Saturday, March 10, 2012

Terrain Correction of SAR Images -- Part 4

A short comment only on the "Range Doppler Terrain Correction." As described in Part 1, the SARSIM algorithm takes the DEM and using orbit parameters of the satellite creates a simulated SAR image from this DEM. The simulated and the real SAR image, which will look very similiar, are coregistered. Through this simulation, the displacement for each location in the original landscape, the DEM, is known, so if the simulated SAR image is transformed back into the original DEM -- and the coregistered SAR image along with it -- the pixels of the SAR image will receive their real, geographical location.

The Range Doppler Algorithm does not simulate a SAR image to coregister this and the original SAR image, but calculates displacement based on orbit parameters and a DEM. The Range Doppler algorithm is much faster in processing scenes. When comparing scenes with both SARSIM and Range Doppler methods, I find no difference in the final product. However, the Range Doppler method does not work for quite a few of my scenes. If I understood ESA correctly, this is due to not accurate enough data in the SAR metadata, such that calculations of the displacement is incorrect. This appears to be special with data from the Arctic regions

I haven't therefore used this one that much, but in other areas of the world it may be worth using the Range Doppler Terrain Correction.

Choose Geometry>Terrain Correction>Range Doppler Terrain Correction  (in Graph Builder choose "Terrain Correction"). The settings are as follows


3 comments:

  1. hey, I'm having trouble with the radiometric normalization with the RD-terrain correction. There is still a significant gradient in the range direction after normalization. My scenes are also from the Arctic. Could you maybe explain what you mean by 'not accurate enough metadata'?

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    1. That metadata issue a different issue, only applying to Range Doppler which for some reasons does not work in the Arctic, but Terrain Correction works. But if you mean Sentinel-1 images, they have quite a large gradient, angular dependent, and as I hear also from others this is not easy to resolve at present. Radarsat has it as well, but less, and not in three stripes as Sentinel-1 has it. It would need assumption on the surface material and its backscatter dependence on the incidence angle.

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    2. PS: you can see this gradient on all processed images at http://www.polarview.aq/arctic as well.

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