Back Geocoding

Sentinel-1 Back Geocoding

This operator co-registers two S-1 SLC split products (reference and secondary) of the same sub-swath using the orbits of the two products and a Digital Elevation Model (DEM).

In resampling the secondary images into reference frame, deramp and demodulation are performed first to the secondary image, then the truncated-sinc interpolation is performed. Finally, the reramp and remodulation are applied to the interpolated secondary image.

Input

• Before running this operator, user should first run S-1 TOPS Split operator to both reference and secondary products to get the same sub-swath and same polarization data for co-registration (e.g. IW1 - VV).
  
• The user should run Apply Orbit File to correct the orbits in both the split products with the Sentinel Precise (or Restitude) orbit files. This step can be skipped if no precise or restitude orbit file is available. Otherwise this step is always recommended in order to achieve better co-registration result.
  

Output

• The output of the operator are the co-registered reference and secondary bands in I/Q format.
  

Parameters Used

• Digital Elevation Model: The DEM type. Currently the following DEMs are supported
  
• • ACE2_5min
  • ACE30
  • ASTER 1sec GDEM
    
  • GETASSE30
  • SRTM 1Sec Grid
  • SRTM 3Sec
  • user provided external DEM file
    
• DEM Resampling Method: Interpolation method for obtaining elevation values from the original DEM file. The following interpolation methods are available
• • nearest neighbour
  • bi-linear
  • cubic convolution
  • 5-points bi-sinc
  • 11-points bi-sinc
  • 21-points bi-sinc
  • bi-cubic
    
• Resampling Type: Interpolation methods for obtaining pixel values from the source image. The interpolation methods available are the same as listed above. Generally the accurate 21-point truncated sinc interpolation is encouraged to use for complex image interpolation. This, of course, will take longer processing time than using other interpolation method. With other interpolation method, however, some oscillation effect could be introduced in the interpolated image and in the coherence image generated later on.  
• Mask out areas with no elevation:  Checkbox indicating if areas without elevation should be masked out from co-registration. It is recommended to select this checkbox to avoid artefacts along the coast in the co-registered images.
  
• Output Deramp and Demod Phase: Checkbox indicating if deramp and demodulation phases should be output as separated bands. This option is mostly for debugging purpose and not recommended for most users.
• Disable Reramp: Checkbox indicating if deramped and demodulated reference and secondary bands should be output. This option is generally not necessary if the coregistered reference and secondary bands will be used in generating interferogram. However, if this option is selected, we suggest you disable the "Mask out areas with no elevation" option. This is because the valid zeros in bands may be treated as invalid noDataValue and causes problem in the later on processing.