The Sentinel-2 Ground Segment and its Products
Colin, Olivier1; Cadau, Enrico1; Collins, Michelle2; Gascon, Ferran1; López Fernández, Borja1; Marchese, Franco2; Teianu, Bogdan3

The GMES Space Component (GSC) is being gradually put together in Europe through a joint initiative of the European Space Agency (ESA) and the European Commission (EC). In this overall framework, the Sentinel-2 system, based on the continuous operations of a constellation of two optical satellites hosting the Multi-Spectral Instrument (MSI), will deliver a new generation of optical data products designed to directly feed downstream services acting in several domains such as land management, agricultural industry, forestry, food security, or disaster control management following floods, volcanic eruptions, landslides, etc.

Within the Sentinel-2 system, the control and maintenance in-orbit of the constellation resources, their day-to-day planning, and the elaboration of their measurements into user-exploitable products is the task of the Ground Segment. To this end, the Sentinel-2 Ground Segment is composed of two main coordinated elements, the Payload Data Ground Segment (PDGS) and the Flight Operation Segment (FOS).

The PDGS, in coordination with the FOS, is in charge of generating recurrently the acquisition plans for the two satellites. The mission plans elaborated by the PDGS are in turn translated into spacecraft schedules and uploaded to each constellation satellite by the FOS. In line with the mission requirements, a systematic and global plan of MSI acquisitions will be implemented aiming at covering all land surfaces in the latitude range between 56° South and 84° North, as well as major islands, inland water bodies and closed seas in a systematic manner. This background plan, offering as basis a global land dataset with maximum revisit time at equator of 5 days (in identical viewing conditions and with two satellites), will be fine-tuned or complemented throughout the mission lifetime according to Sentinel-2 High Level Operation plan (HLOP).

The FOS will then focus on monitoring the correct execution of the plan, as well as executing routine background tasks including orbit maintenance, satellite safety, health monitoring and critical management of the on-board resources (on-board software maintenance, contingency management, etc). To command and control the satellites in orbit, the Sentinel-2 FOS makes use of the generic ESTRACK network of S-Band ground station resources and more particularly of two daily passes over the Kiruna station located in North Sweden. It is complemented by other subsystems, operated locally at ESA's European Space Operation Centre (ESOC) under the supervision of a dedicated Spacecraft Controller.

On the other hand, the Sentinel-2 PDGS operates within the overall GSC Coordinated Data Access system put in place for GMES, and has for primary focus the generation of the Sentinel-2 mission products and their distribution to the GSC users. Besides its planning role outlined earlier, the Sentinel-2 PDGS will recurrently receive the raw measurement data from the two constellation satellites, and transform it into the final high-level products down to their archiving and availability for download through the Internet. The PDGS is also in charge of monitoring the payload and overall mission and products performance along time.

The data volumes generated by the Sentinel-2 constellation directly cascade from the characteristics of the mission, designed to provide a systematic, continuous and global coverage of the earth land surfaces using two satellites in orbit, and to the rich observation capacity of the Multi Spectral Instrument (MSI) on board each Sentinel-2 satellite. In return, the archive of data products which will gradually build up in the PDGS is expected to reach a petabyte scale after less than a year of mission operations with full deployed capacity.

To maximize the products suitability and readiness to downstream usage for the majority of applications, the Sentinel-2 PDGS will systematically generate and archive Level-1C products, which will provide Top of Atmosphere (TOA) reflectance images, orthorectified on a global DEM and UTM geocoded. A complementary atmospheric correction and enhanced cloud classification algorithm is being prototyped and will be provided to users in a software toolbox to be operated on their platforms. This algorithm translates the Level-1C TOA reflectance images into Level-2A Bottom of Atmosphere (BOA) reflectances. Additionally, Level-1B products will also be available for expert users and will provide the radiometrically corrected pixels in sensor geometry with the geometric model appended.

On the other hand, it is expected that according to their specific field of application, the product users will require diverse data access capabilities over the global archive, ranging in geographical coverage (local to continental scale), time coverage (access to recent and historical data), observation frequency, data contents (spectral contents, maximum cloud cover, etc), volume of data, automation of access, etc.

To respond to this anticipated challenge, both in terms of the huge accessible data volumes and functionality of access from the user side, the design of the Sentinel-2 PDGS has been focusing on product handling and data access solutions since the start.

To cope with user demands of products immediately after satellite acquisitions as well as to offer access to the global archive, the PDGS is designed as a distributed system which involves the receiving stations themselves offering access to recent products, and complementary long term archiving centres in charge of safeguarding and publishing the products in the longer term. All distributed archives are federated within a global network whereby any data held in any single archive can be retrieved transparently. Data access performance scalability is further addressed through potential collaborations with third-parties, whereby additional archives can be plugged in this federated network, to mirror selected datasets at given network access points to enhance the download performance for the benefits of the user community.

For data handling on ground, an innovative solution was retained which involves the decomposition of the large dataflows generated by the satellites into slices of data of a defined size, down to their processing and their storage in the archives. These slices of archived data, also called product "granules" (or "tiles" for the case of the Level-1C orthoimagery) allow to catalogue and handle the data efficiently within the PDGS in portions of reasonable size (<500 Megabytes), while giving a fine granularity to the selection of this data by the users according to their specific coverage requirements. The figure below depicts the Level-1C product tiling concept based on a fixed Global Reference System grid defined in UTM geocoded space.

Finally, the end user products will be reconstructed from a selection of these archived product "granules'', according to the desired geographical coverage defined by the user together with other packaging options (e.g. band extracts) aiming at reducing the transported volumes to a minimum. With the same objective of optimizing the user downloads, the actual product reconstruction will be performed directly at the user sites as part of the download process by means of dedicated software running on the user’s computer.

The presentation will describe the main principles of the Sentinel-2 Ground Segment with specific focus on the Sentinel-2 data products it will generate and the mechanisms put in place for user access.