 Instrument functional description:
    The VIS is a set of three low-light-level cameras.  Two of these
    cameras share primary and some secondary optics and are designed to
    provide images of the nighttime auroral oval at visible wavelengths.
    A third camera is used to monitor the directions of the fields-of-view
    of the auroral cameras with respect to the sunlit Earth and return
    global images of the auroral oval at ultraviolet wavelengths.  The
    VIS instrumentation produces an auroral image of 256 x 256 pixels
    approximately every 24 seconds dependent on the integration time and
    filter selected.  The fields-of-view of the two nighttime auroral
    cameras are 5.6 x 6.3 degrees and 2.8 x 3.3 degrees for the low and
    medium resolution cameras, respectively.  One or more Earth camera
    images of 256 x 256 pixels are produced every five minutes, depending
    on the commanded mode.  The field-of-view of the Earth camera is
    approximately 20 x 20 degrees.
 
 Reference:
    Frank, L. A., J. B. Sigwarth, J. D. Craven, J. P. Cravens, J. S. Dolan,
        M. R. Dvorsky, J. D. Harvey, P. K. Hardebeck, and D. Muller,
        'The Visible Imaging System (VIS) for the Polar Spacecraft',
        Space Science Review, vol. 71, pp. 297-328, 1995.
 [Note to first-time users:  The first four variables are of primary interest.
    The displayable 256 x 256 image array is in variable 3.  The correct orien-
    tation of a displayed image is explained in the description of variable 3
    below.]
 
 Data set description:
         The VIS Earth camera key parameter data set is a survey of global
    auroral activity providedby a series of piled images produced by the median-
    filtering of up to five consecutive images.  The displayable image counts
    are in variable 3.
         Some coordinate information is included for viewer orientation.
    Coordinates are calculated for a grid of 18 x 18 points corresponding
    to one pixel out of every 15 x 15 pixel block.  In addition, a rotation
    matrix and a table of distortion-correcting look direction unit vectors
    are provided for the purpose of calculating coordinates for every pixel.
    See the description of variables 14 and 15 below.
         To facilitate viewing of the images, a mapping of pixel value to a
    recommended color table based on the characteristics of the selected
    filter will be included with each image.  See the description of variables
    19, 20, and 21 below.
         A relative intensity scale is provided by the uncompressed count table
    of variable 24.  Approximate intensity levels in kiloRayleighs are given in
    the intensity table of variable 25.  Information on the availability of
    more precisely calibrated intensities can be found on the VIS website at
    URL http://eiger.physics.uiowa.edu/~vis/software/.
 
 Variable descriptions:
 
    1,2. Center time
        The time assigned to an image is the center time of the integration
        period within a resolution of 50 milliseconds.
 
    3. Image counts
        Image pixel counts range from 0 to 255.  They are stored in a two-
        dimensional 256 x 256 byte array.  Images from the Earth camera
        (sensor 0) are conventionally displayed with row 1 at the top, row 256
        at the bottom, column 1 on the left, and column 256 on the right.  The
        conventional image display for the low resolution camera (sensor 1) is
        rotated 180 degrees so that the row 1-column 1 pixel is at the lower
        right corner and the row 256-column 256 pixel is at the upper left
        corner.  When displayed in this manner, the spacecraft spin axis is
        oriented to the right in the display, the X component is defined as
        the center of the image look direction, and the Y component is the
        cross product of the spin axis and the look direction.
 
    4. Sensor number
        0 = Earth camera,
        1 = low resolution camera,
        2 = medium resolution camera.
 
    5. Half integration time
        This is half the length of the integration period for the image,
        measured in milliseconds.
 
    6. Filter
        Twelve filters are available for visible imaging; the filter number,
        1-12, is given here.  Ultra-violet imaging is done with one filter only,
        designated here as filter number 0.  In addition, the peak wavelength
        in Angstroms is given for the selected filter.
 
    7. Presumed altitude of emissions
        The presumed altitude of the emissions seen in the image varies
        with the characteristics of the filter used.
 
