Auxiliary data is all other data (i.e. excluding direct measurements from the instrument and information on platform orbital dynamics) necessary for product processing and generation.
ADFs, such as the Digital Elevation Model (DEM) or the Global Reference Image (GRI), that are used in the generation of Level-1 and Level-2A products, are verified before use to ensure the processing chain is not compromised. The GRI enables the automatic creation of Ground Control Points (GCPs) for Level-1C processing.
For SENTINEL-2, auxiliary data is composed of:
- On-Board Configuration Data (OBCD), comprised of on-board configuration parameters and payload and platform characterisation data
- On-Ground Configuration Data (OGCD), which, for Level-1 and Level-2A products, is composed of Ground Image Processing Parameters (GIPP) and includes the DEM, the GRI, the ECMWF and CAMS auxiliary files, the International Earth Rotation and Reference Systems Service (IERS) Bulletins and the processors versions.
Each OBCD and OGCD file is identified by the type of data, applicability date, version number and the satellite to which it is applicable (SENTINEL-2A, SENTINEL-2B or both). Any update of one or other of the files will result in the other being updated at the same time. All auxiliary data used for Level-1B, Level-1C and Level-2A processing can be identified in the metadata accompanying the product.
ECMWF global forecast data
In addition to the Level-1B auxiliary data, the Level-1C and Level-2A auxiliary data also includes meteorological parameters, temporally and spatially interpolated from raw ECMWF global forecast data sets:
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Total Column Ozone (TCO3) [kg/m2]
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Total Column Water Vapour (TCWV) [kg/m2]
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Mean Sea Level Pressure (MSLP) [hPa]
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10 meter V wind component (10v)
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10 meter U wind component (10u)
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Relative Humidity @ Isobaric Surface
This meteorological auxiliary data are resampled and distributed within the Level-1C and Level-2A tiles. Grid points are provided in latitude/longitude using the WGS84 reference system.
ECMWF Ozone
Within Level-1C products, in addition to Total Column Water Vapour (TCWV) and Mean Sea Level Pressure (MSLP) auxiliary data, Total Column Ozone (TCO3) auxiliary data is also provided. Expressed in units of kg/m2, TCO3 corresponds to the vertical integrated ozone content.
TCO3 is used in the context of atmospheric corrections, to correct ozone gaseous absorption affecting TOA measurements, and forms part of the On-Ground Configuration Data (OGCD) provided for onward processing (i.e. to Level-2A).
The input data content is provided by ECMWF and is derived from temporal and spatial interpolation of the raw global forecast dataset. This data is then resampled to conform to the Level-1C grid and information requirements. Grid points are provided in latitude/longitude using the WGS84 reference system.
Total Column Water Vapour
SENTINEL-2 Water Vapour
One of the parameters included in the Level-1C auxiliary data is the Total Column Water Vapour (TCWV) expressed in kg/m2. This parameter corresponds to the vertically integrated water vapour. TCWV is used in the context of atmospheric corrections, to correct water vapour gaseous absorption affecting TOA measurements.
TCWV data is derived from ECMWF input data resampled and distributed in grid information tiles with the same dimensions as the Level-1C tiles. Grid points are provided in latitude / longitude using the WGS84 reference system.
Mean Sea Level Pressure (MSLP)
One of the parameters embedded in the Level-1C auxiliary data is the Mean Sea Level Pressure (MSLP) expressed in hPa. This parameter corresponds to the atmospheric pressure at sea level.
The MSLP is used in the context of atmospheric corrections to correct TOA measurements for Rayleigh scattering.
It is derived from temporal and spatial interpolation of the raw ECMWF global forecast dataset. This dataset is updated daily. MSLP data is resampled and distributed in grid information tiles with the same dimensions as the Level-1C tiles. Grid points are provided in latitude/longitude using the WGS84 reference system.
More information on these ECMWF inputs can be found at European Centre for Medium-Range Weather Forecasts.
CAMS atmospheric auxiliary data
Since 25 January 2022, the Level-1C and Level-2A contain the Copernicus Atmosphere Monitoring Service (CAMS) atmospheric auxiliary data. Indeed, to improve the atmospheric correction when DDV pixels are missing in the image, a fallback solution is to use CAMS atmospheric auxiliary data.
The following CAMS parameters are expected:
- Total Aerosol Optical Depth at 550nm (aod550): mandatory parameter
- Surface geopotential (z): mandatory parameter
- Black Carbon Aerosol Optical Depth at 550nm (bcaod550)
- Dust Aerosol Optical Depth at 550nm (duaod550)
- Organic Matter Aerosol Optical Depth at 550nm (omaod550)
- Sea Salt Aerosol Optical Depth at 550nm (ssaod550)
- Sulphate Aerosol Optical Depth at 550nm (suaod550
- Total Aerosol Optical Depth at 469nm (aod469)
- Total Aerosol Optical Depth at 670nm (aod670)
- Total Aerosol Optical Depth at 865nm (aod865)
- Total Aerosol Optical Depth at 1240nm (aod1240)
The original CAMS data covering the whole globe (CAMS Global) can be divided in three types:
- Global forecasts (from 1 h to 120 h in the future)
- Global analysis (available within 24 hours)
- Global reanalysis (covers the period 2003 – June 2020 at the time of this TN)
For more information on the CAMS data catalogue and access, please refer to the following website:
https://confluence.ecmwf.int/pages/viewpage.action?pageId=151530675