This data recipe is for users who wish to generate an RTC image from Sentinel-1 data using easy-to-follow instructions in a graphical user interface (GUI).
This recipe was written for version 8 of the SNAP Sentinel-1 Toolbox (S1TBX). While it also works with versions 6 and 7, we recommend updating to version 8, because it generates better RTC products.
Document Contents:
- Background
- Materials List
- Preparing Software and Workspace
- Steps for Generating an RTC Image
- OPTIONAL: View RTC Image using GIS Software
- ArcGIS
- QGIS
Background
Distortions in SAR imagery are induced by the side-looking nature of SAR sensors and are compounded by rugged terrain. Radiometric Terrain Correction (RTC) applies both of the following corrections to generate a superior product for science applications:
Terrain correction corrects geometric distortions that lead to geolocation errors by moving image pixels into the proper spatial relationship with each other based on a Digital Elevation Model (DEM).
Radiometric correction removes the misleading influence of topography on backscatter values.
Materials List
i. S1TBX is integrated into ESA’s Sentinel Application Platform software.
ii. Download the Sentinel Toolboxes installer for your Operating System.
iii. Run the installer and follow the prompts to install the software.
2. Sentinel-1 data file
i. You may download this sample file, or find a different Sentinel-1 GRD data file using ASF’s Vertex data portal. A NASA Earthdata Login username and password are required.
Preparing Software and Workspace
- Open the S1TBX application
2. Make sure that any available updates have been applied. A message will usually display in the bottom right corner when updates are available, but you can also check by selecting Check for Updates from the Help menu.
i. Create a new folder (for example, Sentinel_RTC_S1TBX) to contain the files for this recipe. Download the Sentinel-1 GRD zip file into this new folder.
Note: Do not unzip the GRD granule file. S1TBX uses the .zip file structure and will not recognize the extracted.SAFE directory.
Steps for Generating an RTC Image
A. Open the GRD granule in S1TBX
- Open S1TBX (See Section C).
- From the File menu, select Open Product or click the Open Product icon (Figure 1)
Figure 1: Open the GRD product .zip file in S1TBX
3. Browse to the folder that contains your Sentinel-1 data, and double-click the .zip file (or highlight the file and click <Open>).
Note: If you get an error that no appropriate product reader was found, the zip file contents may have been corrupted. Make sure that all SNAP updates have been applied (Section C), restart S1TBX, and download the GRD granule again.
4. Note that as new products are generated during S1TBX processing, they will be displayed in the Product Explorer window and will be numbered sequentially (in square brackets) in the order in which they are generated (Figure 5).
B. Apply Radiometric Terrain Correction
- Apply Orbit File
i. Highlight the GRD granule in the Product Explorer pane by clicking it once, then select Apply Orbit File from the Radar menu (Figure 2).
Figure 2: Apply Orbit File in Radar Menu
ii. In the I/O Parameters tab, set the Directory path to the same folder that contains your .zip file, and verify that the source is the correct granule (Figure 3).
Figure 3: Apply Orbit File – I/O Parameters
iii. In the Processing Parameters tab, verify that Orbit State Vectors are
set to Sentinel Precise (Auto-Download). This will automatically
find the appropriate Orbit file to use from the ESA archive (Figure 4).
Figure 4: Apply Orbit File – Processing Parameters
iv. Click <Run>. A Writing Target Product popup shows your progress.
Note: Depending on the processing power of your computer, this step can take from less than a minute to several minutes to complete.
v. The output from the Apply Orbit File process now appears as the second product in the Product Explorer pane, with an _Orb tag appended to the filename and prefixed with [2] (Figure 5).
Figure 5: Naming and numbering convention in the Product Explorer
2. Calibrate Data
i. Single-click the _Orb file in the Product Explorer pane.
ii. In the Radar menu, select Radiometric > Calibrate (Figure 7).
iii. In the Calibration dialog box, verify that the source is the _Orb product and that the directory is the same output folder.
Figure 7: Selecting the Calibrate function
in the Radiometric group of the Radar menu
iv. Under the Processing Parameters tab, select Output beta0 band (the next step requires a beta0 product), and remove the check from any of the other output options (Figure 8).
v. Your granule may have multiple Polarizations listed. By default, all the polarizations listed will be processed. To select only specific polarizations, click on each desired. Hint: use the key and your mouse to select multiple polarizations.
Figure 8: Calibration Processing Parameters
vi. Click the <Run> button.
vii. The output will have an _Orb_Cal tag at the end of the filename and [3] in front.
