Orthorectification

Range Doppler Terrain Correction Operator

Due to topographical variations of a scene and the tilt of the satellite sensor, distances can be distorted in the SAR images. Image data not directly at the sensor’s Nadir location will have some distortion. Terrain corrections are intended to compensate for these distortions so that the geometric representation of the image will be as close as possible to the real world.

The geometry of topographical distortions in SAR imagery is shown below.  Here we can see that point B with elevation h above the ellipsoid is imaged at position B’ in SAR image, though its real position is B".  The offset Δ_r between B' and B"  exhibits the effect of topographic distortions.

Terrain Correction allows geometric overlays of data from different sensors and/or geometries.

Orthorectification Algorithm

The Range Doppler Terrain Correction Operator implements the Range Doppler orthorectification method [1] for geocoding SAR images from single 2D raster radar geometry. It uses available orbit state vector information in the metadata or external precise orbit (only for ERS and ASAR), the radar timing annotations, the slant to ground range conversion parameters together with the reference DEM data to derive the precise geolocation information. 

DEM Supported

Currently, only the DEMs with geographic coordinates (P_lat, P_lon, P_h) referred to global geodetic ellipsoid reference WGS84 (and height in meters) are properly supported.

Various different types of Digital Elevation models can be used (ACE2_5Min, ACE_30, ASTER_1Sec, CFEM, Copernicus30m, Copernicus90m, GETASSE30, SRTM 1Sec, SRTM 3Sec GeoTiff).

The STRM v.4 (3” tiles) from the Joint Research Center FTP (xftp.jrc.it) will automatically be downloaded in tiles for  the area covered by the image to be orthorectified. The tiles will be downloaded to the folder .snap\AuxData\DEMs\SRTM_DEM\tiff. The .snap folder is located in your user folder.

Please note that for ACE, Copernicus and SRTM, the height information (being referred to geoid EGM96) is automatically corrected to obtain height relative to the WGS84 ellipsoid. For Aster Dem height correction is not yet applied.

Note also that the SRTM DEM covers area between -60 and 60 degrees latitude. Therefore, for orthorectification of product of high latitude area, different DEM should be used.

User can also use external DEM file in Geotiff format which, as specified above, must be with geographic coordinates (P_lat, P_lon, P_h) referred to global geodetic ellipsoid reference WGS84 (and height in meters).

Pixel Spacing

Besides the default suggested pixel spacing computed with parameters in the metadata, user can specify output pixel spacing for the orthorectified image.

The pixel spacing can be entered in both meters and degrees. If the pixel spacing in one unit is entered, then  the pixel spacing in another unit is computed automatically.

The calculations of the pixel spacing in meters and in degrees are given by the following equations: 

pixelSpacingInDegree = pixelSpacingInMeter / EquatorialEarthRadius * 180 / PI;

pixelSpacingInMeter = pixelSpacingInDegree * PolarEarthRadius  * PI / 180;

where EquatorialEarthRadius = 6378137.0 m and PolarEarthRadius = 6356752.314245 m as given in WGS84. 

Radiometric Normalization

This option implements a radiometric normalization based on the approach proposed by Kellndorfer et al., TGRS, Sept. 1998 where

In current implementation θ_DEM is the local incidence angle projected into the range plane and defined as the angle between the incoming radiation vector and the projected surface normal vector into range plane[2]. The range plane is the plane formed by the satellite position, backscattering element position and the earth centre. 

Note that among σ^₀, γ^₀ and β^₀ bands output in the target product, only σ^₀ is real band while γ^₀ and β^₀ are virtual bands expressed in terms of σ^₀ and incidence angle. Therefore, σ^₀ and incidence angle are automatically saved and output if γ^₀ or β^₀ is selected.

For σ^₀ and γ^₀ calculation, by default the projected local incidence angle from DEM [2] (local incidence angle projected into range plane) option is selected, but the option of incidence angle from ellipsoid correction (incidence angle from tie points of the source product) is also available.

ENVISAT ASAR

The correction factors [3] applied to the original image depend on if the product is complex or detected and the selection of Auxiliary file (ASAR XCA file). 

Complex Product (IMS, APS)

• Latest AUX File (& use projected local incidence angle computed from DEM):

   The most recent ASAR XCA available from installation_folder\auxdata\envisat compatible with product date is automatically selected. According to this XCA file, calibration constant, range spreading loss and antenna pattern gain are obtained.

• • Applied factors:

  1. apply projected local incidence angle into the range plane correction

  2. apply calibration constant correction based on the XCA file
     

  3. apply range spreading loss correction based on the XCA file and DEM geometry
     

  4. apply antenna pattern gain correction based on the XCA file and DEM geometry
     

• External AUX File (& use projected local incidence angle computed from DEM):

   User can select a specific ASAR XCA file available from the installation folder or from another repository. According to this selected XCA file, calibration constant, range spreading loss and antenna pattern gain are computed.

