A radar system looks to the side and records the locations of objects using the distance from the sensor to the object along the line of sight, rather than along the surface. An image collected using this geometry is referred to as a slant range image. Slant range radar data have a systematic geometric distortion in the range direction. The true, or ground range, pixel sizes vary across the range direction because of the changing incident angles. This makes the image appear compressed in the near range, relative to what it would look like if all of the pixels covered the same area on the ground.

Use Slant to Ground Range to resample slant range radar images to ground range pixel sizes with a fixed size, assuming flat terrain. ENVI has slant-to-ground range correction tools specifically for use with SIR-C, AIRSAR, and RADARSAT data, as well as a generic slant-to-ground range tool for other radar images.

For SIR-C and AIRSAR data, ENVI performs slant-to-ground range correction on synthesized images; it does not perform the correction on the entire SIR-C compressed data product file, nor the AIRSAR compressed Stokes matrix file. Because the compressed files contain required information about the sensor orientation, ENVI prompts you to select the original unsynthesized data file from which to extract the necessary information.

  1. From the Toolbox, select one of the following:
    • Radar > AIRSAR > AIRSAR Slant-to-Ground Range
    • Radar > RADARSAT > RADARSAT Slant-to-Ground Range
    • Radar > SIR-C > SIR-C Slant-to-Ground Range
    • Radar > Generic Slant-to-Ground Range
  2. For SIR-C, AIRSAR, and RADARSAT, the Enter Filename dialog appears. For Generic, the Input File dialog appears (skip the next step).

  3. For SIR-C, AIRSAR, and RADARSAT input files, select a file that contains the data acquisition parameters. For AIRSAR and RADARSAT, this information appears in the CEOS header.
    • For AIRSAR and SIR-C, select a compressed Stokes matrix file (.stk). Click OK.
    • For RADARSAT-1, select the image file (.dat). Click OK.
    • For RADARSAT-2, select the image file in GeoTIFF format (image_<n>.tif, one for each polarization channel). Click OK.
    • To manually enter the necessary parameters, click Cancel and enter the parameters in the Slant to Ground Range Parameters dialog that appears.
  4. The Input File dialog appears.

  5. Select an input file and perform optional spatial subsetting, then click OK. The Slant to Ground Range Parameters dialog appears.
  6. For SIR-C, AIRSAR, and RADARSAT input files, ENVI automatically populates the Instrument height (km), Near range distance (km), and Slant range pixel size (m) fields with information from the input file that contains data acquisition parameters.
  7. For generic input files:

    • Enter the sensor altitude in the Instrument height (km) field.
    • Enter a Near range distance (km) value from nadir for the input image.
    • Enter a value for Slant range pixel size (m). This is not the pixel size of the input image.
  8. Enter a value for the desired Output pixel size (m) of the ground range data.
  9. From the Near Range Location drop-down list, select the location of the near range in the image: Top, Bottom, Left, or Right.
  10. From the Resampling Method drop-down list, select Nearest Neighbor, Bilinear, or Cubic Conv.
  11. Output the result to File or Memory.
  12. Click OK.