
.. Label between '.. _' and ':' ; use :ref:`text <label>` for reference
.. _s4-syn-no2-processing:

****************************************
Synthetic Sentinel-4 NO2 data processing
****************************************

This chapter describes the tasks performed for processing 
a set of synthetic Sentinel-4 NO\ :sub:`2` data.


Product description
===================

The Sentinel-4 mission includes instruments that observe trace gasses over Europe
from a geo-stationairy orbit. The intended lauch in in 2023.

* `Sentinel-4 Mission Introduction <https://sentinels.copernicus.eu/web/sentinel/missions/sentinel-4>`_

.. figure:: figs/S4/S4-impression.png
   :scale: 50 %
   :align: center
   :alt: Sentinel-4 mission
   
   *Artist impression of Sentinel-4 at geo-stationary position observing Europe. 
   Shaded area on the right shows extent of the domain used by CAMS Regional Ensemble, 
   which for a large part overlaps with the Sentinel-4 Geographic Coverage Area (GCA).*

The Sentinel-4 instrument will provide at daytime hourly snapshots of trace gasses at a
resolution of about 8x8 km. The amount of data values is therefore much higher than for
the polar orbiting Sentinel-5p instrument.
In the context of the CAMS_61 project, a first evaluation has been made of the resources
needed to process the future Sentinel-4 data within simulation models.
For this, a synthetic data set of Sentinel-4/NO2 data has been created using
simulations from the MONARCH model (BSC), accompanied with realistic error characteristics
and averaging kernels produced using machine-learning.
An extensive description of how the data was produced and evaulated is available in:

* Final report of WP6130 R&D activities *(not available online yet)*

.. figure:: figs/S4/S4-syn-no2-r.png
   :scale: 50 %
   :align: center
   :alt: Synthetic Sentinel-4 NO2
   
   *Example of synthetic Sentinel-4 NO2 retrievals.*


Downloading
===========

The data set be available through a public server.

The data set consists of hourly files for July 2018::

  S4_v3_r/BSC_S4-syn_NO2_20180701_0000.nc
          BSC_S4-syn_NO2_20180701_0100.nc
          BSC_S4-syn_NO2_20180701_0200.nc
                                   :
          listing.csv

Also a *listing* file is included for use with the CSO observation operator.

A catalogue of images of the data could be made similar to the
:ref:`S5p NO2 image catalogue<s5p-no2-catalogue>`.


Configuration of observation operator
=====================================

The *observation operator* described in chapter ':ref:`obsoper`' requires settings from
an rcfile. 
Template settings are included as::

  oper/tutorial_oper_S4-syn.rc
  
The *tutorial* program ``tutorial_oper_S5p.F90`` should be changed to use these settings
rather than the default settings::
  
  !~ input settings for synthetic S4/NO2
  rcfile = 'tutorial_oper_S4-syn.rc'
  ! base of keywords in rcfile:
  rcbase = 'tutorial.S4-syn.no2'

Without further changes, the tutorial program will create (dummy) simulations of the
synthetic Sentinel-4 retrievals.

A catalogue of images of the, by the tutorial, simulated data could be made similar to the
:ref:`simulated S5p NO2 image catalogue<s5p-no2-sim-catalogue>`.
