[INTERNAL] Basic SAR data processing using ENVI SARscape

 

This Help Article lists and describes the key steps that should be completed in ENVI SARscape to process SAR data in order to generate geocoded, calibrated products, starting from raw data. Due to the specific acquisition geometry of SAR raw data (i.e sensor data), they cannot be directly analyzed and need to be processed first, using a SAR tool such as ENVI SARSCAPE.


Below is an example of the processing chain that could be used to convert RAW data into geocodes, calibrated data for image analysis.

Note: this processing chain is valid for data acquired using the same geometry (i.e. same sensor and same acquisition mode). If it is not the case please refer to the SARscape help for more details or contact or technical support (support@exelisvis.com)

 

 

Step 1 - Focusing:

This steps consists in converting raw data to Single Look Complex (SLC) images. If your data are already focused you can skip this step and start at step 2.

SAR data are acquired by an antenna that is moving along the azimuth direction (i.e. flight sensor direction). A signal is regularly emitted from this antenna. Thus  successive signals can illuminate a single target along the azimuth direction. It implies that several signals are backscattered from this single target to the sensor. The signal from on single target is then spread out along the azimuth direction.

In order to improve the range resolution of SAR data, the signal emitted along the range direction is modulated in phase. Such signal illuminates a large area on the ground that contains several targets. The signal backscattered from one single target is then combined with the signal of the neighbour targets. The signal is also spread out along the range direction.

The signal recorded by a SAR sensor for one single target is spread out along the azimuth and the range direction: it requires to be processed to retrieve the impulsive response of this single target. This is  what the Focusing step will to by compressing the phase of the raw signal along the azimuth and range directions. 

The output image is called the Single Look Complex (SLC) images. It contains both the phase and the amplitude of the SAR signal.

Technical note: - The Azimuth phase compression is based on the Doppler theory: the distance between the sensor and the target changes when the sensor is moving along the azimuth direction. Consequently  the phase of the different backscattered signals from one single target will change too. - The Range phase compression is based on the distance between each illuminated target and the sensor: this distance is function of the range direction. So the phase of the backscattered signal for each single target will change too along the range direction These phase changes are used to complete the phase compression and rebuilt the impulsive response of each target in the image - The focusing will also compensate the range migration effect: since the sensor is moving along the azimuth direction during SAR acquisition, the distance between the target and the sensor changes and the recorded signal presents a range migration that is compensated during the focusing step. - The focusing is applied to all targets of a SAR images using convolution products in the FFT domain.

 

 

 

Step 2 - Multilooking:

This step consists in converting Single Look Complex images to Power or Amplitude images. If your data are already power/amplitude images you can skip this step and start at step 3.

 

The multilooking step has 2 aims:
- it removes some of the speckle (i.e. grainy appearance of most of SAR data)
and
- it allows to output an image with a similar resolution in range and azimuth (i.e. with a squared pixel)

The output image is a power or amplitude or intensity image: it does not include the phase information anymore.

Technical note: SLC images are often difficult to interpret because of the speckle effect. The speckle gives a grainy appearance to SLC image. It is due to the system acquisition: when a transmitted signal reaches the ground, it interacts with different targets. For example in a field the signal interacts with each grass component. It results in constructive and destructive interferences in the backscattered signal and an homogeneous area will present a grainy appearance on the SLC image.  For more details about the multilooking step, you can read the Help Article #4265

 

 

Step 3 - Filtering:

This steps consists in applying filters to power/amplitude/intensity images to remove the residual speckle after multilooking the images.

Different kinds of filters are available in ENVI SARscape:
- Standard filters - similar to optical filter- for a single image,
- Specific SAR filters - such as the Frost filter- for a single image,
- Multitemporal filters for multiple SAR data. Such filters requires data acquired in the same acquisition geometry in input. 

If you are working with multiple images acquired in the same acquisition geometry the multitemporal filters are usually given the best results.  

 

 

Step 4 - Geocoding:

This step consists in projecting an image in acquisition geometry into a ground map geometry. The geocoding tool complete the 3 following steps
- the geolocalization to locate each pixel in the ground map geometry, based on geographic coordinates
- the geocoding to convert geographic coordinates to Cartesian coordinates
- the resampling of the image in the new ground map geometry

 

 

Step 5- Calibration and normalization

This step is completed at the same time as geocoding in ENVI SARscape, by checking the appropriate boxes in the geocoding interface.

The calibration is based on the radar equation. It is essential to quantitatively compare different SAR data.

Normalization allows to compensate the difference in the backscattered signal intensity due to the incident angle in the function of the range location

 

Technical note:  The calibration compensates 3 different effects - based on the radar equation-: * the range spread loss * the antenna gain pattern * the scattering area (A). The normalization compensates the backscattered coefficient decreasing in function of the range, due to the incidence angle variations.

 

 

Step 6- Mosaicking: If you work with different SAR you can mosaic them using ENVI SARscape.

 

 

 

After completing this processing chain, your data are ready for quantitative image analysis and classifications using ENVI for example.