Nominal incidence uncorrected sigma nought

Microwave Toolbox
1

Hello Folks,

My question is not directly connected with the S1TBX but I’m looking for retrieving the incidence uncorrected sigma nought. As far as I understand, the data we can download from SSDH server are already [partially] calibrated and so the “nominal” incidence angle is taken into consideration at this step. Am I right? If so, I would need to get access to the uncorrected sigma nought value. I’d like to perform some numerical modelling of the backscatter function and use the S1A data as ground truth. I found this very interesting document: https://sentinel.esa.int/documents/247904/685163/S1-Radiometric-Calibration-V1.0.pdf but I still did not find if the “nominal” inc. angle (the one which is in the xml ancillary file) is (or is not) taken into consideration. Any thoughts about this?

Thank you

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The incidence uncorrected sigma nought is, if I understand your question correctly, beta0. The relationship between sigma0 and beta0 is: sigma0 = beta0 * sin(inc angle) and so beta0 does not include the incidence angle correction. The calculation of beta0 is given in Section 4 for the document you mention, i.e. beta0 = DN^2/Abeta^2 where DN is the product pixel amplitude values and Abeta is one of the calibration LUT’s in the product annotation.

Note that if you want the beta0 of a distributed region (rather than a single pixel), it is important to calculate the average pixel intensity of your region (average DN^2) rather than the square of the average pixel amplitude.

2 Likes
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Hello Peter,

thank you for your reply.

I was actually not familiar with the “Beta” notation, and so I’ve missed it in the documentation.
Ok, so it seems that I have everything now.

Thanks so much for your help!

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@peter.meadows

A somewhat related question: I am looking for information on the Lookup Table used by the Sentinel-1 SAR processor at the end of the image formation process to convert the pixel values into DNs. Do you know if any documentation on this is available?

To clarify, I am referring to a Sentinel-1 document equivalent to the following Radarsat-2 one:
http://www.mdacorporation.com/docs/default-source/technical-documents/geospatial-services/pg-tn-52-7196_rsat2_application_lut_v1-3-clean.pdf?sfvrsn=4

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Only one Application Look Up Table has been for Sentinel-1 and thus, unlike Radarsat, there are no specific LUTs for different surface types such as sea or land. The calibration document mentioned above describes the conversion from DN to beta0, sigma0 or gamma or indeed to the original DN value prior to the application of the Application LUT. More information can be found in Sections 9.18 and 9.19 of this document: https://sentinel.esa.int/documents/247904/349449/Sentinel-1+Level-1+Detailed+Algorithm+Definition+1.6

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How can I get DN’s of sentinel-1_slc data?
Is there any value (DN) in *.Tiff files other than just identification array ! or
Any chance to compute DN by any formula…!

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Please go through some of the S1TBX tutorials.

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URL broken

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The latest S1 Level1 Detailed Algorithm Definition document can be found at https://earth.esa.int/web/sentinel/user-guides/sentinel-1-sar/document-library/-/asset_publisher/1dO7RF5fJMbd/content/sentinel-1-level-1-detailed-algorithm-definition or have a look thorough documents at https://earth.esa.int/web/sentinel/user-guides/sentinel-1-sar/document-library