Orthorectification Tutorial

Terrain Correction

The Terrain Correction Operator will produce an orthorectified product in the WGS 84 geographic coordinates. The Range Doppler orthorectification method [1] is implemented for geocoding SAR images from a single 2D raster radar geometry. It uses available orbit state vector information in the metadata or external precise orbit, the radar timing annotations, the slant to ground range conversion parameters together with the reference DEM data to derive the precise geolocation information. Optionally radiometric normalisation can be applied to the orthorectified image to produce σ⁰, γ⁰ or β⁰ output.

The Ellipsoid Correction RD and Ellipsoid Correction GG Operators will produce ellipsoid corrected products in the WGS 84 geographic coordinates. The Terrain Correction Operator should be used whenever DEM is available. The Ellipsoid Correction (RD and GG) should be used only when DEM is not available.

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. 

Products Supported

• ASAR (IMS, IMP, IMM, APP, APM, WSM) and ERS products (SLC, IMP) are fully supported.
• RADARSAT-2 (all products)
• TerraSAR-X (SSC only)
• Cosmo-Skymed

DEMs Supported

Right now 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.

STRM v.4 (3” tiles) from the Joint Research Center FTP (xftp.jrc.it) are downloaded automatically for  the area covered by the image to be orthorectified. The tiles will be downloaded to the folder C:\AuxData\DEMs\SRTM_DEM\tiff or the folder specified in the Settings.

The Test Connectivity functionality under the Help tab in the main menu bar allows the user to verify if the SRTM downloading is working properly.

Please note that for ACE 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 already 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 the product being 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 the 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 \auxdata\envisat 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 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):

  The User can select a specific ASAR XCA file available from the installation folder \auxdata\envisat 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].
   

Steps to Produce Orthorectified Image

1. From the Geometry menu select Terrain Correction. This will call up the dialog for the Terrain Correction Operator (Figure 1).
2. Select your source bands.
3. Select the Digital Elevation Model (DEM) to use. You can select 30 second GETASSE30 or ACE DEMs if they are installed on your computer. Preferably, select the SRTM 3 second DEM which has much better resolution and can be downloaded as need automatically if you have an internet connection. Alternatively, you could also browse for an External DEM tile. 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. Select the interpolation methods to use for the DEM resampling and the target image resampling.
5. Optionally select the Pixel Spacing in meters for the orthorectified image. By default the pixel spacing computed from the original SAR image is used. For details, the reader is referred to Pixel Spacing section above.
6. Optionally select the Pixel Spacing in degrees for the orthorectified image. By default it is computed from the pixel spacing in meters. If any of the two pixel spacing is changed, the other one is updated accordingly. For details, the reader is referred to Pixel Spacing section above.
7. Optionally select Map Projection. The orthorectified image will be presented with the user selected map projection. By default the output image will be expressed in WGS84 latlong geographic coordinate.
8. Optionally select to save the DEM as a band and the local incidence angle.
9. Optionally select to apply Radiometric Normalizatin to output σ⁰, γ⁰ or β⁰ of the orthorectified image. 
10. Press Run to process the data.

Figure 1. Terrain Correction operator dialog box.

Below are some sample images showing the Terrain Correction result of an ASAR IMS product ASA_IMS_1PNUPA20081003_092731_000000162072_00351_34473_2366.N1, acquired on October 3, 2008, imaging the area around Rome in Central Italy.

The ASAR IMS image has been multi-looked with 2 Range looks and 10 Azimuth Looks before to be orthorectified.

The DEM employed is the SRTM 3 second Version 4 and since the SRTM height information is referred to geoid EGM96, not WGS84 ellipsoid, correction has been applied to obtain height relative to the WGS84 ellipsoid (this is done automatically)

Figure 2 is in the original SAR geometry after multi-looking 2-10.

The orthorectified image and its radiometric normalised image σ0 are shown in Figure 3  and Figure 4 respectively.

Figures 5 and 6 are a zoom of the figure 3 and 4.

The radiometric scale is in dB/m^2.

Figure 3. Original SAR Geometry after multi-looking 2-10. 

                 

                                                                                        Figure 3. Orthrectified image.                                                                                                                                              Figure 4. Radiometric normalized image.

    

                                                                                           Figure 5. Zoom in of the orthorectified image.                                                                                                          Figure 6. Zoom in of the radiometric normalized image.  

After Terrain Correction your SAR data will be closer to the real world geometry and you will be able to overlay layers from other sources correctly.

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