark subtraction,
readout correction, non-linearity and flat field corrections.  UVVISCAL
gives the user the option of writing the output image values as radiance
values or as reflectance(I/F) values.


ALGORITHM
---------

General algorithm applied to data for radiometric correction is
described below:


Step 1. Global Offset Correction

     Step_1_DN = raw_DN - C4*OFFSET_MODE_ID - C5
       C4 = -8.177
       C5 = 15.56

Step 2. Divide by gain

     Step_2_DN = Step_1_DN / g(GAIN_MODE_ID)
         GAIN_MODE_ID      g(GAIN_MODE_ID)
              1                 1.0
              2                 2.907
              4                 6.906

Step 3. Dark current correction

     Step_3_DN = Step_2_DN - (DC + C3)
       C3 = 7.13


  Non-linearity correction

   XMUL = ACO + BCO*Step_3_DN + CCO*(Step_3_DN**2) + DCO*(Step_3_DN**3)

     Corrected_Step_3_DN = Step_3_DN * XMUL

       ACO = 1.062
       BCO = -.1153E-02
       CCO = .6245E-05
       DCO = -.1216E-07

Step 4.  Temperature-Dependent Offset Correction

     Step_4_DN = Corrected_Step_3_DN - C2 * u
       C2 = .003737 * exp(.0908*(T-273.15))
       T = Corrected FOCAL_PLANE_TEMPERATURE (The value in the labels is
           not correct.  The value used is taken from an interpolated
           set of values from ACT)
       u = t + readout time
       t = EXPOSURE_DURATION + .0494
       readout time = 60.05 + .05*(line # - 1)

Step 5.  Frame Transfer Correction

     Step_5_DN = Step_4_DN - ro
     ro = column_sum*dt/(t+288*dt)
     column_sum = sum of all 288 Step_4_DN values
     dt = frame transfer time per row = .00068
     t = EXPOSURE_DURATION + .0494

Step 6.  Flat-Field and Exposure Time Normalization

     Step_6_DN = Step_5_DN / (FF*t)
       FF = Filter-dependent flat-field
       t = EXPOSURE_DURATION + .0494

 The units of Step_6_DN are now in counts/ms.

Step 7.  Normalize to Sun-Moon distance of 1 AU

     Step_7_DN = Step_6_DN * (SOLAR_DISTANCE/AU)**2
       AU = 149597870 km

Step 8a.  Conversion to 1 AU Relative Radiance

     L = Step_7_DN / C1 ( L = Radiance in mW/sr-cm**2 )

   CENTER_FILTER_WAVELENGTH   C1
            415 nm           1.39
            750 nm           2.57
            900 nm           4.35
            950 nm           4.76
            1000 nm          2.77

Step 8b. Conversion to Reflectance

     R = Step_7_DN * CR

 The following CR values were derived by Eric Eliason on 6/20/96.  These
 are slight improvements over Carle Pieter's values because Eric used
 the most current flat files and Alfred McEwen's non-linearity
 correction.  For more information, see Eric's memo from 6/20/96.


   CENTER_FILTER_WAVELENGTH   CR
            415 nm          .021406
            750 nm          .012266
            900 nm          .010674
            950 nm          .010831
            1000 nm         .024271


PROGRAM STRATEGY
----------------

UVVISCAL reads the keyword label area from the input file to obtain
various processing parameters in order to radiometrically correct
the image. The following keywords are extracted from the keyword
label area:



                    TABLE OF IMAGE KEYWORDS USED BY UVVISCAL
                    --------------------------------------

      INSTRUMENT_ID           -    Camera (Should be UVVIS)
      GAIN_MODE_ID            -    Instrument gain mode
      OFFSET_MODE_ID          -    Instrument offset mode
      EXPOSURE_DURATION       -    Exposure time of camera
      BAND_BIN_FILTER_NAME    -    Instrument filter name
                                   (A,B,C,D,E,F)
      SC_SUN_POSITION_VECTOR  -    The sun vector is used to determine
                                   the distance of the planet from the
                                   sun at the time the image was
                                   recorded.
      FOCAL_PLANE_TEMPERATURE -    Focal plane array temperature
                                   (celsius)


After obtaining the image keywords from the image label area and
determining all processing parameters, UVVISCAL opens the appropriate
calibration files and processes the image.