3. Flatten Terrain Radiometrically
i. Single-click the _Orb_Cal product in the Product Explorer pane.
ii. In the Radar menu, select Radiometric > Radiometric Terrain Flattening (Figure 9).
Figure 9: Radiometric Terrain function in the
Radiometric group of the Radar menu
iii. Verify that the source is the _Orb_Cal product and that the output directory is correct.
iv. In the Processing Parameters tab, the Digital Elevation Model is already listed; it is selected automatically to match the geolocation of the GRD granule.
Note: If you processed multiple polarizations in the previous step, there will be a band listed for each polarization. Again, if you want to process all of them, don’t click in the list box. To select a specific polarization, click on the desired band, or use the button and mouse to select multiple bands.
v. Click the <Run> button. This step may take up to an hour to complete.
vi. The output will have an _Orb_Cal_TF tag at the end of the filename for product [4].
4. Apply Geometric Terrain Correction
i. Single-click the _Orb_Cal_TF product in the Product Explorer pane.
ii. In the Radar menu, select Geometric > Terrain Correction > Range- Doppler Terrain Correction (Figure 10).
Figure 10: Range-Doppler Terrain Correction in the Radar menu
iii. Verify that the source is the _Orb_Cal_TF product and that the output directory is correct.
iv. In the Processing Parameters tab, change the following settings
(Figure 11):
1) Click the button next to Map Projection. Select UTM / WGS 84 (Automatic) from the Projection dropdown menu and click <OK>. The correct UTM zone will be automatically selected based on the granule location and displayed for the Map Projection.
2). If desired, you may set the pixel spacing to a value other than the source pixel spacing.
3). Note that if you processed multiple polarizations in previous steps, there will be multiple Source Bands listed.
4). Leave all remaining settings as the default unless you want to output additional products, such as the DEM or incidence angle bands.
Figure 11: Range Doppler Terrain Correction
v. Click the <Run> button. This step may take on the order of 30 minutes to complete.
vi. The output will have an _Orb_Cal_TF_TC tag at the end of the filename for the product [5].
5. View the RTC image in S1TBX
i. Double-click the _Orb_Cal_TF_TC product in the Product Explorer pane to expand the product contents.
ii. Expand the Bands item.
iii. Double-click on the image file named Gamma0_VV. The image will open in the S1TBX interface at the bottom, and you may use other application tools to interact with the image. You may wish to explore the Tool Windows available in the View menu (Figure 12).
iv. The Navigation window allows you to pan and zoom.
v. The World View window shows where the granule is located on a globe-like view, while World Map shows the granule location on a flattened map of the world.
vi. The Colour Manipulation window displays the values contained in the image and includes tools for visualizing the image and data values.
Note: If the granule you processed has very rugged terrain, you may notice NoData pixels in mountainous areas. This is a consequence of the Sentinel-1 look angle and shadows created by high-relief topography. This effect was more pronounced in products processed by versions 6 and 7 of S1TBX, but there are significantly fewer NoData pixels in products processed by version 8 (Figure 13).
Figure 13: Comparison of NoData pixels (shown in red)
in RTC products from Version 6 (left) and Version 8 (right)
6. Save the S1TBX Session (Optional)
i. Under the File menu, select Session — Save Session As.
ii. Navigate to the directory where you would like to save the session and enter a name for the session file.
iii. Saving a session retains all of the products generated during the RTC process, allowing for easy viewing of the different steps, and amendments to the settings. This is especially useful if you are not able to finish the entire process in one sitting and want to save your work to continue later.
iv. To re-open a session:
1) Under the File menu, select Session — Open Session.
2) Navigate to your saved session and double-click to open.
OPTIONAL: View RTC Image using GIS Software
The _Orb_Cal_TF_TC outputs are in ENVI format, comprised of a header file (.hdr) and an image file (.img). To view the data in a GIS, both files must be present in the same folder. If you have not made any changes to the _Orb_Cal_TF_TC product folder, these files will be stored together, allowing the image to be added easily to either ArcGIS or QGIS.
ArcGIS
- Open ArcMap and use the Catalog window to navigate to the folder containing your S1TBX products. To open the Catalog window, select it from the Windows menu (Figure 14).
Figure 14: Opening the Catalog window in ArcGIS
i. Right-click on Folder Connections and select Connect To Folder (Figure 15).
