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