• • Applied factors:

  1. apply projected local incidence angle into the range plane correction

  2. apply calibration constant correction based on the selected XCA file
     

  3. apply range spreading loss correction based on the selected XCA file and DEM geometry
     

  4. apply antenna pattern gain correction based on the selected XCA file and DEM geometry
     

Detected Product (IMP, IMM, APP, APM, WSM)

• Latest AUX File (& use projected local incidence angle computed from DEM):

  The most recent ASAR XCA available from the installation folder compatible with product date is automatically selected. Basically with this option all the correction factors applied to the original SAR image based on product XCA file used during the focusing, such as antenna pattern gain and range spreading loss, are removed first. Then new factors computed according to the new ASAR XCA file together with calibration constant and local incidence angle correction factors are applied during the radiometric normalisation process.

• • Applied factors:

  1. remove antenna pattern gain correction based on product XCA file

  2. remove range spreading loss correction based on product XCA file
     

  3. apply projected local incidence angle into the range plane correction
     

  4. apply calibration constant correction based on new XCA file

  5. apply range spreading loss correction based on new XCA file and DEM geometry

  6. apply new antenna pattern gain correction based on new XCA file and DEM geometry
     

• Product AUX File (& use projected local incidence angle computed from DEM):

   The product ASAR XCA file employed during the focusing is used. With this option the antenna pattern gain and range spreading loss are kept from the original product and only the calibration constant and local incidence angle correction factors are applied during the radiometric normalisation process.

• • Applied factors:

  1. apply projected local incidence angle into the range plane correction

  2. apply calibration constant correction based on product XCA file
     

• External AUX File (& use projected local incidence angle computed from DEM):

   User can select a specific ASAR XCA file available from the installation folder or from another repository. Basically with this option all the correction factors applied to the original SAR image based on product XCA file used during the focusing, such as antenna pattern gain and range spreading loss, are removed first. Then new factors computed according to the new selected ASAR XCA file together with calibration constant and local incidence angle correction factors are applied during the radiometric normalisation process.

• • Applied factors:

  1. remove antenna pattern gain correction based on product XCA file

  2. remove range spreading loss correction based on product XCA file
     

  3. apply projected local incidence angle into the range plane correction
     

  4. apply calibration constant correction based on new selected XCA file

  5. apply range spreading loss correction based on new selected XCA file and DEM geometry

  6. apply new antenna pattern gain correction based on new selected XCA file and DEM geometry
     

Please note that if the product has been previously multilooked then the radiometric normalization does not correct the antenna pattern and range spreading loss and only constant and incidence angle corrections are applied. This is because the original antenna pattern and the range spreading loss correction cannot be properly removed due to the pixel averaging by multilooking.

If user needs to apply a radiometric normalization, multilook and terrain correction to a product, then user graph “RemoveAntPat_Multilook_Orthorectify” could be used.

ERS 1&2

For ERS 1&2 the radiometric normalization cannot be applied directly to original ERS product.

Because of the Analogue to Digital Converter (ADC) power loss correction , a step before is required to properly handle the data. It is necessary to employ the Remove Antenna Pattern Operator which performs the following operations:

 For Single look complex (SLC, IMS) products

• apply ADC correction

For Ground range (PRI, IMP) products:

• remove antenna pattern gain
• remove range spreading loss
• apply ADC correction

After having applied the Remove Antenna Pattern Operator to ERS data, the radiometric normalisation can be performed during the Terrain Correction.

The applied factors in case of "USE projected angle from the DEM" selection are:

1. apply projected local incidence angle into the range plane correction
2. apply absolute calibration constant correction
3. apply range spreading loss correction based on product metadata and DEM geometry
4. apply new antenna pattern gain correction based on product metadata and DEM geometry

To apply radiometric normalization and terrain correction for ERS, user can also use one of the following user graphs:

• RemoveAntPat_Orthorectify
• RemoveAntPat_Multilook_Orthorectify

RADARSAT-2

• In case of "USE projected angle from the DEM" selection, the radiometric normalisation is performed applying the product LUTs and multiplying by (sin DEM/sin el), where DEM is projected local incidence angle into the range plane and el is the incidence angle computed from the tie point grid respect to ellipsoid.
• In case of selection of "USE incidence angle from Ellipsoid", the radiometric normalisation is performed applying the product LUT.

These LUTs allow one to convert the digital numbers found in the output product to sigma-nought, beta-nought, or gamma-nought values (depending on which LUT is used).

TerraSAR-X

• In case of "USE projected angle from the DEM" selection, the radiometric normalisation is performed applying
  
  1. projected local incidence angle into the range plane correction
  2. absolute calibration constant correction
• In case of " USE incidence angle from Ellipsoid " selection, the radiometric normalisation is performed applying
  
  1. projected local incidence angle into the range plane correction
  2. absolute calibration constant correction

Cosmo-SkyMed

• In case of "USE projected angle from the DEM" selection, the radiometric normalisation is performed deriving σ_⁰Ellipsoid [7] and then multiplying by (sinθ_DEM / sinθ_el), where θ_DEM is the projected local incidence angle into the range plane and θ_el is the incidence angle computed from the tie point grid respect to ellipsoid.
• In case of selection of "USE incidence angle from Ellipsoid", the radiometric normalisation is performed deriving σ_⁰Ellipsoid [7]
  

1. The local incidence angle is defined as the angle between the normal vector of the backscattering element (i.e. vector perpendicular to the ground surface) and the incoming radiation vector (i.e. vector formed by the satellite position and the backscattering element position) [2].
2. The projected local incidence angle from DEM is defined as the angle between the incoming radiation vector (as defined above) and the projected surface normal vector into range plane. Here range plane is the plane formed by the satellite position, backscattering element position and the earth centre [2].
   

