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25 February 2019 at 18:41 #1328
S3VT-OC meeting summary: 20th February 2019
Radiometric gains for OLCI-A:
- Gain trends are stabilising except for band 1 (strongest on camera 4).
- Model currently departing from the gain evolution, update foreseen in April.
- Most bands RMS below 0.1, all below 0.4. Expect evolution of model to bring all within 0.1%.
Radiometric gains for OLCI-B:
- With reference to calibration on 26/01/2019.
- Band 1 also most strongly evolving.
- No aging of diffuser taken into account – more calibrations needed to do this.
- Calibrations done during yaw campaign are being used to update bidirectional reflectance distribution function (BRDF) model of OLCI diffuser (coming in April).
Geometric calibration for OLCI-A:
- Recalibration done on 14/03/2018
- Overall performance 0.3/0.4 RMS.
- Spikes related to SLSTR decontamination
- Check cyclic report to see details: https://sentinels.copernicus.eu/web/sentinel/technical-guides/sentinel-3-olci/data-quality-reports
Geometric calibration for OLCI-B:
- Persistent drift due (entirely) to along-track degradation. Calibration done in Dec 2018 for L1 public release.
- Across track pointing, no drift, <0.1 RMS
- Another geometric calibration coming in March.
Level 1 updates:
- Geometric calibration in March
- Update of OLCI-A radiometric gain evolution model.
- New BRDF diffuser and evolution model for OLCI-B.
- Cyclic reports on the status of OLCI-A and B are available through: https://sentinels.copernicus.eu/web/sentinel/technical-guides/sentinel-3-olci/data-quality-reports
L2 product status for OLCI-A:
- Validation continuing with Aeronet-OC.
- In general results are reliable at global spatial scales (at 412, 443, 590 nm).
- Stable in oligotrophic waters when compared to other sensors (given differences in band spectral centres)
L2 product status for OLCI-B:
- Not very accurate comparisons at the moment – needs System Vicarious Calibration (SVC).
- Difficulties with MOBY operations during 2018 and unstable matchups with OLCI-B
- Few matchups, difficulty in obtaining reliable SVC gains.
- Decision will need to be made to coincide with L2 public release end of March.
- Sentinel-3B Routine Operations Readiness Review in March will follow the OLCI-B L2 public release
Marco Peters – C2RCC in SNAP
Discussions were had at previous S3VT meetings as to how the operational processing can be recreated using SNAP. It was noted that there are differences between the chlorophyll-a and other products generated using C2RCC in SNAP, and those available in the standard level-2 OLCI product. This observation also has impacts on those who wish to use and/or validate the level 2 reflectance from this processor. Marco Peters from Brockmann Consult was asked to comment on the sources of differences between the operational processing chain, and the implementation of C2RCC in SNAP.
Overview of OLCI L2 processing chain is available in OLCI handbook (see: https://www.eumetsat.int/website/wcm/idc/idcplg?IdcService=GET_FILE&dDocName=PDF_DMT_907205&RevisionSelectionMethod=LatestReleased&Rendition=Web)
In the operational processing chain, there are steps between L1B product values and entrance to C2RCC, which are either not present or implemented separately in SNAP.
Different pre-processing steps:
- Radiance to reflectance conversion, pixel pre-classification, gaseous correction, pixel post-classification, smile correction, application of System Vicarious Calibration gains.
- Currently, pixel classification and smile correction are not applied in SNAP.
- In the operational processing the SVC gains are applied after the pre-processing steps.
- In SNAP, a GPT graph has been supplied to S3VT which applies the SVC gains but the gains are applied at top of chain, directly to the L1B product radiances.
Climatology for temperature and salinity:
- Another source of differences
- Operational processing uses a climatology for which the Auxiliary Data File OL_2_CLP_AX is available to S3VT users
- In SNAP, values are constant for whole scene when applied in C2RCC.
Question – how big are differences?
- Hard to tell with the reflectances as these are not produced in the operational chain.
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