Multilooking necessary when geocoding?


#1

Hello,
I am fairly new to the SAR processing and I am trying to learn the basics through the ESA Sentinel Toolbox tutorials, which are a valuable resource.
In the very first tutorial “SAR Basic Tutorial”, I don’t understand the role of the Multilooking process specially when compared with Geocoding.
According to the tutorial, the “Multilooking can be an optional step, since it is not necessary when terrain correcting an image”. But in the Terrain correction step of the tutorial, the input image is the multilooked and speckled image.
So what I would like to know is if the Multilooking is actually a optional step or it is usually applied in pre-processing.
Thanks in advance
Niccolò


Pre-Processing Sentinel-1-Data
#2

Multilooking is not mandatory. It fixes the inconsistend pixel sizes caused by the angle of the incoming signal and produces an image with “squared” pixels. As a nice side effect, speckle is reduced to a certain degree.
When you cange the number of looks in the module you see the changes in your total resolution. If your data has a good resolution, TSX for example, you can apply multi-looking to reduce it to about 10 meters and still have sufficient resolution for most applications. However, if your data resolution is already coarse or if you need the spatial resolution for the search for certain objects I would personally advise to skip the multilooking.
As you said, the final geocoding is performed in the RangeDoppler Terrain Correction module at the end of your pre-processing. You can apply it on both multilooked or non multilooked data and (besides of some resampling) it won’t change your resolution any more.


Image from the Sentinel-1's SLC product
#3

Thank you for your quick and exaustive reply!


#4

But if the data is GRD, the pixel is square. It is not like this? Correct me if I’m wrong. Thanks


#5

yes, after multi-looking the pixels are of equal spacing in range and azimuth direction.