SNAP Data Processors - MERIS L1b Fourth to Third Reprocessing Adaptation Algorithm Specification |

### Algorithm Specification

#### Adaptation of spectral bands

The 15 spectral bands in the MERIS L1b Fourth Reprocessing product, M01_radiance,.., M15_radiance are simply adapted by band name to the 15 spectral bands as specified in the MERIS L1b Third Reprocessing product, radiance_1,..., radiance_15. Spectral band properties, such as description, unit no data value etc., are also adapted to the third reprocessing conventions. A solar flux value is also set as property for each radiance_* band: For each spectral band, the mean of the corresponding solar flux band in the fourth reprocessing product is computed.

#### Adaptation of the 'detector_index' band

The 'detector_index' band is simply copied from the fourth into the third reprocessing product. The band properties are set following the third reprocessing conventions.

#### Adaptation of tie point grids

The tie point grids are simply copied from the fourth into the third reprocessing product and adapted by name where possible. However, there are a few specific differences to be considered, as listed below:

- For the sun and view azimuth angles, the value range needs to be converted from [-180..180] to [0..360]. I.e., negative values need to be increased by 360 in the third reprocessing product.
- For total ozone, the values need to be converted from kg/m2 to DU.
- The 'rel_hum' tie point grid in the third reprocessing product provides the relative humidity at 850 hPa. This is obtained from the 'humidity_pressure_level_14' tie point grid in the fourth reprocessing product. The fourth reprocessing product provides the relative humidity as a 20-level profile.
- The fourth reprocessing product does not provide equivalent tie point grids for the 'dem_rough', 'lat_corr' and 'lon_corr' tie point grids in the third reprocessing product. Therefore, these tie point grids are not provided in the result product of this adaptation.

#### Adaptation of the quality flag

The 'quality_flags' band in the MERIS L1b Fourth Reprocessing product is converted to a 'l1_flags' band as specified in the MERIS L1b Third Reprocessing product. All single flags in the 'l1_flags' band are also available in the 'quality_flags', thus the adaptation is a simple extraction of the relevant bits and mapping them into the 'l1_flags' coding.

#### Adaptation of the product metadata

The product metadata structure is complex and rather different for the fourth and third reprocessing products. In the current processor version, just a small metadata subset is extracted from the fourth reprocessing product and written into the 'MPH' metadata section of the third reprocessing product. In a future processor version, the metadata of the third reprocessing product will be extended, depending on user demands.

### References

[1] European Space Agency: MERIS Product Handbook, Issue 3.0, 1 August 2011
https://earth.esa.int/eogateway/instruments/meris
[2] European Space Agency: Release notes of Envisat MERIS 4th reprocessing data, 14 Jul 2020. https://earth.esa.int/eogateway/news/release-of-envisat-meris-4th-reprocessing-data-new-online-dissemination-service