Saturday, March 10, 2012

Terrain Correction of SAR Images -- Part 1

A characteristic of side-looking SAR image is the so-called foreshortening and layover, a reflected signal from a mountaintop reaches the sensor earlier or at the same time as the signal at the foot of the mountain. This results in the typical look of mountains that seem to have "fallen over" towards the sensor:

In the original image to the left, a pixel is basically displaced depending on its elevation above sea-level, so it is important to remove this layover as seen in the image above to the right. The freely available SNAP SAR Toolbox (previous version known as the  NEST SAR Toolbox ) is in many ways a great tool for satellite image processing and makes it very easy terrain-correct SAR images in a fully automatic process.

The 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. (It's actually quite easy in principle, but not sure this description is clear...)

Below is the original ESA SAR image as loaded into SNAP (or NEST) displaying the typical layover: 

Before the terrain correction, I apply the newest orbit file (Utilities>Apply Orbit), calibrate the values (SAR Tools>Radiometric Correction>Calibrate; but not in dB since the terrain correction needs linear values!) and the run a speckle filter, median 3x3 (SAR Tools>Speckle Filtering>Single Image)

Now to the actual terrain correction. Choose Geometry>Terrain Correction>SAR Simulation Terrain Correction. In the first tab 1-Read you choose the product to be corrected: 

The second tab defines the output. Unfortunately the default output filename in this case is only "SarSimTC.dim", I follow the SNAP (NEST) naming convention where all methods applied are contained in the filename, such as "ORIGINALNAME_AppOrb_Calib_Spk_SarSimTC.dim" but this has to be typed manually:

In the "3-SAR simulation" tab, one can choose various DEM such as GETASSE and SRTM, but in my case I choose an "External DEM" and specify the file path. I set the "no data value" to 9999.0, otherwise all ocean surface will be NAN. There is something unusual in this tab -- if you do not highlight any of the source bands, the first one only will be processed, in this case "Amplitude_VV". If other bands also should be processed, both source bands (in this case Amplitude_VV and Amplitude_VH) must be highlighted by choosing and clicking them!

The "4-GCP Selection" tab I leave the given values: 

Finally, the "5-SARSIM-Terrain-Correction" tab. For my purpose, I choose 20m resolution for the output image, the map projection of Svalbard/Spitsbergen WGS1984 / UTM 33N and prefer nearest neighbour interpolation:

Now I can choose "process" and this particular run takes 6.5 minutes on a Windows 7 64-bit computer for one source band. For two source bands it is much, much longer (80 minutes in this run!) and may not work if the computer has too little RAM, so the best and fast way is to process individual source bands separately. (The Range Doppler algorithm for removing layover which I will discuss in a later  is faster but does not work for some scenes at high latitudes(?)).

 I choose "Utilities>Dataset Conversion>Linear to dB" to get desibel values and get this final result. The data fits perfectly to cartographic shapefiles of the coastlines and to the other geolocations.

compared again to the original SAR image the difference is easily visible!

A problem in the current version: If you -- after having processed a particular scene -- choose a different scene under "1-Read" as input having differently named source bands, the source band list under "3-SAR Simulation" does not update -- you have to close the whole window and start all over -- part 2 and following describe how to process a large number of scenes.

The next postings will discuss how to run all this from command line and do batch processing.


  1. This post was great help as i am a beginner in nest

  2. Thanks for this tutorial. It is indeed a very simplified explanation and helped me to understand the concept easily. Can you further clear out whats the basic difference between three types of terrain correction(1. Range doppler TC, 2. SAR simulation and 3. SAR simulation TC) in SNAP?