Ellipsoid Correction

Ellipsoid Correction GG Operator

The operator implements the Geolocation-Grid (GG) method [1]. The implementation is exactly the same as for the Range Doppler Terrain Correction operator except that the slant range is computed from slant range time tie point of the source product instead of using DEM.

Major Processing Steps

Some major steps of the algorithm are given below:

  1. Get the latitudes and longitudes for the four corners of the source image;
  2. Determine target image boundaries based on the scene corner latitudes and longitude;
  3. Get range and azimuth pixel spacings from the metadata of the source image;
  4. Compute target image traversal intervals based on the source image pixel spacing;
  5. Compute target image dimension;
  6. Get tie points (latitude, longitude and slant range time) from geolocation LADS of the source image;
  7. Repeat the following steps for each cell in the target image raster:
    1. Get latitude and longitude for current cell;
    2. Determine the corresponding position of current cell in the source image and the 4 pixels that are immediately adjacent to it;
    3. Compute slant range R for the cell using slant range time and bi-quadratic interpolation;
    4. Compute zero Doppler time T for the cell;
    5. Compute bias-corrected zero Doppler time Tc = T + R*2/C, where C is the light speed;
    6. Compute azimuth index Ia using zero Doppler time Tc;
    7. Compute range image index Ir using slant range R;
    8. Compute pixel value x(Ia,Ir) using bi-linear interpolation and set it for current sample in target image.

Parameters Used

The following parameters are used by the operator:

  1. Source Band: All bands (real or virtual) of the source product. User can select one or more bands. For complex product, i and q bands must be selected together. If no bands are selected, then by default all bands are selected.
  2. Image resampling method: Interpolation methods for obtaining pixel values from source image.
  3. Map Projection: The map projection types. The orthorectified image will be presented with the user selected map projection.

The output will be expressed in WGS84 latlong geographic coodinate.


[1] Small D., Schubert A., Guide to ASAR Geocoding, Issue 1.0, 19.03.2008