This recipe will go over the steps to create a mosaic of two GRD products in different swaths. Rather than taking individual images of each area, the Sentinel-1 satellites intake large swaths or paths of data as they orbit the Earth. In order to facilitate the use of Sentinel-1 products and avoid unruly image sizes, the swaths of data are divided into more manageable Alaska Satellite Facility framed slices. Because of the manner in which the Sentinel-1 satellites acquire images, creating a mosaic of these products in the horizontal direction requires a different set of steps than in the vertical direction. For steps on how to create a mosaic of products located in the same path, see Alaska Satellite Facility’s swath mosaic recipe.

On the boundary of two paths or adjacent frames, there is considerable overlap and the lateral edges of the products may also have a significant amount of border noise. This will have to be eliminated before the frames themselves can be combined. The mosaicking of multiple swaths relies on resampling of the overlap area, patching them together. Because the mosaic process for horizontally aligned products results in a rough amalgamate of the two or more swaths, the resultant image is no longer a product that can be processed normally. However, some operations like speckle filtering will still work. This same procedure can be applied for assembled swaths, like the product created in Alaska Satellite Facility’s swath mosaic recipe.

Preliminary Pre-Processing

Open the Products in Sentinel-1 Toolbox

1. Open the Sentinel-1 Toolbox.
2. Use the Open Product button in the top toolbar and browse to the location of the downloaded GRD products.
3. Press and hold the Ctrl button (Command key for Mac OS X) to select the .zip files and press Open.

Note: Do not extract the files, if you did unzip the files, select the manifest.safe file from each Sentinel-1 product folder.

View the Products

1. In the Products Explorer, you will see the opened products.
2. Double-click on a product to expand it.
3. Expand the Bands folder and you will find two bands for each polarization: an amplitude band, which is actually the product, and a virtual intensity band, which is there to assist you in working with the GRD data.
4. To view the data, double-click on Amplitude_VH band.
5. Zoom in using the mouse wheel and pan by clicking and dragging the left mouse button. 

Steps to Generate a Mosaic

Note: After this step is complete, you will not be able to perform most SAR operations on your resultant image. If you need to perform any other processes, they should be done to the individual products rather than the mosaic.

SAR Mosaic Wizard

1. In the Product Explorer, Ctrl-click (Command-click for Mac OS X) each of the last produced products in the previous step. If you removed border noise, select the _Bdr products. If not, select the _Orb products. Keep the output format as BEAM-DIMAP.
2. Navigate to File > Close Other Products.
3. From the top toolbar, navigate to Radar > SAR Wizards > SAR Mosaic Wizard. A corresponding window will open outlining the process. When going through each stage of this procedure, be sure not to close this window. Red text will show up after each sub-step, indicating a finished process, but only at the very end should you exit the SAR Mosaic Wizard window. Otherwise, you will have to start from the beginning.
4. Click Next.
5. In the SAR Mosaic Wizard: Step 1 Input window, click on the Add Opened button, the icon consisting of a plus sign in front of a rectangular window. This will add both of the swaths to the list. Keep the other parameters as the defaults.
6. Specify the output folder
7. Click Next.
8. In the SAR Mosaic Wizard: Step 2 Viewing the footprint window, you can view the footprint of your products. Zoom in to the location of your products by using the mouse wheel. To move the view, right-click and hold while dragging.
9. Click Next. The Batch Process Terrain Correction window will appear.
10. This batch process will apply another series of pre-processing operations to each of the swaths. The swaths will be calibrated to sigma naught, multi-looked and geocoded.
    • In the I/O Parameters tab, ensure the output directory is correct.
    • If your products are above 60° North, you will need to open the Terrain-Correction tab and select a different Digital Elevation Model, such as ACE30. In this case, our products are near the equator and you can leave the tabs and parameters alone. A DEM from the SRTM 3Sec set will be used. The other parameters can be left as they are.