Calibration - Sentinel-2 MSI Technical Guide - Sentinel Online
The Sentinel-2 calibration activities under Sentinel-2 MPC responsibility, correspond to the process of updating on-board and on-ground configuration parameters and algorithms.
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 at 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.
There is no secondary sun-diffuser on-board Sentinel-2 for degradation checking purposes and consequently 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
Geometric calibration activities allow determination of all Ground Image Processing Parameters (GIPP) of the geometric calibration model which aims to ensure better geometry is maintained for Sentinel-2 images. 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 update of these parameter values that may occur at launch and during the in-orbit phase, and to remove the effects of navigation errors and surface topography on the ortho-rectified product during standard processing using a simplified approach.
The geometric calibration activities are the set of methods used to estimate the parameters of the Sentinel-2 geometric model.
Nominal geometric calibration activities include annual generation of the Global Reference Image (GRI). Identified contingency radiometric calibration activities include:
- absolute calibration of the viewing frames for the reference band
- relative calibration of the viewing frames for non-reference bands
- absolute calibration of the focal plane for the reference band
- relative calibration of the focal plane for non-reference bands.
To meet the multi-temporal registration and absolute geo-location requirements, a GRI is generated and used for automatic extraction of GCP for systematic refinement of the geometric model at the end the Level-1B processing.
In the following table, the different activities of calibration at Level-1 are summarised:
|Radiometric||Dark Signal Calibration||Nominal||
|Absolute Radiometric Calibration||
|SWIR Detectors Re-arrangement Parameters Generation|
|Relative Gains Calibration||Contingency||MPC/CC|
|Crosstalk Correction Calibration||MPC/ESL|
|Geometric||Global Reference Image Generation||Nominal||
|Absolute Calibration of the Viewing Frames for the Reference Band||Contingency||MPC/ESL|
|Relative Calibration of the Viewing Frames for Non-Reference Bands|
Absolute Calibration of the Focal Plane
for the Reference Band
Relative Calibration of the Focal Plane
for Non-Reference Bands
These calibration activities are related to the validation activities (cal/val). The validation includes:
- radiometric validation:
- equalization validation
- absolute radiometry vicarious validation
- absolute radiometry cross-mission validation
- multi-temporal relative radiometry vicarious validation
- inter-band relative radiometric uncertainty validation
- pixel response validation
- MTF validation.
- geometric validation:
- geo-location uncertainty validation
- multi-temporal registration uncertainty validation
- multi-spectral registration uncertainty validation.
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)