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 data set comprises all Earth camera images for
the selected time period.  EJ-ER type files have images that have been
processed to remove the effects of penetrating radiation.  In addition,
the images have been flat-fielded and fixed pattern noise has been
removed.  Image pixels are median filtered with the images immediately
before and after in time.  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
   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.
       assignments:
               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
           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
Updated TEXT section bug
Updated some variables
Added an ADID number, same as K1
changed linear validmin 0->10, validmax 255->60 to suppress dayglow - 4/12/01 - REM
changed log validmin 0->1, validmax 255->18 to suppress dayglow - 4/12/01 - REM

Polar
!___Magnetospheric Data
!___ITM Data including Earth Imaging and Ground-Based

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

Presumed emission altitude (also quickly shows times when images are available). 

Earth Camera UV Images (quasi-log cnts), small format display with click-expand (~1 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.

Earth Camera UV Images (kRay), small format display with click-expand (~1 min. res.)

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

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)