Auxiliary Data

The Auxiliary Data describes all auxiliary information that is used by the ESA Copernicus Ground Segment for the processing of MSI data. The auxiliary data required in MultiSpectral Instrument (MSI) data generation are:

• Ground Image Processing Parameters (GIPP)
• Digital Elevation Model (DEM) (see below)
• Global Reference Image (GRI)
• European Centre for Medium-Range Weather Forecasts (ECMWF): ozone, surface pressure and water vapour data required for Level-1C processing
• Copernicus Atmosphere Monitoring Service (CAMS) auxiliary parameters
• International Earth Rotation & Reference Systems service (IERS) data
• Precise Orbit Determination (POD) data.

Datatake

The continuous acquisition of an image from one SENTINEL-2 satellite in a given MSI imaging mode is called a "datatake". The maximum length of an imaging datatake is 15,000 km (continuous observation from northern Russia to southern Africa).

Datastrip

Within a given datatake, a portion of image downlinked during a pass to a given station is termed a "datastrip". If a particular orbit is acquired by more than one station, a datatake is composed of one or more datastrips. It is expected that the maximum length of a datastrip downlinked to a ground station is approximately 5,000 km.

DEM

Since March 2021, the Digital Elevation Model (DEM) used for the orthorectification process of the L1C is the Copernicus DEM GLO-90m. The Copernicus DEM (COP-DEM) is a Digital Surface Model (DSM) which represents the surface of the Earth including buildings, infrastructure and vegetation and is based on the radar satellite data acquired during the TanDEM-X Mission.

More details about the DEM and how to access the data are available at https://spacedata.copernicus.eu/collections/copernicus-digital-elevation-model.

Global Reference Image (GRI)

The GRI is a set of mono-spectral (B4, red channel) L1B SENTINEL-2 images acquired globally, which has been processed to provide accurate relative coherence and an improved geolocation.  The GRI is used during the Level-1 processing as a source of reference Ground Control Points (GCPs). Homologous points found between the GRI and the L1B products within the processing chain, are used to refine the geometric model.

Consequently, the GRI have an absolute geolocation performance which allows respecting the following two specifications:

• The geolocation of Level-1C products refined with the GRI is better than 12.5 m CE95

• The multi-temporal registration performance of refined products is better than 0.5 pixels.

Granules and Tiles

A granule is the minimum indivisible partition of a product (containing all possible spectral bands).

For Level-0, Level-1A and Level-1B, granules are sub-images of a detector with a given number of lines along track. A granule covers approximately 25 km across-track and 23 km along-track.

For Level-1C, the granules, also called tiles, are 110 km² ortho-images in UTM/WGS84 projection. Earth is subdivided on a predefined set of tiles, defined in UTM/WGS84 projection and using a 100 km step. However, each tile has a surface of 110x110km² in order to provide large overlap with the neighbouring.

Quality Indicator

A medium that accompanies the data and can be used to obtain information necessary to determine the suitability of a product for a certain use or application. Examples of quality indicators are: defective pixels mask, cloud masks, on-line quality control reports, etc.

Timeliness

SENTINEL-2 nominal timeliness corresponds to the on-line availability of products between 3 and 24 hours after sensing.

True Colour Images (TCI)

Following the introduction of the Compact Naming Convention on the 6th of December 2016 (see the SENTINEL-2 User Guide Product Naming Convention page for details), a TCI (True Colour Image) in JPEG2000 format is included within the Tile folder of Level-1C products in this format⁽¹⁾. The TCI is an RGB image built from the B02 (Blue), B03 (Green), and B04 (Red) Bands. The reflectances are coded between 1 and 255, 0 being reserved for 'No Data'. The saturation level of 255 digital counts correspond to a level of 3558 for L1C products or 2000 for L2A products (0.3558 and 0.2 in reflectance value respectively).

(1) Products acquired before the 6th of December 2016, and products reprocessed within the Systematic Processing and Reprocessing Services still use the old format Naming Convention.