SAR Simulation

SAR Simulation Operator

The operator generates simulated SAR image using DEM, the Geocoding and orbit state vectors from a given SAR image, and mathematical modeling of SAR imaging geometry. The simulated SAR image will have the same dimension and resolution as the original SAR image.

Major Processing Steps

Some major steps of the simulation procedure are listed below:

1. First a DEM image is created from the original SAR image. The DEM image has the same dimension as the original SAR image. The pixel value of the DEM image is the elevation of the corresponding pixel in the original SAR image.
2. Then, for each cell in the DEM image, its pixel position (row/column indices) in the simulated SAR image is computed based on the SAR model.
3. Finally, the backscattered power σ₀ for the pixel is computed using backscattering model.

DEM Supported

Right now only the DEMs with geographic coordinates (Pא, Pא, Ph) referred to global geodetic ellipsoid reference WGS84 in meters are properly supported. 

Since the height information in ACE and SRTM is referred to geoid EGM96, not WGS84 ellipsoid, correction has been applied to obtain height relative to the WGS84 ellipsoid. 

User can also use external DEM file which, as specified above, must be WGS84 (Pא , Pא, Ph) DEM in meters.

Layover-Shadow Mask Generation

Besides producing simulated image, this operator can also generate layover-shadow mask for the simulated image using the 2-pass algorithm given in section 7.4 in [2]. For details of layover-shadow mask generation, reader is referred to SAR Simulation Terrain Correction operator.

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 producing simulated image. If no bands are selected, then by default all bands are selected. The selected band will be output as a band in the target product together with the simulated image.
2. Digital Elevation Model: DEM types. Choose from the automatically tiled DEMs or specify using a single external DEM file by selecting "External DEM".
3. DEM Resampling Method: Interpolation method for obtaining elevation values from the original DEM file. The following interpolation methods are available: nearest neighbour, bilinear, cubic convolution, binsinc and bicubic interpolations.
4. External DEM: User specified external DEM file. Currently only WGS84-latlong DEM in meters is accepted as geographic system.
5. Save Layover-Shadow Mask as band: Checkbox indicating that layover-shadow mask is saved as a band in the target product.

Detailed Simulation Algorithm

Detailed procedure is as the follows:

1. Get data for the following parameters from the metadata of the SAR image product:
2. • radar wave length
   • range spacing
   • first_line_time
   • line_time_interval
   • slant range to 1st pixel
   • orbit state vectors
   • slant range to ground range conversion coefficients
3. Compute satellite position and velocity for each azimuth time by interpolating the orbit state vectors;
4. Repeat the following steps for each cell in the DEM image:
5. 1. Get latitude, longitude and elevation for the cell;
   2. Convert (latitude, longitude, elevation) to Cartesian coordinate P(X, Y, Z);
   3. Compute zero Doppler time t for point P(x, y, z) using Doppler frequency function;
   4. Compute SAR sensor position S(X, Y, Z) at time t;
   5. Compute slant range r = |S - P|;
   6. Compute bias-corrected zero Doppler time tc = t + r*2/c, where c is the light speed;
   7. Update satellite position S(tc) and slant range r(tc) = |S(tc) – P| for the bias-corrected zero Doppler time tc;
   8. Compute azimuth index Ia in the source image using zero Doppler time tc;
   9. Compute range index Ir in the source image using slant range r(tc);
   10. Compute local incidence angle;
   11. Compute backscattered power and save it as value for pixel ((int)Ia, (int)Ir);

Reference:

[1] Liu H., Zhao Z., Lezek K. C., Correction of Positional Errors and Geometric Distortions in Topographic Maps and DEMs Using a Rigorous SAR Simulation Technique, Photogrammetric Engineering & Remote Sensing, Vol. 70, No. 9, Sep. 2004

[2] Gunter Schreier, SAR geocoding: data and systems, Wichmann-Verlag, Karlsruhe, Germany, 1993