A Word on Sentinel-1 Interferometric Wide Swath (IW) Data

The Interferometric Wide (IW) swath mode is the main acquisition mode over land for Sentinel-1. It acquires data with a 250km swath at 5m x 20m spatial resolution (single look). IW mode captures three sub-swaths using the Terrain Observation with Progressive Scans SAR (TOPSAR) acquisition principle.

With the TOPSAR technique, in addition to steering the beam in range as in ScanSAR, the beam is also electronically steered from backward to forward in the azimuth direction for each burst, avoiding scalloping and resulting in homogeneous image quality throughout the swath. A schematic of the TOPSAR acquisition principle is shown below.

View the Products

In the Product Explorer window, you will see the products listed.

• Double-click on each product to expand the view.
• Double-click Bands to expand that folder for each product.

Orbit auxiliary data contain information about the position of the satellite during the acquisition of SAR data. Orbit data are automatically downloaded by Sentinel-1 Toolbox and no manual search is required by the user.

The Precise Orbit Determination (POD) service for Sentinel-1 provides Restituted orbit files and Precise Orbit Ephemerides (POE) orbit files. POE files cover approximately 28 hours and contain orbit state vectors at fixed time steps of 10-second intervals. Files are generated one file per day and are delivered within 20 days after data acquisition.

If Precise Orbits are not yet available for your product, you may select the Restituted orbits, which may not be as accurate as the Precise Orbits but will be better than the predicted orbits available within the product.

• In the Back-Geocoding tab, select the Digital Elevation Model (DEM) to use and the interpolation methods. The default is the SRTM 3 Sec DEM and this can be used for this tutorial. Areas that are not covered by the DEM or are located in the ocean may optionally be masked out.
  • Select the option to Output Deramp and Demod Phase if you require Enhanced Spectral Diversity to improve the coregistration.
• In the Write tab, set the Directory path to your working directory.
• Click Run to begin coregistering the data. The resulting coregistered stack product will appear in the Product Explorer window with the suffix Orb_Stack.

Visualize Interferometric Phase

Once the interferogram product is created (product [4] in the Product Explorer, appended with Orb_Stack_ifg), expand the product and double-click the Phase_… band to visualize the interferometric phase (as in the Visualize a Band section). When zoomed in, you may still see the demarcation zones between bursts in this initial interferogram. This will be removed once TOPS Deburst is applied.

What it Means:

Interferometric fringes represent a full 2π cycle of phase change. Fringes appear on an interferogram as cycles of colors, with each cycle representing relative range difference of half a sensor’s wavelength. Relative ground movement between two points can be calculated by counting the fringes and multiplying by half of the wavelength. The closer the fringes are together, the greater the deformation on the ground.

Flat terrain should produce constant or only slowly varying fringes. Any deviation from a parallel fringe pattern can be interpreted as topographical variation.

TOPS Deburst

To seamlessly join all bursts in a swath into a single image, we apply the TOPS Deburst operator from the Sentinel-1 TOPS menu.

• Navigate to the Radar > Sentinel-1 TOPS menu.
• Select the S-1 TOPS Deburst option.
• Keep the default values, ensuring that product [4] (tagged _Orb_Stack_ifg) is selected as the Source and the output Directory path is correct.
• Click Run.

The resulting product will be appended with Orb_Stack_ifg_deb.

Topographic Phase Removal

To emphasize phase signatures related to deformation, topographic phase contributions are typically removed using a known DEM. In Sentinel-1 Toolbox, the Topographic Phase Removal operator will simulate an interferogram based on a reference DEM and subtract it from the processed interferogram.

• Navigate to the Radar > Interferometric > Products menu.
• Select the Topographic Phase Removal option.

The Sentinel-1 Toolbox will automatically find and download the DEM segment required for correcting your interferogram of interest.

• Keep the default values (check that the source is product [5]) and click Run.

After topographic phase removal, the resulting product (appended with Orb_Stack_ifg_deb_dinsar) will appear largely devoid of topographic influence.

Multi-looking

The first step to improve phase fidelity is called multi-looking. Navigate to the Radar dropdown menu.

• Select the Multilooking option (bottom of the menu). A new window opens.
• Ensure that the source is set to product [6] (_dinsar) and the output Directory is correct. 
• Click on the Processing Parameters tab.
  
  Note: The coherence band (starting with coh_) will be essential if your intention is to unwrap the interferogram or create a deformation map. Coherence is used to verify the legitimacy of the derived phase data; typically data with coherence values less than 0.3 are thrown out. In this interferogram, coherence is highest in urban areas and lowest in vegetated areas.
   
• Use the Ctrl button to select the i, q, and coh bands from the list as the Source Bands to be multi-looked.

