The “Sentinel-3 data extraction” service for L1 radiometric validation

The S3ETRAC service delivers to calibration experts, appropriate OLCI and SLSTR data and associated statistics for the radiometric validation activities.

The S3ETRAC service initiated during the Sentinel-3A commissioning phase is now operated by ACRI-ST in the frame of the Sentinel-3 Mission Performance Centre. The successive versions of the service are presented here.

The main users of the S3ETRAC products are: ESA, CNES, RAL, VITO and ACRI-ST. They exploit these products in order to check and monitor the calibration of the reflective spectral bands of the Sentinel-3 optical sensors (OLCI and SLSTR). The data extracted by S3ETRAC are used through several calibration approaches to assess the absolute calibration, the interband calibration or long-term calibration trending …

The S3ETRAC service provides S3ETRAC products over 5 surface types:

  • Desert (both OLCI and SLSTR)
  • Snow (both OLCI and SLSTR)
  • Ocean – Rayleigh (both OLCI and SLSTR)
  • Ocean – Sunglint (both OLCI and SLSTR)
  • Deep Convective Clouds (only for OLCI)
The S3ETRAC product format/content is detailed in the S3ETRAC Product Description Document.

The data are extracted over a series of pre-defined sites. These sites have been selected for their appropriate optical properties to validate the radiometry of optical sensors: e.g. spatial and temporal radiometric stability for desert and snowy sites, maximization of the Rayleigh scattering over oceanic surface, etc. The list of these sites and their geographical limits are available here.

The S3ETRAC service is organised in several main steps:

  • Product selection: A Sentinel-3 product is processed if it intersects one site (and if the acquisition is done during the day for SLSTR). The S3ETRAC service only processes OLCI L1 RR and SLSTR L1 RBT products including consolidated meteo data (non critical time (i.e., NT) products)
  • Data selection: All the pixels of the site matching a series of filtering criteria defined by surface type are selected. For example, the criteria minimize disturbing effects such as clouds, bad atmospheric and meteo conditions, etc. Please, refer to the CNES ATBDs here .
  • Statistics computation: If the percentage of selected pixels is greater than a threshold, statistics are computed per site or per macro-pixel depending on the site type.
  • S3ETRAC product formatting: Two output files are delivered per Sentinel-3 input file. The first one corresponds to the main S3ETRAC product in netCDF 4.0 format including all the required information to assess the calibration. The second one in XML format provides a summary of the statistics per data selection criteria. The format of the S3ETRAC product is available here.
  • S3ETRAC products delivery: The S3ETRAC products are delivered on a ftp site (access reserved) constantly updated. The products are organised per sensor and per surface type.

S3ETRAC operations statistics are regularly updated and available here