NOTES ON TAE INPUT PARAMETERS
-----------------------------
Most of the TAE input parameters can be left as their default value for
UVVISCAL.  The only TAE parameters required by UVVISCAL are the FROM and
TO parameters which indicate the input and output file names. The other
TAE parameters provide optional processing strategies.  For most
applications, program NAIFLAB should always be run prior to UVVISCAL
to insure the correct keywords exist in the image labels.


PROGRAMMER:  Tracie Sucharski

Input file name

NONE

Output file name

NONE

Convert to I/F reflectance
values

YES

Use corrected
FOCAL_PLANE_TEMPERATURE

YES

Dark current constant
(Optional)

--

Dark current file
(Optional)

--

Array of flat-field
coefficients
(Optional)

--

High_Instrument_Saturation
(Default is ISIS Definition)

--

Output pixel type

2

Output pixel data range

0.0,1.0

User comment

" "

ADDITIONAL NOTES:

Parm	Description
FROM	Specify the input file to be corrected.
TO	Specify the output file. The result will be the radiometrically corrected image. The output values can either be output as absolute radiance or converted to reflectance (I/F) values. The ouput file can be 8, 16, or 32 bit which is defined by the OTYPE parameter shown below.
CONV	This option allows the output values to be written as absolute radiance or converted to reflectance values (I/F).
TEMPCOR	Use the corrected FOCAL_PLANE_TEMPERATURE from ACT instead of the value in the labels.
DCCONST	This constant will replace the array of dark current coefficients in the derived dark current file. In most cases, the default should be used for this parameter to indicate the derived dark current file is to be used. The name of the derived dark current file is $ISISDATA/dcfile.cub
DCFILE	This parameter allows the user to use an alternate dark current file. In most cases, the default should be used for this parameter to indicate the derived dark current file is to be used. The name of the derived dark current file is $ISISCLEMDATA/dark_5_15_96.cub.
FFFILE	This parameter allows the user to use an alternate flat-field file. In most cases, the default should be used for this parameter to indicate the derived flat-field file is to be used. There is a different version of the flat file for compressed and uncompressed images.The name of the derived flat-field file is $ISISCLEMDATA/lu[filter]_[un]comp_flat_long.cub. An example for a compressed B filter image is $ISISCLEMDATA/lub_comp_flat_long.cub.
HISAT	High_Instrument_Saturation value. The default is the ISIS definition of High_Instrument_Saturation which is 255 for 8-bit data.
OTYPE	Output pixel data type. Permitted values are: 0 - output type is same as input file pixel type 1 - 8-bit (integer with type conversion parameters) 2 - 16-bit (integer with type conversion parameters) 3 - 32-bit (floating point)
ORANGE	Output pixel data range. If the output pixel type is 1 (8-bit integer with type conversion parameters) or 2 (16-bit integer with type conversion parameters), then the type conversion parameters in the output file will be set to values that allow representing the specified range of output values. Output values outside this range will be stored as the special "representation saturation" value. The ORANGE parameter is ignored if the output pixel type is 3 (32-bit floating point) since type conversion parameters are not applicable to floating point pixel values. If both ORANGE(1) and ORANGE(2) are 0.0, then the type conversion parameters in the output file will automatically be set to allow representing the same range of values as can be represented in the input file. (The user will be required to supply a specific range for ORANGE if the input pixel type is 3 (32-bit floating point) and the output pixel type is 1 (8-bit with type conversion parameters) or 2 (16-bit with type conversion parameters)).
USERNOTE	Comment from the user. This will be recorded in the ISIS session log file and also in the History entry that is put into the History object of the output file.