Incidence angle correction of Sentinel 1

Gamma Naught is implemented in SNAP by first calibrating to Beta0 and then using the Terrain Flattening.
If I recall correctly there were some changes between its first implementation in NEST and the module today but it still works fine in moderate relief.
Have a look at this topic: S1 radiometric correction

Hi, ABraun Thanks for your reply.
But I’m still confused. Based on my understading of Gamma Naught, the Terrain Flattening will adjusts the influence of changes in the incidence angle caused by the local topographic factors, not the effects of incidence angle in different range distsnce. And it perform better than Normalization in Range doppler TC. I’m not sure whether i understand it correctly. I’d like to normalize SAR data to a fixed incidence angle, :worried:
Look forward to your reply.

yes, Terrain Flattening only compensates for topographically induced radiometric distortions. Maybe I misunderstood your question, sorry.

Thank you very much @ABraun,I learned a lot in the forum,:grinning:

Hi Chen,

… there is no incidence angle correction…

Just to clarify - a incidence angle correction is required when converting DN to sigma0 but this correction is not applied to S1 products themselves. It needs to be applied by the user or the software they are using.

… I think that the sin(incidence angle) term is just like the term sin(θ).

These two equations are identical.

…if you want to get Gamma naught (not Sigma naught)just replace it with tan(incidence angle).Right?..

Yes you are correct.

In addition, the effect from incidence angle shouldn’t be ignored when comparing the sigma0 of a feature at a different incidence angle. So, it is necessary to correct the incident angle after radiation calibration.

Yes this correct as well.


totally agree!

I think that doing radiometric terrain flattening is the best option for most users. The fact remains that most targets scatter non-isotropically so they will look different when viewed from differing incidence-angles. The resulting difference in backscattering is signal and not something that should be normalised away, in my opinion. If this causes problems in your analysis you should treat the time-series separately (one series per incidence-angle).


That means the S1 product whether SLC product or GRD product, no incidence angle correction is considered(during Calibration).When I read the contents about Calibration in the Help of SNAP, the incidence angle correction is mentioned for ASAR、ERS-1、Radarsat-2、etc,but not for the Sentinel-1.[quote=“peter.meadows, post:17, topic:9639”]
These two equations are identical.

As for the correction method,I have searched for some literature,could you give me some advice?:slight_smile:

Thanks for your reply and help me solve some confusion.
Best regards


As you said,complete angle correction is very difficult, but can we try to do some compensation? I have searched for some papers about this.Maybe I ended up failing.:worried:

Thanks for your reply.

Please summarize this topic.
How to normalize the incidence angle?

come on… It takes about 5 minutes to read it yourself :slight_smile:

How to make a constant incidence angle throughout the the image/ scene?

Start by reading this paper:

Going to gamma0 “corrects” the backscatters for local incidence angle, which is dependent on local topography and viewing angle. That’s what we call here terrain flattening. Most of the current users stick to sigma0, which is only terrain-corrected, so not taking into account the effect of local incidence angle on the backscatter magnitude.

But as already mentioned in this topic, you will always have the effect of anisotropic scattering, so just a different interaction of the radar beam with the scatterer on the ground. This is something you cannot really correct for. On the other hand, this gives us rich information, so it’s not all that bad!

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Hello all I have faced a problem which confused me a lot I am making correlation between local incidence angle and back scattering coefficient.and I used two consecutive images of sentinel one and I gat high R2=0.9 for the first image however the R2 for the next image at exactly the same place goes down R2= 0.00005 is it acceptable i mean does it can vary (the local incidence angle) i need your help please.

how did you compute the R2?
I mean, how many points are used and which tools did you use for it?

I used 15 points for both case.Then i extracted the value of the backs scattering coefficient and the local incidence angle from the image finally plot it in excel

could you please show both plots? 0.9 and 0.00005 are a bit too off…

Hello Everyone,

This has been a very interesting discussion! I am however, a bit confused.

I have a question about the relationship of incidence angle and Sigma0.

I am using Sentinel-1 GRD data with IW and EW swaths. I am looking into the land and sea-ice types and want to see if Sigma0 would be better suited for the study or Gamma0.

During the pre-processing, the terrain correction step allows for 3 different local incidence angles, which will be added to the pre-processed Sigma0 and Gamma0 products as separate bands.

In order to see the dependence between the Sigma0 and local incidence angle, I plotted the Sigma0_HH band against the local incidence angle and incidence angle from ellipsoid and got the following results

  1. Incidence Angle From Ellipsoid

  1. Local Incidence Angle

The above two plots show completely different dependency. Figure 1, with incidence angle from ellipsoid shows inverse correlation between Sigma0 and incidence angle from ellipsoid from 30.5 to 38 degrees but beyond 38 degrees the relationship is not clear.

In Figure 2, the relationship does not seem to be there at all.

  1. Is there a dependence between Sigma0 and incidence angle?
  2. Does the Sigma0 need to be corrected for local incidence angle? If yes, then is Gamma0 a way to go about it (as Gamma0 includes the correction already)?
  3. Is Gamm0 a better band for land and sea-ice?

Please let me know if there is any clarification needed from my side.


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How can I make the incidence angle normalization

by applying Radiometric Terrain Flattening: Radiometric & Geometric Correction Workflow

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