User's Guide

SUNSAT

Generate zenith and relative azimuth angles

Function:

Generate zenith and relative azimuth angles for an AVHRR LAS image. The bands that can be created are:

   Satellite zenith
   Solar zenith
   Absolute difference of the satellite and solar azimuths

An output image is created containing these new bands.

Parameters:

IN
Input image. The AVHRR LAS image for which view angles are to be calculated. The image must be in satellite space and must have an associated AVHRR Data Descriptor Record (ADDR) file.

OUT
Output image. The image that will contain the specified angles.

NPIXELS(100)
Number of pixels. The number of pixels within an increment. Output pixel angle values will be calculated along each line at increments specified by this parameter. Interpolation will be used to determine the value of the pixels lying between the calculated pixels. If every pixel value is to be calculated, NPIXELS should be set to one.

ODTYPE(BYTE)
Output data type. The program does all computations with floating-point data. If another data type is selected, the output data will be scaled as follows:

Data type   Band   Scaling
---------------------------------
  BYTE        1    Add 90 degrees
             2-3   None
  I*2        1-3   Multiply by 10
  I*4        1-3   Multiply by 10
  R*4        1-3   None

BANDS(1,2,3)
Band numbers. Specifies which angles will be computed. They may be placed in the output image in any order.

  = 1: Satellite zenith
  = 2: Solar zenith
  = 3: Absolute difference of
       satellite and solar
       azimuths

Examples:

  1. LAS> sunsat in=avhrrimg out=sunsatangs npixels=100 odtype=i*4

    SUNSAT computes satellite zenith, solar zenith, and relative azimuth angles angles for every 100th pixel in each line of the input image and interpolates for all pixels in between. A three-band I*4 image is created. The first two bands contain the satellite and solar zenith angles, and the third band contains the absolute difference between the satellite and solar azimuths.

  2. LAS> sunsat in=avhrrimg out=sunsatangs npixels=1 odtype=i*4 bands=2

    SUNSAT creates an I*4 image containing satellite zenith angles for AVHRRIMG. Since NPIXELS has been set to one, the angle is computed for every pixel in the image; no interpolation is performed.

Description/Algorithm:

The AVHRR orbital model is called to calculate the satellite zenith, solar zenith, and the relative azimuth angles for every NPIXELS'th pixel across each line in the input image. The angle values for each pixel lying between these pixels are then interpolated.

Nonfatal Error Message:

  1. [sunsat-warn] Nonfatal error encountered

    A nonfatal error was encountered during processing. Processing continues.

Fatal Error Messages:

  1. [sunsat-fatal] Fatal error encountered

    A fatal error was encountered during processing. Processing is terminated. The error message that is displayed preceding this message is the specific error that was encountered.

  2. [sunsat-nmrimg] Only one input image is allowed

    Only one input image is allowed. Specify the parameter IN accordingly and rerun the application.

User Notes:

  1. A related function is ANGINTERP. While SUNSAT creates view angle bands for an AVHRR LAS image in satellite space, ANGINTERP creates view angles for an AVHRR image that has been reprojected.