Standard Atm Correction procedures for OLCI


#1

Hello,

Can you point me to the documentation that specifies the what the standard atm correction procedure is for the Lvl 2 products for OLCI? In the S3 documentation, it states there is a Baseline atm correction, or you can choose a NN atm correction procedure. Currently, it does not seem like that option to choose is available yet. The reflectance output from a lvl 2 image are Oa_reflectances. Are these the baseline correction or the NN correction? Also, is the NN correction the same as the C2RCC NN correction that is applicable to lvl 1 data? As well as the CHL_NN product from a lvl 2 image? Or is the NN procedure producing the products in the lvl 2 product a completely different NN than C2RCC?

Thanks,
Jeremy


#2

For documentation I would recommend the OLCI ATBDs which are accessible from the S3-OLCI Document library
https://sentinel.esa.int/web/sentinel/user-guides/sentinel-3-olci/document-library
Here you find the “Sentinel-3 OLCI Atmospheric corrections over Case 1 Waters ATBD” and the “Sentinel-3 OLCI Bright Waters Atmospheric Correction ATBD” which together describe the standard AC, i.e. the water leaving reflectances that are included in OLCI L2 Water products are generated according to these algorithms.

In the document library you also find the “Sentinel-3 OLCI Alternative Atmospheric Correction ATBD”. This is an alternative which is internally used by the ground segment processor to generate water leaving reflectances which are used to retrieve the concentrations in Case 2 waters, i.e. input to the neural net. However, these water leaving reflectances are not written into the OLCI L2 water product distributed by ESA & EUMETSAT

The ground segment Level 2 processing and C2RCC processing (using default settings) are basically the same. There are preprocessing steps, like the smile correction, or the gaseous absorption correction, which can be different. However, the neural nets used in the ground are exactly the same as the ones in C2RCC, and also the processing logic and thresholds used by the two different processors are the same.

Of course, the C2RCC processor allows you to modify the default setting of many processing parameters, like the conversion from IOP to concentrations, or the thresholds used for flagging. Also, the C2RCC allows output of variables not available in the ground segment. When choosing non-defaults you get different result than the ground segment products.

I hope this helps!


#3

Hi, marpet. Thank you so much for the documents you provided. I have read a lot.
At present, I still have the following questions:
(1)

Does this indicate that there is a criterion for judgment to distinguish between using BPAC (Bright Pixel Atmospheric Correction) and CWAC (Clean Water Atmospheric Correction) algorithms? I want to know what the criteria which ESA currently are using.
Also, I have seen the flag of OLCI L2 product which is “WQSE_lsb.BPAC_ON”. What I understand is the pixel whose “BPAC_ON” flag is “Ture” was corrected by the BPAC algorithm. Is that right?

(2)
Since the L2 product starts at 2017-07-05, how could I perform the BPAC algorithm for the images before 2017-07-05? Does SNAP have related plugins? If not, what should I do?

(3)
Last question, could you tell me how to determine the processing version of the image which we can download from the hub due to there were many changes in the preprocessing of the image products.

I will be greatly appreciated if you can answer for me. Thank you again.


#4
  1. I can’t give you details about this. I don’t know the internals of the algorithm. It should be described somewhere in the documents.
    If you can’t find, please try asking ESA directly: https://esatellus.service-now.com/

  2. No this is not included in SNAP.

  3. I think the most suitable information is the PUG version. The IPF version is not stated in the metadata.


#5

Thanks for your help very much.
Would you like to tell me how to perform BPAC for L1B product, especially for products before 2017-07-05?


#6

It is not possible. There is no processing tool available at the moment, as far as I know.


#7

OK! Thank you anyway.:grinning:


#8

Hi, marpet.

I still feel confused about the PUG version and the IPF version. Actually, I do not know their definition.
Moreover, will the difference between the different processing version be large? If I want to do research with OLCI images, do I need to follow the latest processing version of the product (i.e., I have to re-process the images when the new version is released)?
I will appreciate it if you could respond to me.