    8. Platform pitch angle
        This is the platform pointing angle of rotation around the spin
        axis, measured from nadir.
 
  9,10. Geographic coordinates
        Geographic north latitude and east longitude are provided for the
        pixels at these image array locations: every 15th row starting
        with row 1 and ending with row 256, and every 15th column starting
        with column 1 and ending with column 256, for a total of
        18 x 18 coordinate pairs.
 
 11,12. Spacecraft position and velocity vectors, GCI
        The spacecraft position vector and velocity vector in GCI
        coordinates are for the image center time as given in variables
        1 and 2.
 
   13. Spacecraft spin axis unit vector, GCI
 
 14,15. Image-to-GCI rotation matrix and look direction vector table
        The rotation matrix may be used with the look direction vector table to
        obtain pointing vectors in GCI coordinates for each pixel.  The
        resulting vectors may be used to calculate coordinates for the observed
        positions of the pixels.  Software for this purpose is available at URL
         http://eiger.physics.uiowa.edu/~vis/software/.  The general method
         used is described below.
 
        In the image coordinate system, the X axis is the center line-of-sight
        or look direction; the Y axis is the cross product of the spin axis an
        the X axis; and the Z axis is the cross product of the X axis and the
        Y axis.  When the display orientation conventions in the variable 3
        description are applied, the low resolution camera image is rotated so
        that both Earth camera and low resolution camera images are displayed
        with Y axis pointing up and Z axis pointing toward the right.
 
        To obtain the coordinates of the observed position of a pixel,
        calculate the intersection of the line-of-sight with the surface
        of an oblately spheroidal Earth at the altitude given as
        variable 7.  The equation of the spheroid is
            X**2/(A+ALT)**2 + Y**2/(A+ALT)**2 + Z**2/(B+ALT)**2 = 1
            where A is the Earth radius at the equator,
                  B is the Earth radius at the pole, and
                  ALT is the given altitude.
        The line-of-sight equations are
            (X-SCX)/DX = (Y-SCY)/DY = (Z-SCZ)/DZ
            where (SCX,SCY,SCZ) is the spacecraft position vector GCI, and
                    (DX,DY,DZ)  is the look direction unit vector GCI.
        Solve the line-of-sight equations for two variables in terms
        of the third; substitute into the spheroid equation; and use the
        quadratic formula to solve for the third variable.  Select
        the solution point closer to the spacecraft.
 
   16. Zenith angle of center line-of-sight at presumed altitude
        This is the angle between the geocentric vector through the
        observed point, assuming the altitude given as variable 7,
        and the reverse of the image center line-of-sight vector.
 
   17. Sun position unit vector, GCI
 
   18. Solar zenith angle at observed point of center line-of-sight
        This is the angle of the sun from zenith at the observed point
        of the center line-of-sight, assuming the altitude given as
        variable 7.
 
   19. RGB color table
        This is the recommended color table to be used with the
        limits given in variables 20 and 21.
 
 20,21. Low and high color mapping limits
        The low and high color limits are recommended for remapping
        the color table entries, as follows:
            For pixel values less than the low limit, use the color
                at table position 1.
            For pixel values greater than or equal to the low limit
                and less than or equal to the high limit, use the color
                at table position (pix-low)/(high-low) x 255 + 1.
            For pixel values greater than the high limit, use the color
                at table position 256.
 
   22. Data quality flag
        The data quality word has bits set to 1 when the listed
        conditions are true.  Bit #31 is the most significant bit in the
        word, and it will not be used as a flag.  These are the bit
        assignments:
            bit 0 - image data frame sync error
            bit 1 - image data frame counters error
            bit 2 - image data fill frame flag.
 
   23. Post gap flag
        The post gap flag has these possible values:
            0 - no gap occurred immediately prior to this record,
            1 - the gap occurred because the instrument was not in
                  a mode that allowed for the production of images for the
                  selected sensor,
            2 - the gap occurred because level zero data were missing,
            3 - the gap occurred because level zero data were too
                  noisy to extract images.
 
