Instrument Calibration

The SENTINEL-2 calibration activities under Sentinel-2 OPT-MPC responsibility, correspond to the process of updating on-board and on-ground configuration parameters and algorithms.

In the following table, the different activities of calibration at Level-1 are summarised:

 *(OPT-MPC: Optical Mission Performance Cluster, ESL: Expert Support Laboratories

Radiometric Calibration Activities

The radiometric calibration activities allow determination of the parameters of the radiometric calibration model, which aims to convert the electrical signal measured by the instrument, transformed in digital count, into physical radiance measured at the sensor.

Radiometric calibration activities are the set of methods used to estimate the parameters of the SENTINEL-2 radiometric model.

Nominal radiometric calibration activities include:

• dark signal calibration (every 2 weeks)
• relative gains calibration (every month)
• absolute radiometric calibration (every month).

Identified contingency radiometric calibration activities include:

• SWIR detector re-arrangement parameters generation
• cross-talk correction calibration
• MSI refocusing.

Nominal radiometric activities are based on the exploitation of the on-board sun diffuser images (for absolute radiometric and relative gain calibration) and images acquired over ocean by night (for dark signal calibration). The on-board sun diffuser is a full field/full pupil diffuser, called the Calibration and Shutter Mechanism (CSM), which is integrated with SENTINEL-2 instruments for radiometric calibration to guarantee high quality radiometric performance. This On-Board Calibration Device (OBCD) collects the sunlight after reflection by the diffuser to prevent the instrument from viewing the sun directly in orbit and from contamination during launch.

As there is no secondary sun-diffuser on-board, the use of the sun-diffuser is optimised to reduce exposure to sun irradiance as much as possible and its consequent degradation. The stability of the diffuser is monitored in the context of the vicarious and cross-mission validation activities.

Geometric Calibration Activities

The geometric calibration activities allow to ensure the maintenance of the better geometry possible of the images, according to the user requirements. It aims at determining all ground image processing parameters (GIPP) involved in the MSI geometric model.

The parameters of the geometric model are:

• orientation of the viewing frames
• lines of sight of the detectors of the different focal planes.

These parameters have been estimated before launch. The purpose of the geometric calibration activities is to take into account any updates of these parameters values that may occur during the satellite lifetime, and to remove the effects of navigation errors and surface topography on the ortho-rectified product during standard processing.

In order to meet the multi-temporal registration and the absolute geolocation requirements, a Global Reference Image (GRI) is used for the automatic extraction of tie-points for the systematic refinement of the geometric model.

Reference Documents:
Sentinel-2: ESA's Optical High-Resolution Mission for GMES Operational Services

GMES/Copernicus SENTINEL-2 Calibration and Validation Plan for the Operational Phase (GMES-GSEG-EOPG-PL-10-0054)