Sentinel-1A data preprocessing

Microwave Toolbox
#1

Hi everyone,

I am new with Sentinel-1A data. I have downloaded some GRD data with names like this:

S1A_IW_GRDH_1SDV_20151125T193206_20151125T193231_008765_00C7D9_9BA0.zip

I would like to know whether the following steps of pre-processing have been applied or need to be conducted for these products (GRD products).

(1) Radiometric calibration.
(2) Multi-looking.
(3) Speckle filtering.
(4) Terrain correction.

Thank you for help!

Regards, Gao

Processing steps for Sentinel-1 GRD product
Processing graph for Geometric + Radiometric corrections for Terrain
#2

Hi Gao,

(1) Radiometric calibration.
Yes S1-A products has been radiometrically calibrated - see ESA document at: https://sentinel.esa.int/web/sentinel/user-guides/sentinel-1-sar/document-library/-/asset_publisher/1dO7RF5fJMbd/content/sentinel-1-radiometric-calibration-of-products-generated-by-the-s1-ipf

(2) Multi-looking.
For GRD products multi-looking as been performed - see Table 5-1 on page 5-2 of the ESA document at:
https://sentinel.esa.int/documents/247904/0/Sentinel-1+Product+Definition+V2.6. Multi-looking is not performed for complex products.

(3) Speckle filtering.
This is not performed on S1-A products.

(4) Terrain correction.
This is not performed on S1-A products.

Peter

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#3

Actually, you will still need to apply the LUT (via the calibration operator of the toolbox) if you want to have Sigma0, Gamma0 or Beta0.

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#4

Hi Peter,

Thank you so much for your exhaustive reply. I have another question: I found that there are only two polarizations in the product, namely VV and VH. How about other polarizations such as HH? (Sorry, I am new with SAR data ^ ^) Thank you so much!

Regards, Gao.

#5

Hi lveci,

Thank you for your reply! :grinning: Since I am new with SAR data, I am not very clear about the differences among Sigma0, Gamma0, and Beta0 data. Could you please give me an idea about the differences in them? And could you please tell me if I want to distinguish vegetation such as crops and grasses, which kind of data (Sigma0, Gamma0, or Beta0) is better? Thank you for your help! :relaxed:

Best regards, Gao.

#7

Gao, the difference between gamma/sigma/beta is described in detail in David Small’s paper; the summary is that you probably want gamma-nought, but with both geometric and radiometric corrections for the terrain.

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#8

Hi Gao,

S1 data is acquired in one of four polarisation combinations. These are either (i) VV, (ii) HH, (iii) HH and HV or (iv) VH and VV. These first two are these are single polarisation acquisitions while the latter two are dual polarisation acquisitions. The first polarisation in say HV refers to the transmit polarisation while the second is the receive. Thus HV is transmit H and receive V.

So the polarisations in your product depends on what was used to acquire the data. For your example:

S1A_IW_GRDH_1SDV…

this is a dual polarisation product (the D in the 1SDV part of the file name) with V transmit polarisation (so this product will include VH and VV polarisations).

Peter

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#9

Hi Gao,

welcome to the world of SAR :slight_smile:
you can check the observation strategy of Sen-1 here:
https://sentinel.esa.int/web/sentinel/missions/sentinel-1/observation-scenario
This explains also a bit the rationale behind the polarization and when which mode is used.

If you want to use your data for classifications you should also consider the Terrain Flattening inside your workflow which compensates for illumination effects along slopes. You will need to apply the calibration to beta0 beforehand. After the Flattening the backscatter refers then to gamma0 (i.e. RTC product in the Small article).

Best,
Andreas

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#10

Hi Benjimin and AVollrath,

@benjimin @AVollrath

Thank you for your help. I have another question: I do not know the unit of gamma nought values. I guess it should be dB, but I am not sure since the values are quite low (in the range of 0.01~0.1). Could you help me on this, please?

Thanks!

Best regards, Gao.

#11

hello,could give a examlpe in snappy on the do_apply_the LUT? In fact ,I want to have Gamm0 band!

#12

@peter.meadows I saw an tutorial that It says that we should apply radiometric calibration and multi-looking on SAR data.
http://sentinel1.s3.amazonaws.com/docs/S1TBX%20SAR%20Basics%20Tutorial.pdf
But you mentioned that SAR data are already radiometric calibrated and multi-looked.
Which one is correct?

#13

Hi,

Whether you need to perform mulit-looking in the SAR Tool Box depends on the product type. The GRD products (ground range detected) are already multi-looked and so there is no need to perform this step in the tool box. The SLC products (single look complex) are 1 look products and so it will be beneficial to perform multi-looking in the tool box to reduce the visual impact speckle.

S1 product are radiometrically calibrated in the sense that all the required information is included in the product to calculate the radar cross-section of a feature in the scene. However, the user (or the tool box) still needs to convert the product pixel values to radar cross-section.

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