Because the phase band is virtual, it is only a temporary visualization of the interferogram. After multi-looking is performed, this band will disappear, but it will be restored in the following Goldstein Phase Filtering step.

• In the Number of Range Looks field, enter 6, which will result in a pixel size of about 26.5 meters.

In essence, multi-looking performs a spatial average of a number of neighboring pixels (in our case, 6×2 pixels) to suppress noise and proportion the image correctly. This process comes at the expense of spatial resolution.

• Click Run. The resulting product name is appended with Orb_Stack_ifg_deb_dinsar_ML.

Phase Filtering

In addition to multi-looking, we perform a phase filtering step using a state-of-the-art filtering approach.

• Navigate to Radar > Interferometric > Filtering.
• Select Goldstein Phase Filtering.
• Ensure the source is set to product [7] (_ML) and that the output Directory path is correct.
• Keep the default values and click Run.

The resulting product name is appended with Orb_Stack_ifg_deb_dinsar_ML_flt.

After phase filtering, the interferometric phase is significantly improved, and the dense earthquake deformation-related ring pattern is now clearly visible.

This interferogram is now ready to be unwrapped. Instructions are available via ASF’s How to Phase Unwrap an Interferogram data recipe. If you came to this recipe via the Deformation Map Recipe, proceed with the steps iterated there. You can also continue with the geocoding and export steps if you just want a wrapped phase image.

Geocoding and Exporting in a User-defined Format

To make the data useful to geoscientists, the interferometric phase image needs to be projected into a geographic coordinate system using a DEM-assisted geocoding step. If your intention is to unwrap and further process the interferogram, this step should be skipped and performed at a later point.

Geocoding

• Navigate to Radar > Geometric > Terrain Correction.
• Select the Range-Doppler Terrain Correction option.
• In the I/O Parameters tab of the Range-Doppler Terrain Correction window, select product [8] (or the product generated in the previous step) as the Source.
• In the Processing Parameters tab:
  • Select the Phase band from the Source Bands list as the Source Band to be geocoded.
  • Adjust the pixel spacing if desired.
  • Click Run.
• If desired, re-run the process for the Intensity band.

The resulting product name is appended with Orb_Stack_ifg_deb_dinsar_ML_flt_TC. The resulting geocoded interferogram of subswath IW1 is located below.

Export Data

The final geocoded data can be exported from Sentinel-1 Toolbox in a variety of formats. 

In addition to GeoTIFF and HDF5 formats, KMZ and various specialty formats are supported. The figure below shows a KMZ-formatted interferogram overlaid on Google Earth.

• Select the Geocoded product to export in the Product Explorer.
• To find the export options, select Export from the File menu.
  • Note that if the Phase and Intensity bands were geocoded together, you will generate a 2-band GeoTIFF if you select that product.
  • If you want to export a one-band product or generate a KMZ file, you will need to geocode the Phase band by itself and select that single-band product for export.

Optional — Combining Subswaths

Create a Geocoded Differential Interferogram of the Kumamoto Earthquake by Merging Subswaths IW1 and IW2:

To create the merged product, run the steps below, noting the new step.

• Run the Coregistration step: This time select IW2 in the TOPS Split and TOPS Split(2) operator tabs to coregister an InSAR pair for subswath IW2.
  • The Subswath designation must match under the two TOPS Split tabs, or the process will not run.
  • Make sure to create a new filename under the Write tab to avoid overwriting the IW1 stack result.
• Run the Interferogram Formation step: Using the new coregistered IW2 stack as input, create an IW2 subswath interferogram.
• Run the Debursting step: Deburst the IW2 interferogram.
• New Step — Merge Subswaths:
  • To merge deburst SLC subswaths, go to the Radar > Sentinel-1 TOPS menu and select the S-1 TOPS Merge option.
  • Select the deburst IW1 (Orb_Stack_ifg_deb) and deburst IW2 interferograms as inputs. Use the plus icon to browse from a directory, or the next button to add all the layers open in your project, then remove all but the two deburst products from the list.
• Run the remaining steps to create the final merged interferogram.

Interferogram Interpretation

The interferometric phase carries a wealth of information about surface deformation (strength and direction of motion) and the location of the surface rupture. The phase map is also a proxy for other earthquake-related parameters such as the energy released during an event and the amount of shaking experienced across the affected area.

Image Anomalies

SAR images sometimes suffer unavoidable radio frequency interference (RFI) from microwave sources on the ground transmitting in the same frequency. This is more common in maritime settings but it is also visible in the later image in this SLC pair. Consequently, this region on the image will generate inaccurate phase information. The coherence band will indicate this and provide a method for masking the anomaly out of interferogram or later products using band maths. Alternatively, selecting another SLC at a later point in time may decrease the coherence and quality of the entire interferogram.