Figure 15: Creating a new Folder Connection in ArcMap
ii. Navigate to your product’s parent directory and click <OK>.
iii. Open the _Orb_Cal_TF_TC data folder and drag the Gamma0_VV.img file into your project. Build pyramids if desired (this will take a while, but it makes zooming and panning faster later on).
NOTE: Only the .img file will appear in the Catalog window; the .hdr file is hidden from view but used by ArcGIS to properly display the .img file.
2. The image may appear very dark in GIS. You can change the stretch properties to display the image in a more visually useful way. Or, you can refer to the ASF Data Recipe How to View RTC Images in ArcGIS for instructions on converting the values from power to another scale (amplitude or dB) for easier viewing.
Note: Converting to a different scale creates a new raster (image) with different pixel values. Changing the stretch only adjusts how the image is displayed, without changing the actual pixel values.
3. To change the stretch without creating a new image:
i. Right-click on the layer in the Table of Contents, select Properties (Figure 16).
Figure 16: Opening Layer Properties
ii. In the Symbology tab, under the Stretched option, set the Stretch Type to Standard Deviations (the default is Percent Clip).
iii. The default value for the number of standard deviations is 2.5, which is fine for many RTC images. You may find that a different number works better for your image; try a few others and use the <Apply> button to find the value that you like best for your image. When you’ve decided on a value, click the <OK> button.
iv. When using the Identify tool on a stretched image, the main Identify window will still display the original power value. The expanded feature properties will display both the Stretched value and the original (power) pixel value.
QGIS
- Open QGIS and use the Browser panel to navigate to your data folder (Figure 17).
i. If your Browser panel is not open, right-click in the open grey area by the toolbars and select Browser Panel from the dropdown list.
Figure 17: Navigate to your folder using the Browser panel
ii. Expand the _Orb_Cal_TF_TC.data folder and drag the Gamma0_VV.img
file into your Layers panel.
Note: Unlike ArcGIS, the file structure in QGIS is transparent, so you can see files in the Browser panel that are not GIS-readable. You can see that each .img file has an associated .hdr file. As in ArcGIS, you only need to add the .img file, and it will recognize the associated .hdr file and use the contents appropriately to display the image.
2.. As in ArcGIS, you can adjust the appearance of the image by changing the Stretch settings. Or, refer to the ASF Data Recipe How to View RTC Images in QGIS for instructions on converting the values from power scale to another scale (amplitude or dB) for easier viewing.
Note: Converting to a different scale creates a new raster (image) with different pixel values. Changing the stretch only adjusts how the image is displayed, without changing the actual pixel values.
3. To change the stretch without creating a new image:
i. In QGIS 2.18, the default Min/Max stretch value settings defaulted to the Cumulative count cut setting, which tends to render RTC images fairly well.
ii. With the release of QGIS 3.0, the default settings changed so that the stretch Min/Max values match the actual range of data values in the image by default. This will generally result in a very dark RTC image.
iii. You can adjust the stretch settings in either case, but it will be more necessary in QGIS 3.X.
Note: The symbology legend beneath the layer in the Layers panel will display the range of stretched values, but if you use the Identify Features tool (located under the View dropdown), it will only display the original power value.
QGIS 2.18
- Right-click the Gamma0_VV layer in the Layers panel and select Properties.
- Under the Style tab, expand the Load min/max values section.
i. The default in QGIS 2.18 is the Cumulative count cut option, which generally displays well. Or, you can adjust the values, click the <Load> button to recalculate the min/max values, and click the <Apply> button at the bottom of the dialog box to view how the changes impact the image display. Click <OK> to keep the change and finish.
ii. Another approach is to use the Mean +/- standard deviation option. The default of 2 is usually okay, but in some images, you may want to experiment with other values. Click the <Load> button to calculate changes and use the <Apply> button if you’d like to test the settings before clicking <OK> to finish.
QGIS 3.x
- Right-click the Gamma0_VV layer in the Layers panel and select Properties.
- Under the Symbology tab, expand the Min / max values settings section. The default in QGIS 3.x are the minimum and maximum values, which tend to display very dark RTC images.
i. To use the same settings as the QGIS 2.18 default values, select the Cumulative count cut option. The default values are probably appropriate, but you can change the values and use the <Apply> button to view the effects before clicking <OK> to close the Layer Properties window.
ii. Another approach is to use the Mean +/- standard deviation
option. The default of 2 is usually okay, but in some images,
you may want to experiment with other values. Use the <Apply> button to view the effects before clicking <OK> to close the Layer
Properties window.