Satellite Image Display

The satellite image display program is xsat program. This program reads three formats of satellite images:

• McIDAS Area files
  Area files come in various sizes and resolutions. The image file includes a navigation block to aid programs in determining the location of the image so that data may be overlaid on the image. This navigation block contains the orbit information of the satellite which is used to locate the satellite over the earth's surface and the information needed to translate scan lines and image elements to locations on the earth's surface. Since the image is navigated, it can be remapped. The image retains the original digital information so that enhancements may be applied and temperatures may be inferred from the infrared images.
  
• NOAAPORT image files
  NOAAPORT images are preset image files that are formatted similar to GRIB files.   There is a product definition block which defines the satellite, image type, and projection.  The image then follows as a sequence of 8 bit pixel values which can be enhanced using any of the standard enhancement techniques.  The image can be navigated and remapped.
  
• Unisys image files
  This is a generic image format used by Unisys for transmission of satellite imagery.  In most cases, these are sectorized images with no navigation. Pseudo-navigation can be added with the grid_domain resource.  Some of the images are remapped to a Lambert Conformal projection and these can be remapped.  The image contains a small header in ASCII which describes the product.  The 8 bit image data follows.
  
• WXP image files
  These are general purpose image files. This contains 8 bit image data plus a navigation block equivalent to the projection grid given in grid files. Since the image is navigated, it can be remapped. The image has 8 bits of resolution and may be enhanced with any of the standard enhancement techniques.

Once the image is read in, the colors are determined from the color table and enhancement scheme. The pixels (intensities) within the image are then converted to the appropriate colors. The recommended color table is "sat.clr" which provides a 50 level gray scale for the image. Other color tables are available and up to 256 colors can be specified. The color fill resource can be used to enhance the satellite image.  Several enhancement (.enh) files are provided. The image may also be dithered for display on displays that have depth less than 8 bits. The image may also be set up for overlay on top of another image where black regions on the image become transparent allowing the underlying image to show through.

The image is display in its original form.  This means that if the image is too large for the window, only the upper left portion of the image will display.  To view more of the image, use the plot_domain resource.   There are two useful methods:

• zoom:clat:clon:factor
  This will zoom the image by the factor listed centered on the specified clat,clon.   If the image has no navigation, this will be the pixel location to center on.
• full plot domain
  By specifying a full plot domain, the image will be remapped to that domain.   Region keywords can also be used.

Enlargements are done by linear pixel interpolation.

After the remapping, the final plot domain of the image that will display in the window is determined.  This can be used for overlay plots.

The image may also be printed using a Postscript interface built into the xsat program. This creates a gray scale image based on a 640x512 sized window. Also, a color enhanced image can be printed in color postscript. Navigation is used to overlay a map for final output.

xsat -cu=la -if=sat_ir -pd=us -ct=sat.clr -cof=sat_mb.enh -pp=irbar

For further information about WXP, email technical-support@weather.unisys.com
Last updated by Dan Vietor on July 21, 1998