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 calibration. If no bands are selected, then by default all bands are used.
2. Digital Elevation Model: DEM types. Please refer to DEM Supported section above.
3. External DEM: User specified external DEM file. Currently only DEM in Geotiff format with geographic coordinates (P_lat, P_lon, P_h) referred to global geodetic ellipsoid reference WGS84 (and height in meters) is accepted.
4. 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, bi-sinc and bi-cubic interpolations.
5. Image Resampling Method: Interpolation methods for obtaining pixel values from the source image. The following interpolation methods are available: nearest neighbour, bi-linear, cubic and bi-sinc interpolations.
6. Pixel Spacing (m): User can specify pixel spacing in meters for orthorectified image. If no pixel spacing is specified, then default pixel spacing computed from the source SAR image is used. For details, the reader is referred to Pixel Spacing section above.
7. Pixel Spacing (deg): User can also specify the pixel spacing in degrees.  If the value of any of the two pixel spacing is changed, the other one is updated automatically. For details, the reader is referred to Pixel Spacing section above.
8. Map Projection: The map projection types. By default the output image will be expressed in WGS84 latlong geographic coordinate.
9. Mask out areas without elevation: Checkbox indicating that areas (e.g. sea) where DEM is not available will be masked out as no data value. 
10.  Output complex data: Checkbox indicating that the terrain correction result will be saved in complex format. The geocoded phase will not be phase-corrected and therefore its use for InSAR applications is strongly discouraged. 
11. Selected source band: Checkbox indicating that orthorectified images of user selected bands will be saved without applying radiometric normalization.
12. DEM as a band: Checkbox indicating that DEM will be saved as a band in the target product.
13. Latitude and Longitude: Checkbox indicating that latitude and longitude bands will be saved in the target product. 
14. Incidence angle ellipsoid: Checkbox indicating that the tie point grid incidence angle will be saved as a band in the target product. 
15. Local incidence angle: Checkbox indicating that local incidence angle will be saved as a band in the target product.
16. Projected (into the range plane) local incidence angle: Checkbox indicating that the projected local incidence angle will be saved as a band in the target product.
17. Layover Shadow Mask: Checkbox indicating that the layover and shadow mask will be saved in the target product. 
18. Apply radiometric normalization: Checkbox indicating that radiometric normalization will be applied to the orthorectified image.
19. Save Sigma0 as a band: Checkbox indicating that sigma0 will be saved as a band in the target product. The Sigma0 can be generated using projected local incidence angle, local incidence angle or incidence angle from ellipsoid.
20. Save Gamma0 as a band: Checkbox indicating that Gamma0 will be saved as a band in the target product. The Gamma0 can be generated using projected local incidence angle, local incidence angle or incidence angle from ellipsoid.
21. Save Beta0 as a band: Checkbox indicating that Beta0 will be saved as a band in the target product.
22. Auxiliary File: available only for ASAR. User selected ASAR XCA file for radiometric normalization. The following options are available: Latest Auxiliary File, Product Auxiliary File (for detected product only) and External Auxiliary File. By default, the Latest Auxiliary File is used. Details about the corrections applied according to the XCA selection are provided in Radiometric Normalisation – Envisat ASAR section above.

Reference:

[1] Small D., Schubert A., Guide to ASAR Geocoding, RSL-ASAR-GC-AD, Issue 1.0, March 2008

[2] Schreier G., SAR Geocoding: Data and Systems, Wichmann 1993

[3] Rosich B., Meadows P., Absolute calibration of ASAR Level 1 products, ESA/ESRIN, ENVI-CLVL-EOPG-TN-03-0010, Issue 1, Rev. 5, October 2004

[4] Laur H., Bally P., Meadows P., S�nchez J., Sch�ttler B., Lopinto E. & Esteban D., ERS SAR Calibration: Derivation of σ0 in ESA ERS SAR PRI Products, ESA/ESRIN, ES-TN-RS-PM-HL09, Issue 2, Rev. 5f, November 2004 

[5] RADARSAT-2 PRODUCT FORMAT DEFINITION - RN-RP-51-2713 Issue 1/7: March 14, 2008

[6] Radiometric Calibration of TerraSAR-X data - TSXX-ITD-TN-0049-radiometric_calculations_I1.00.doc, 2008

[7] For further details about Cosmo-SkyMed calibration please contact Cosmo-SkyMed Help Desk at info.cosmo@e-geos.it