   24. Expanded count table
        The image pixel counts are quasi-logarithmically compressed to the
        range 0-255.  This table gives the average of the uncompressed range
        for each compressed count value.  Table entries 1-256 correspond to
        compressed counts 0-255 respectively.
 
   25. Intensity table
        Approximate intensity levels in kiloRayleighs are given for each
        compressed count value.  Table entries 1-256 correspond to compressed
        counts 0-255 respectively.  Information on the availability of more
        precisely calibrated intensities can be found on the VIS website at
        URL http://eiger.physics.uiowa.edu/~vis/software/.
 
 Supporting software:
    Supporting software is available on the VIS website at the URL
    http://eiger.physics.uiowa.edu/~vis/software/.  Included is an IDL program
    that displays the images with the recommended color bar and provides
    approximate intensities and coordinate data for each pixel.

Initial development
modified linear validmin 0=>15, linear validmax 255=>50 to suppress dayglow for UVI testing - 4/12/01 - REM
modified log validmax 255=>15 to suppress dayglow - 4/12/01 - REM

VIS Summary PNG Collection (combined Visible and Earth Camera images at U. Iowa

Presumed altitude of emissions (also quickly shows times when images are available)

Earth Camera UV Images (quasi-log cnts), small format display with click-expand (~4 min. res.)

Image_Counts contains the displayable image.  The counts have been quasi-logarithmically compressed by the instrument.  Approximate uncompressed value for Image_Counts(i,j) is ExpandedCount(Image_Counts(i,j)+1).  Approximate intensity in kR is Intens_Table(Image_Counts(i,j)+1).The appearance of the actual count value 255 is rare.  When displaying an image,it works best to use the fill value as an overflow (i.e. brightest) value.

 --> Larger format display with click-expand, no geographic registration 

 --> (USE SHORT TIME SPANS) Azimuthal projection to geographic (fixed sun orientation)

 --> (USE SHORT TIME SPANS) Azimuthal projection to magnetic LT and invariant lat

 --> (USE SHORT TIME SPANS) Movie display of images, no geographic registration

 --> (USE SHORT TIME SPANS) Movie, azimuthal projection to geographic (fixed sun orientation)

 --> (USE SHORT TIME SPANS) Movie, azimuthal projection to magnetic LT and invariant lat

Earth Camera UV Images (kRay), small format display with click-expand (~4 min. res.)

 --> Larger format display with click-expand, no geographic registration

Sensor number: 0=Earth Camera,1=Low Resolution Camera,2=Medium Resolution Camera

Number of Earth Camera images filtered together to produce this image

Filters #1-12 are visible wavelengths; filter #0 is UV for Earth camera images

Platform angle of rotation around spin axis, measured from nadir in tenths of degrees

Geographic N. latitude for pixels at every 15th row and column from 1 to 256

Geographic E. longitude for pixels at every 15th row and column from 1 to 256

X component is look direction,Y component is the spin axis  cross X

Solar zenith angle at intersection of center line-of-sight with Earth

RGBColorTable should be remapped for displaying an image using the low and high limits given for each image in Limit_Lo and Limit_Hi.Image_Counts count values less than Limit_Lo use the color at table position 1.  Count values greater than Limit_Hi use the color at table position 256.  For count values greater than or equal to Limit_Lo and less than or equal to Limit_Hi, the table position is (Count-Limit_Lo)/(Limit_Hi-Limit_Lo) x 255 + 1.At the selected table position C, the color components are Red at RGBColorTable(1,C), Green at RGBColorTable(2,C), and Blue at RGBColorTable(3,C).

MSB will not be used as a flag; see TEXT for other bit assignments

Expanded count table: quasi-logarithmically uncompressed pixel counts

Image_Counts contains pixel counts which have been quasi-logarithmically compressed by the instrument.  Approximate uncompressed value forImage_Counts(i,j) is ExpandedCount(Image_Counts(i,j)+1).

Approximate intensity in kR for Image_Counts(i,j) isIntens_Table(Image_Counts(i,j)+1)