    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

    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',

 [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

         The VIS key parameter data set is a survey of auroral activity

    provided by a series of single images showing a significant area of the

    auroral zone.  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.

         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

         A relative intensity scale is provided by the uncompressed count table

    of variable 27.  Approximate intensity levels in kiloRayleighs are given in

    the intensity table of variable 28.  Information on the availability of

    more precisely calibrated intensities can be found on the VIS website at

        The time assigned to an image is the center time of the integration

        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

        This is half the length of the integration period for the image,

        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

        The presumed altitude of the emissions seen in the image varies

        The field stop may partially occlude the field of view of the low

        or medium resolution cameras.  The position is given in 1.5 degree

        This is the platform pointing angle of rotation around the spin

        For the low or medium resolution camera, the two-axis mirror

        position is given in steps measured from the instrument calibration

        switches.  The boresight of the instrument is located at step 68 in

        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

        The spacecraft position vector and velocity vector in GCI

        coordinates are for the image center time as given in variables

 17,18. 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

        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

            X**2/(A+ALT)**2 + Y**2/(A+ALT)**2 + Z**2/(B+ALT)**2 = 1

            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

   19. 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.

   21. 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

        The low and high color limits are recommended for remapping

            For pixel values less than the low limit, use the color

            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

        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

            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

            2 - the gap occurred because level zero data were missing,

            3 - the gap occurred because level zero data were too

        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

        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

    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

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

Presumed emission altitude (also quickly shows time periods when images are available)

Visible Image (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

Visible Image (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

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

Mirror pointing angle out of s/c X-Y plane in steps of ~.08660 degrees

Mirror pointing angle of rotation around spin axis, w/r/t platform position, in steps of ~.09375 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)


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