SNAP Data Processors - OLCI Anomaly Flagging Algorithm Specification |

### Algorithm Specification

An inconsistency has been identified in OLCI data around areas of saturation. Due to partial saturarion anomalously low radiances occur in some bands for pixels at the edge of saturated areas, and they do not correspond to geophysical expectations. The issue is present over bright surfaces, and currently detected at the edges of clouds for both OLCI-A and OLCI-B instruments.
Due to an issue in a base library, it can happen (very rare cases have been observed) that the altitude values calculated are beyond the valid data range (e.g. +29153 m)
This processor identifies both issues in the data and adds an additional flag-data variable that flags pixels identified being affected by these issues so that these can be excluded from further downstream processing.

#### Partial saturation (Spectral Inconsistency) Detection

Partial saturation (inconsistent spectra) are detected by calculating the spectral slope between band-centers and thresholding the maximal absolute value of the slope with an appropriate value (0.15 / nm). For this approach, the spectral measurements in atmospheric absorption bands are excluded form the calculation. The radiances are first converted to reflectances, and a spectral slope is calculated for bands [1-12, 16-18, 21]. If the maximal absolute value of the slope values exceeds the threshold, the pixel is flagged.

#### Altitude Outlier Detection

Altitude outliers are detected by checking that the data for each pixel is within the geophysical meaningful range of [-11050 m, 8850 m]. If an altitude value is outside of this range, the pixel is flagged.

### References

[1] Alexis Deru, and Ludovic Bourg:
OLCI anomalous spectral samples
S3MPC.ACR.MEM.087, 16/10/2020