## 3 Calibration Status of Sentinel-2
### Geometric Calibration  
Geometric calibration activities aim to fulfill and validate the following mission requirements:  
* absolute geolocation performance better than 12.5 m (circular error at 95%);
* multi-temporal relative geolocation accuracy better 3 m (circular error at 95%);  
Figure 5: Results of the radiometric validation of Sentinel-2 calibration: top figure for MSI-A, bottom figure for MSI-B. Results are plotted for all the spectral bands and for the different methods. For S2A, the selected methods are the Rayleigh calibration, the in-situ calibration, the desert pseudo invariant calibration sites (PICS) calibration and the inter-sensor calibration with Landsat-8. For MSI-B, the methods are the Rayleigh calibration, the desert PICS calibration and the inter-sensor calibration with MSI-A.  
* relative co-registration between any pair of spectral bands better than 0.3 pixel of the coarser spatial sampling distance (circular error at 99.7%). Calibration activities include:
* calibration of the relative viewing directions of each detector pixel;
* adjustment of on-board time lag;
* monitoring and adjustment of the spacecraft line-of-sight model;
* image geometric refinement using a global reference image.  
The first two activities were performed during the commissioning period of each S2A and S2B unit, and no subsequent adjustment was made.  
The multispectral registration performance is continuously monitored: a degraded performance could indicate the need to recalibrate the line of sight. The method uses cloud-free images over flat terrain. Matching image patches between different spectral bands are identified and the shift vectors are computed. Point cloud plots are produced to identify potential biases (see Figure 6 below) and along-track Fourier transform of the error is used to detect potential oscillations in the line of sight.  
The third activity is to determine the three rotation angles (roll, pitch, yaw) characterizing the pointing bias of the instrument reference line-of-sight with respect to the Attitude and Orbit Control System (AOCS) frame. These angles are still evolving for both satellites.  
The absolute geolocation performance is constantly monitored using a set of ground control points around the globe. Every month, approximately, the point cloud of errors is plotted in the along-track/across-track frame. If the cloud shows a bias in the along-track (respectively across-track) direction, a calibration of the pitch (respectively roll) angle is required. The yaw angle is more difficult to monitor; a yaw bias can be detected by looking at the variation of the along-track error component along the swath.  
To compute the new calibration angles, images in the sensor frame (LIB) and another set of ground control points is used. The angles are obtained by minimizing the global error over the set of control points.  
In the near future, a similar procedure will be performed to refine the geometric model of all Sentinel-2 images. The geometric refinement process uses as a reference a set of carefully georeferenced images, the GRI. For each image data strip, the viewing model will be adjusted to minimize the shift between the current image and the reference image. This procedure and the associated Global Reference Image are currently being validated. Aligning all images with the same GRI is expected to reduce the multi-temporal error from 12 m today to less than 3 m.  
Figure 6: Point cloud of the multi-spectral co-registration error for an S2A product.