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ASAR Image Orthorectification

April 3rd, 2009 Category: Earth Observation

Figure 1 - 3D Model

Figure 1 - 3D Model

These images of Reunion Island provide a clear example of the improvements made through the use of the orthorectification technique.
The original product has been processed, orthorectified and projected using the Chelys SRRS (Satellite Rapid Response System). The orthorectification process is now fully automatic and transparent. The Digital Elevation Model used is the Nasa SRTM v4 (Shuttle Radar Topography Mission).

The proper DEM is selected according to the image resolution: from 15 arcseconds up to 1 arcsecond (only available for USA). The performance of the system are kept in real-time. The entire correction process takes less than 20 seconds per product.

Once the image has been geometrically corrected, its projection can be mapped with absolute precision using the DEM to simulate the third dimension. The result is a new, more interactive way to navigate a SAR image. An example of a 3D model is shown in Figure 1 (the Xvid codec, if missing, can be downloaded here).

Synthetic Aperture Radar and Orthorectification

Reunion Island - Original and Orthorectified Image

Reunion Island - Original and Orthorectified Image

Figure 2 - Synthetic Aperture Radar

Figure 2 - Synthetic Aperture Radar

SAR (Synthetic Aperture Radar) instruments transmit radar signals and then measure how strongly those signals are scattered back. An analogy with photography can be made: when it’s dark, a camera’s flash sends out light and then the film records objects that the flash illuminates. In both cases the SAR and the camera are not dependent upon the sun because they provide their own illumination.

The time it takes for a transmitted signal to travel to an object and back tells you how far away the object is. If you transmit a signal and receive two separate “echoes,” you can use the time difference between when you record the first and second responses to determine the distance between the two sensed objects (dependent on where you stand). In this way the spaceborne SAR measures how far objects are from the spacecraft and the distance between the two objects, along the direction the spacecraft is looking (Figure 2). These distances are said to be recorded in slant range, since they are measured in a direction which is at an angle/slant to the ground (Figure 3).

However, SAR uses a side-looking imaging mode whose angle is much greater than that of the optical image. This mode lends a great influence to geometric distortion of SAR image.

Figure 3 - Slant Range vs Ground Range

Figure 3 - Slant Range vs Ground Range

If you have information about a region’s topography, like a Digital Elevation Model (DEM), you can make the slant to ground range conversion more sophisticated. In effect this terrain correction can compensate for foreshortening by spreading data representing the mountain’s facing side into more pixels and compacting returns from the back face into fewer pixels. It’s nearly impossible, though, to reliably extract the separate returns from data values representing the facing slope. Sometimes people try to compensate for shadowing as well. Knowing the mountain’s slopes, they can approximate how the strength of backscattered signals were affected by the changed incidence angle and adjust results accordingly. These procedures, though inexact, can greatly improve SAR image analysis.

SAR’s ability to pass relatively unaffected through clouds, illuminate the Earth’s surface with its own signals, and precisely measure distances makes it especially useful for the following applications:

  • Sea ice monitoring
  • Cartography
  • Surface deformation detection
  • Glacier monitoring
  • Crop production forecasting
  • Forest cover mapping
  • Ocean wave spectra
  • Urban planning
  • Coastal surveillance (erosion)
  • Monitoring disasters such as forest fires, floods, volcanic eruptions, and oil spills

Some of the larger current research projects include: mapping the Antarctic continent; mapping the Amazon rainforest; using interferometric analysis for predicting or analyzing earthquakes and volcanic activity; and generating “Arctic Snapshots” of the Arctic ice extent.

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