Multilook Operator

Generally, a SAR original image appears speckled with inherent speckle noise. To reduce this inherent speckled appearance, several images are incoherently combined as if they corresponded to different looks of the same scene. This processing is generally known as multilook processing. As a result the multilooked image improves the image interpretability. Additionally, multilook processing can be used to produce an application product with nominal image pixel size.

Multilook Method

There are two ways to implement the multilook processing:

The multilooked images can be produced by space-domain averaging of a single look image, either with or without specific 2D kernels by convolution.
The multilook images can be produced by frequency-domain method using the sub-spectral band width.

This operator implements the space-domain multilook method by averaging a single look image with a small sliding window.

Selecting Range and Azimuth Looks

In selecting the number of range looks and the number of azimuth looks, user has two options:

GR square pixel: the user specifies the number of range looks while the number of azimuth looks is computed based on the ground range spacing and the azimuth spacing. The window size is then determined by the number of range looks and the number of azimuth looks. As a result, image with approximately square pixel spacing on the ground is produced.
Independent looks: the number of looks in range and azimuth can be selected independently. The window size is then determined by the number of range looks and the number of azimuth looks.

Multilook Complex Product

When the input product is complex, if you want to multilook the intensity image, you should select the intensity band by highlighting it in the Source Bands window. If no bands are selected, then the I and Q bands are selected by default and multilooked. The virtual intensity band generated from the multilooked I and Q bands is NOT the multilooked intensity band.
When the input is a complex interferogram product and you are interested in multilooking the phase, you should NOT select the phase band. Instead, you should select the I and Q bands. The desired phase band is generated from the multilooked I and Q bands. Otherwise, artefacts, such as the bell-shaped histogram, will be introduced in the multilooked phase band.

Parameters Used

The following parameters are used by the operator:

Source Band: All bands (real or virtual) of the source product. User can select one or more bands for producing multilooked images. If no bands are selected, then by default all bands are selected.
GR Square Pixel: If selected, the number of azimuth looks is computed based on the user selected number of range looks, and range and azimuth spacings are approximately the same in the multilooked image.
Independent Looks: If selected, the number of range looks and the number of azimuth looks are selected independently by the user.
Number of Range Looks: The number of range looks.
Number of Azimuth Looks: The number of azimuth looks.
Mean GR Square Pixel: The average of the range and azimuth pixel spacings in the multilooked image. It is computed based on the number of range looks, the number of azimuth looks and the source image pixel spacings, and is available only when 'GR Square Pixel' is selected.
Output Intensity: This checkbox is for complex product only. If not checked, any user selected bands (I, Q, intensity or phase) are multilooked and output individually. If checked, user can only select I/Q or intensity band and the output is multilooked intensity band.

Reference: Small D., Schubert A., Guide to ASAR Geocoding, RSL-ASAR-GC-AD, Issue 1.0, March 2008