SNAP Data Processors - OLCI Sensor Harmonisation Algorithm Specification |

### Algorithm Specification

The two OLCI instruments operating on the Sentinel 3A and 3B platforms share a common instrument design. Nevertheless, there are small differences in the spectral characteristics, between the different cameras of each OLCI instrument, as well as between the OLCI-A and B instruments. Further, the radiometric and spectral calibration has uncertainties which eventually lead to small differences in the instrument measurements. Together this can impact downstream processing.
To be able to quantify the instrument characteristic deviations ESA maneouvered the Sentinel 3B platform in an early phase of the mission into a "tandem configuration" where it followed the Sentinel 3A platform on the same orbit with a 30 seconds delay. This configuration allows for measurements over the same ground targets with almost the same atmospheric conditions, hence for comparable measurements.
The recommendations resulting from the analysis of acquisitions taken during the tandem phase are implemented in this processor. These are a "homogenisation" process and a "sensor cross-calibration".

- Homogenisation: the homogenisation aims at correcting the inter-camera differences of each OLCI instrument. It applies a camera specific gain correction on the radiances measured so that measurements of all cameras appear to be measured with the same camera sensitivity (so called "flat fielding").
- Cross calibration: this process applies the homogenisation process and on top of this applies sensor inter-calibration coefficients on the radiances measured. It does this by implementing the linear-regression approach described in reference [1] using the per-pixel detector wavelength.

### References

[1] Nicolas Lamquin, Sebastien Clerc, Ludovic Bourg, and Craig Donlon:
OLCI A/B Tandem Phase Analysis, Part 1: Level 1 Homogenisation and Harmonisation
Remote Sensing, 12.11 (2020): 1804.
[2] Nicolas Lamquin, Alexis Deru, Sebastien Clerc, Ludovic Bourg, and Craig Donlon:
OLCI A/B Tandem Phase Analysis, Part 2: Benefits of Sensors Harmonisation for Level 2 Products
Remote Sensing, 12.17 (2020): 2702.