SAR Processing In Geomatica OrthoEngine
Because of its all-weather and all-day monitoring capabilities, Radar remote sensing offers a number of advantages for Earth-surface and feature observation. Radar sensors can see through cloud and rain and at night, allowing for remote observation at any time. Some features, such as ice, ocean waves, and geological structures can often be seen better in radar images than in optical images.
Over the years, radar remote sensing has provided comprehensive, timely, and accurate information of the Earths surface. Due to the unique nature of radar systems, specific tools are required to extract useable information from the data. The comprehensive radar functions in Geomatica allow you to fully exploit the power of radar for use in a wide variety of applications.
The SAR Specific Model
For orthorectification of SAR data, Geomatica uses the RADAR Specific Model which uses the additional parameters in the orbit data of SAR data to diminish amount of ground control points (GCPs) required. The extra parameters maintain the positional accuracy and high levels of detail in the model, but the number of GCPs needed is reduced to few or none. This math model does not use tie points since each scene is computed using the GCPs of that scene only. If you have more than eight well distributed GCPs, both Toutin's Model and the SAR Specific Model will give you similar results.
The following table describes sensor and product levels supported by the SAR Specific Model.
|ALOS PALSAR||L1.5 SGF|
|Envisat ASAR||ASA_IMM_1P: Level 1b Image Mode Medium Resolution|
|ASA_IMS_1P: Level 1b Image Mode SLC|
|ASA_IMP_1P: Level 1b Image Mode PRI|
|ASA_APM_1P: Level 1b Alternating Polarization Mode Medium Resolution|
|ASA_APS_1P: Level 1b Alternating Polarization Mode SLC|
|ASA_APP_1P: Level 1b Alternating Polarization Mode PRI|
|ASA_WSM_1P: Level 1b Wide Swath Mode Medium Resolution|
|RADARSAT||SLC : Single Look Complex|
|SGF : SAR georeferenced Fine Resolution|
|SGX : SAR georeferenced Extra-Fine Resolution|
|SSG : SAR Systematic Geocorrected|
|SPG : SAR Precision Geocorrected|
|Georeferenced for images produced in Canada PRI level for images produced by ESA|
|Level 1A (SCS) : Single Look Complex Slant Range|
|Level 1B (MGD) : Detected Ground Multi-Look|
|ERS||Georeferenced for images produced in Canada|
|PRI level for images produced by ESA|
About ALOS PALSAR
PALSAR provides higher performance than the previous JERS-1 SAR sensor. In addition to the fine resolution in a convention mode, PALSAR has a ScanSAR observation mode which enables the sensor to acquire a 250 to 350km width of SAR images (depending on the number of scans) at the expense of spatial resolution.
This swath is three to five times wider than conventional SAR images. Another advantage of PALSAR is the polarimetric characteristic. Full polarimetric SAR enables more detailed measurements compared to conventional single polarization SAR.
There are numerous applications for PALSAR data. Examples are land area basin mapping, coastal area basin mapping, monitoring of environment and natural disasters such as oil spill and flooding.
Polarmetric application study is steadily in progress in fields such as forest fire monitoring, classification of vegetation (height), water content in vegetation, monitoring of snow cover, condition of ice, flood monitoring and soil moisture.
Setting up A PALSAR Project In OrthoEngine
The example gives you an opportunity to work with Geomatica OrthoEngine to carry out orthorectification of PALSAR L1.5G data. Sample PALSAR data for use with this tutorial can be downloaded from http://www.alos-restec.jp/sampledata_e.html
Creating the Project
The first step in correcting your data is to create a project within OrthoEngine. The project file contains information about the images being corrected, as well as projection information and information about the model used to correct your data.
There are three main steps for creating a project:
- Selecting the math model
- Setting the projection
- Importing images
The following instructions outline the steps for creating a project specific to working with PASLAR data. This workflow is also much the same for other supported SAR sensors such as RADARSAT and ASAR.
Starting OrthoEngine Windows Systems
To start OrthoEngine on Windows systems click the OrthoEngine icon on the Geomatica toolbar. The main OrthoEngine toolbar opens.
Creating a Project and Importing Radar Data
OrthoEngine works on a project-by-project basis. Therefore, you need to open an existing project or create a new project before you gain access to the functions within OrthoEngine. In this lesson, you create a new PALSAR project.
To create a new project:
- Click New on the File menu. The Project Information dialog box opens.
- Click Browse. The File to Create dialog box opens.
- Choose a location to store your PALSAR Orthoengine project.
- Enter the name PALSAR.prj in the File name field.
- Click Open. The File to Create dialog box closes. The path and filename appear in the File name field in the Project Information dialog box.
- Enter the name PALSAR Project in the Name field.
- In the Description field, type PALSAR ortho project.
- In the Math Modelling Method pane, select Satellite Orbital Modelling.
- In the Options pane, select the ASAR/PALSAR/RADARSAT Specific Model.
- Click Accept. The Project Information dialog box closes. The Set Projection dialog box opens.
Setting the Output Projection Parameters
The projection information needs to be set at the beginning of each project. In the Set Projection dialog box, enter the projection information for project.
