Basic Processing of Radarsat-1 data in Snap ESA

Introduction

On April 1st, 2019. NRCAN released 35000 Radarsat-1 scenes for open-access, with anticipation of releasing more in the future. This allows anyone to access SAR scenes covering Canada as far back as 1995 for free! This is incredible progress, however, very little documentation exists on Radarsat-1 file conventions or processing steps to create a usable product. This tutorial looks to fill this gap; provides users with basic background on Radarsat-1 and its file conventions, along with instructions on downloading Radarsat-1 scenes, and basic processing techniques within open-source SNAP ESA software.

Retrieving a Radarsat-1 Product

EODMS Account

Before we begin working in SNAP, we first have to download a Radarsat-1 product from NRCAN. We begin at the Earth Observation Data Management System. A main screen as per Figure 1 should greet you. In order to download products, an account must first be created. This can be done by clicking on the Register (Required to Order) link under the Users subsection on the left-hand side of the page. Follow the on-screen instructions to create your account, confirm your email, and change your password.

If you already have an account, log into EODMS by clicking on the Login link under the Users subsection on the left-hand side of the page.

Finding a Scene

Once an account is created, we can begin searching for a scene. Begin by clicking the Search under the Getting Started subsection on the left-hand side. What will appear are a series of grey boxes indicating the steps needed to locate and download our scene. Each numbered step below corresponds to the like numbered grey box.

1. The first step is to select our area of interest. This can be done is numerous ways, as indicated by the eight different drop-down menu's under this step. For this example, Draw an Area will be used. Information on the use of each tool available to select an area of interest can be found by selecting the ? beside each drop-down.

A. Begin by using the map on the right half of the screen to zoom into an area of interest, for this example I will be focusing on the oilsands operations around Fort McMurray Alberta.

B. Select the Draw and Area drop-down menu and then the Rectangle option. Draw the rectangle around an area that you wish to be included in your scene. See Figure 2 for an example.

2. Once an area of interest is selected, our date range must be selected. Select the Date Range option and input dates between which you wish to find a scene. For this example, I am looking for any images in 2012.

3. Now, we must select which type of data we are looking for. Select ONLY the RADARSAT-1 Open Data Products check-box, this can be found under the Radar Satellites Subsection.

4. The next step is to select specific options we wish our data to have. This includes a certain incidence angle range, beam modes, pass direction and so on. This is optional as it is dependent on the specific needs of the user. Under the Beam Mode selection box, select Standard.

NOTE: Due to a bug within SNAP, we can only work with Standard beam mode imagery. Standard beam-mode imagery is not available everywhere, so this may affect your search results.

5. Once the other steps are completed, select the Search button. If results are found, a new Results tab will open showing all matching results. If no results are found, refine your search dates and/or search area until results are found.

Downloading the Scene

The Results will list all scenes that match your search criteria, and will look similar to Figure 3. Hover over each result to highlight its footprint on the map. Select a scene you wish to work with and check the Order box. A window as per Figure 4 will appear. Check the box under the Select item(s) tab and then the Update Cart button. This will close the window and open a forth tab titled Cart. In this tab you will see your scene listed. Click the Submit Satellite Product Order button. This will open a tab asking for the Order Priority and Delivery Destination(s). Leave both of these as their defaults and click the Order button.

Once submitted, an email will be sent confirming your order, along with a notice that another email will sent once the scene is ready for order. This second email could take some time to arrive depending on the amount of traffic on EODMS when your order is sent in. Once this email arrives, select the HTTPS link within the email, this will open a tab in your internet browser. In this tab, click the link beside the file icon, followed by the link ending in .zip to download your scene. Save it to a place that is easily accessible later

Snap ESA Introduction

To load our scene into SNAP, navigate to and select File->Import->SAR SENSORS->RADARSAT 1. Navigate to the location your scene was extracted into using the file browser and double-click the corresponding file. Due to the age of the data, there is a chance an error will occur on the import step due to corruption. If this is the case, you will need to go back to the previous steps and download a new scene.

Subset

The first step within SNAP that should be conducted is to create around your specific study area. This is not a required step, as the full scene can be used for this tutorial, just be aware that using the full scene will require longer processing times than using a subset.

Begin by navigating to Raster->Subset. A window as per Figure _ should appear. Within this interface, we can subset in three ways: Drawing a subset area, Pixel coordinates, or Geographic coordinates. If your subset area does not need to be precise, the easiest method is to simply manipulate the subset bounding box on the map. If precision is necessary, enter the geographic coordinates. If entering coordinates, make sure your subset region bounding box appears on the map, if not, your study area may not be within the image bounds. Once your preferred subset region has been set, hit OK. A new product will appear within the Product Explorer. Open up the Amplitude band as described in the previous step, notice how the image has been clipped to only contain your study region.

Speckle Filter

The next step we need to conduct in processing our scene is to run a Speckle Filtering operation. Speckle is the salt-and-pepper effect that appears in radar imagery. Generally this is considered error or noise and should be averaged out to remove false returns and achieve a better insight on return from the ground. Select the subsetted product (or the original scene that was imported into SNAP if subsetting was not conducted), and then navigate to Radar->Speckle Filter->Single Product Speckle Filter. A window will appear with two tabs as per figure _.

The first tab contains input and output parameters, most of which should be auto filled. Make sure the correct input file is selected, the output name can be left the same, change the directory to your working directory using the .... The output format that should be used in intermediate steps is BEAM-DIMAP, SNAP's format.

The second tab contains the operation settings. For this example, Select Amplitude as our source band, a Boxcar filter, and X & Y filter size of 5.

With these parameters set, hit Run. A new object will appear within the Product Explorer once the task is completed. Open up your Amplitude band has before, and notice how the filtering has smoothed out our image. Below the product explorer, open the Colour Manipulation tab and look at the histogram. Compare it to the histogram of the non-filters product and notice how it has become a smoother and better defined bell. Feel free to try other filtering methods and filter sizes to compare and contrast their differences.

Slant-Range to Ground-Range

The final processing step is to convert our scene from slant-range to ground-range. Our scene in its current form is orientated on an imaginary slant relative to the sensor in range and azimuth coordinates. This means the image is positioned relative to the sensors flight path and not general map coordinates. To convert our image to mapping coordinates, the image must be re-sampled in a process known as an ellipsoid correction. To perform this operation, select the filtered product in the Product Explorer, and navigate to Radar->Geometric->Ellipsoid Correction->Geolocation-Grid.

Same as the previous step, a window with two tabs will appear as summarized in Figure _. The settings for the first tab are the same as the previous step. In the second tab, select the Amplitude band, and pick a re-sampling method (I recommend Bilinear Interpolation), and finally select a map projection. I good option to select for this setting is UTM / WGS84 (Automatic), which will determine which UTM zone to use automatically. Once prepared, hit Run. A new tab will appear in the Product Explorer once the process is completed. The product is now ready for export.