Sentinel-2 Calibration/Validation During In-Orbit Commissioning and Operational Phases
Martimort, Philippe¹; Isola, C.¹; Gascon, F.^2
1ESA/ESTEC, NETHERLANDS; ²ESA/ESRIN, ITALY

GMES and Sentinel-2 Introduction
The GMES programme is a European initiative for the implementation of information services dealing with environment and security, based on observation data received from Earth Observation (EO) satellites and ground based information [http://www.copernicus.eu]. Within this context, ESA is responsible in particular, for the implementation of the GMES Sentinel missions, feeding the GMES services with operational EO data [http://www.esa.int/gmes]. The GMES Sentinel-2 optical imaging mission will be devoted to the operational monitoring of land and coastal areas. This system will deliver a new generation of optical data products designed to directly feed downstream services related to land monitoring, emergency management and security.
User needs are not homogeneous and are dependent of each application field (e.g. forest monitoring, cartog-raphy, photo-interpretation). Therefore, as an example, the ability to detect and quantify changes in the Earth's environment depends on sensors that can provide calibrated (known accuracy and precision) and consistent measurements of the Earth's surface features through time. Calibration and Validation (often referred as Cal/Val) corresponds to the process of updating and validating on-board and on-ground configuration parameters and algorithms to ensure that the product data quality requirements are met. To meet the baseline products quality requirements, a well-defined Calibration and Validation (Cal/Val) plan will be systematically applied during both in-orbit commissioning and operational phases.
During the in-orbit commissioning phase (Phase E1) led by the ESA/ESTEC Sentinel-2 Project Team, as soon as the satellite has reached a stable configuration during the Satellite In Orbit Verification (SIOV) phase, the MSI Calibration/Validation phase will start. The planned duration for the In-Orbit commissioning phase including SIOV and MSI Cal/Val is three months.
The objectives of the MSI Cal/Val during the In-Orbit commissioning phase are to assess the radiometric and geometric image quality of the MSI data products and to fine tune the on-board and on-ground image processing parameters to achieve the best performance levels and to compare them with respect to the Sentinel-2 mission and system requirements. This will include in particular the execution of all calibration modes of the MSI (absolute calibration with the sun diffuser, dark calibration over ocean at night, vicarious calibration over ground test sites, cross-calibration with other spaceborne optical missions) and the on-ground processing of a significant number of data products up to Level-1 and in some cases up to Level-2, using a set of facilities deployed in Europe, in cooperation between ESA and CNES. Those activities will culminate with the In-Orbit Commissioning Review (IOCR) where the Sentinel-2 SIOV and the MSI Cal/Val results will be presented and assessed with a view to conclude on the handover from phase E1 to phase E2.
During the operational phase (Phase E2), the Sentinel-2 Mission Performance Centre (MPC), as integrating part of the mission ground segment, will be in charge of ensuring that mission performances are met in terms of data quality. MPC activities will include: Calibration (CAL) - the objective of which is to update on-board and on-ground configuration data in order to meet product quality requirements; Validation (VAL) - the objective of which is to assess, by independent means with respect to the methods and tools used for calibration, the quality of the generated data products. Validation functions provide feedback to calibration and data processors corrective and perfective maintenance activities; Quality Control (QC) - the objective of which is to routinely monitor the status of the sensor and to check if the derived products meet the quality requirements along mission lifetime; Data processors and tools corrective and perfective maintenance (PTM) - the objective of which is to manage the updates of the processors, quality control tools, calibration tools and all auxiliary files in order to meet the product data quality requirements; End-to-end system performance monitoring (E2ESPM) - objective is to monitor the performance of the Sentinel-2 system operations (including the instrument) and assessing the system with respect to the operations plan.
The two main MPC service entities are: a single MPC/CC (Coordinating Centre) and a set of MPC/ESL (Expert Support Laboratories). The MPC/CC entity primarily covers the routine Calibration, Quality Control and End-toend System Performance Monitoring of the MPC Service plus supports the functions performed by the MPC/ESL. The MPC/ESL scientific, expertise and engineering support to the mission. In particular, the ESLs are responsible for (or supporting) activities such as the Level-1/2A processors calibration/validation, the Level-1/2A processing baseline maintenance or the Sentinel-2 instrument configuration.
Calibration and validation activities during Phase E2 are described in the Sentinel-2 Calibration and Validation Plan for the Operational Phase (Phase E2). Foreseen calibration and validation activities are presented in Tables 1 and 2, where are divided between those that are performed nominally during the operational phase and those that are performed in case of contingency. It is also indicated for each activity whether they will be performed by an MPC/ESL, the MPC/CC or by a combined approach with involvement of both MPC elements. The periodicities indicated in the following Tables will be fine tuned following the completion of the commissioning phase.
Activity Nominal / Contingency Performer (perio-dicity)
Level-1 Radiometric Calibration Dark Signal Calibration Nominal MPC/CC (2
weeks) Relative Gains Calibration Nominal MPC/CC (1
month) Absolute Radiometric Calibration Nominal MPC/CC (1 month) SWIR Detectors Re-arrangement Parameters Genera-tion Contingency MPC/CC
Crosstalk Correction Calibration Contingency
MPC/ESL
MSI Refocusing Contingency MPC/ESL
Geometric Cali-bration Global Reference Images Generation Nominal
MPC/ESL (1 year) Absolute Calibration of the Viewing Frames for the Reference Band Contingency MPC/ESL Relative Calibration of the Viewing Frames for Non-Reference Bands Contingency MPC/ESL Absolute Calibration of the Focal Plane for the Refer-ence Band Contingency MPC/ESL Relative Calibration of the Focal Plane for Non-Reference Bands Contingency MPC/ESL Table 1: Level 1 Calibration Activities. "MPC/ESL" refers to activities provided as a service and performed by expert teams in the domain. "MPC/CC" refers to the activities performed in an automated manner assisted by operators within the MPC/CC facility.
Activity Nominal / Contingency Performer (periodicity) Level-1 Radiometric Validation Equalization Validation Nominal MPC/ESL (6 months) + MPC/CC (1 month) Absolute Radiometry Vicarious Validation Nominal MPC/ESL (6 months) + MPC/CC support (10 days) Absolute Radiometry Cross-Mission Valida-tion Nominal MPC/ESL (3 months) + MPC/CC support (10 days) Multi-temporal Relative Radiometry Vicarious Validation Nominal MPC/ESL (3 months) + MPC/CC support (10 days) Inter-band Relative Radiometric Uncertainty Validation Nominal MPC/ESL (6 months) + MPC/CC support (10 days) SNR Validation Nominal MPC/ESL (6 months)+MPC/CC (1 month) Pixel Response Validation Nominal MPC/CC (1 day, 1 month)
MTF Validation Nominal MPC/ESL (1 year) Geometric Validation Geolocation Uncertainty Validation Nominal MPC/ESL (1 year) + MPC/CC (10 day) Multi-spectral Registration Uncertainty Validation Nominal MPC/ESL (6 months) + MPC/CC (10 day) Multi-temporal Registration Uncertainty Validation Nominal MPC/ESL (6 months) + MPC/CC (10 day)
Global Reference Images Validation Nominal MPC/ESL (1 year)
Table 2: Level 1 Validation Activities. "MPC/ESL" refers to activities provided as a service and performed by expert teams in the domain. "PDGS MPC/CC" refers to the activities performed in an automated manner assist-ed by operators within the PDGS MPC/CC (Mission Performance Centre).