- Select UTM from the pull-down menu in the Output Projection pane. The UTM Zones dialog box opens.
- Under the Datums tab, select D000.
- Click Accept. The UTM Zones dialog box opens.
- Click to select Zone 53 and click Accept.
- Click to select Northern Hemisphere.
- Click Accept.
- For the Output Pixel Spacing, type 6.25.
- For the Output Line Spacing, type 6.25. The value of 6.25 meters represents the desired resolution of the ortho images and the mosaic file.
Setting the GCP Projection
- Click Set GCP Projection based on Output Projection in the GCP Projection section. The GCP Projection adopts the same settings used for the Output Projection.
- Click Accept. The Set Projection dialog box closes.
Reading Satellite Images from a CD
Now that you have created the project file, you need to images to your project file. OrthoEngine reads the raw satellite data, saves the imagery into a PCIDSK file, and adds a binary segment containing the ephemeris data (orbit information) to the file. The project file will then contain the filename and location of each input image.
To input the images to the current project:
- From the Processing Step menu on the main toolbar, select Data Input.
- A new toolbar with two icons appears on the main panel. You can input data from either CD-ROM or PCIDSK file.
- On the Data Input toolbar, click Read CD-ROM data. The Read CD-ROM dialog box opens.
- For the CD Format, select PALSAR (CEOS).
- Click Select beside CD Image Format. A File Selection window opens.
- Navigate to the folder containing your RAW L1.5 PALSAR data
- Select the file *.DAT file. (e.g. PASL15L0701120614590704240000_SGF.dat)
- Click Open. The path and filename are updated in the dialog box.
- For the Requested Channels, click 1.
- For the PCIDSK File name, browse to your desired output directory and give the file an output *.pix filename. A report file will also be created.
- Click Read. Once the file is read, it is part of your project.
- Repeat steps 4 to 11 to read data from additional raw scenes you may want to add.
Saving your Project
- Click File on the Menu bar.
- Click Save on the File menu.
The project file PASLAR.prj is saved in the specified folder. In addition, OrthoEngine automatically creates a backup file every 10 minutes. The backup file uses the same file name as your project file, but with a .bk extension.
Tip: If you need to revert to the backup file, rename the backup file so that it uses the .p rj extension. OrthoEngine can load this project file in the normal way.
Orthorectifying PALSAR Data
When working with the ASAR/RADARSAT Specific Model, ground control points (GCPs) are optional. Without any GCPs, the model is calculated based on the satellite’s positioning information. The addition of ground control points, although not necessary, will refine the model and improve its accuracy. This means, however, that as long as you have a DEM, or have extracted a DEM from stereo radar images, you can still orthorectify your radar data without any ground control.
Setting up for Orthorectification
The Ortho Image Production dialog box allow you to set up and schedule the images for orthorectification. Several images can be selected and processed in batch mode.
To set up the images:
1. From the Processing Step menu, select Ortho Generation.
2. The Ortho Generation toolbar appears.
3. On the Ortho Generation toolbar, click Schedule ortho generation. The Ortho Image Production dialog box appears.
4. Select all images in the Available Image list.
5. Click the arrow to move all the photos to the Images to Process list.
6. By default, the ortho image will be named ofilename.pix. You could also enter a different filename in the Corrected Image pane.
Selecting the Digital Elevation Model
- In the DEM pane, click Browse beside Source.
- Locate the folder containing the DEM you are going to use for orthorectification and select it. If you do not have a DEM for your dataset you can use the demworld.pix file located in the Geomatica etc directory
- Click Open.
- The Database Channels dialog box opens.
- Select the layer containing the DEM
6. Click Select in the Database Channels dialog box.
The Database Channels dialog box closes and the DEM is selected.
Orthorectifying the RADARSAT-1 Scenes
To generate the ortho images:
- At the bottom of the Ortho Image Production dialog box, click Generate Orthos.The Ortho Production Progress monitor opens and shows the status of the orthorectification process for each image. After all the orthos are generated.
- Click Close at the bottom of the progress monitor.
The message Ortho done appears beside each image in the Available Images pane, indicating that the original images are now orthorectified.
Note: This is a good time to save your project file.
Viewing the orthorectified image in OrthoEngine:
- From the OrthoEngine File menu, select Image View. A Database File Selection window opens.
- From the Radar_Data folder, select output image.
- Click Open.
A Viewer opens displaying the orthorectified and radiometric terrain corrected image. Note the large No Data areas in the image due to layover.
There are a number of other SAR processing workflows available in the Geomatica suite of software. For example, The RADAR DEM Extraction package allows you to create Digital Elevation Models (DEMs) from stereo RADAR data. Image correlation is used to extract matching pixels in two overlapping images and then use the sensor geometry from a computed math model to calculate x, y, and z positions. RADAR DEM extraction allows you to batch epipolar generation, batch the DEM extraction process, geocode DEMs, and create absolute or relative DEMs. DEM extraction in OrthoEngine is possible if you have stereo imagery.
Other workflows such as Radiometric Correction, SAR texture analysis, SAR Classification, and SAR Change Detection are possibly in Geomatica 10.1 as well Below is an image of some sample SAR workflows possible within the Geomatica Visual Modeler environment.