Mission Performance

Sentinel-3 OLCI mission performance

Instrument performance

The OLCI-A and OLCI-B instrument health is excellent. The sensors temperatures are perfectly well controlled. The nominal radiometric diffusers ageing shows the expected magnitude and spectral behaviours: around 0.4% after 4 years for OLCI-A at 400 nm (Oa01), down to 0.1% at 510 nm (Oa05) and undetectable above; below 0.25% for OLCI-B.

The instrument sensitivity evolution so far is limited to less than 2.5% (OLCI-A) and no evidence of degradation can be demonstrated: the variation of the instrument sensitivity seems more correlated with a potential spectral evolution of the correcting filters – inside the spectrometers – than to darkening of the optics or loss of sensitivity of the CCD sensors. Sensitivity evolution of OLCI-B is similar to that of OLCI-A, and maybe with a slightly higher magnitude for the 400 nm channel. The regularly monitored instrument SNR performance is well within requirement.

Spectral Calibration is monitored thanks to dedicated acquisition campaigns. The in-flight spectral campaigns reveal a high agreement of the in-flight characterisation with the pre-flight spectral calibration for both A and B sensors, with differences of the OLCI channels centre smaller than 0.1 nm, except for channels Oa01 (400 nm) and Oa21 (1020 nm), with up to 0.2 nm.

A small temporal evolution is observed, different for each camera but approximately identical at all wavelengths; the observed changes for OLCI-A after 4 years are smaller than 0.2 nm, and even 0.1 nm for cameras 3 and 5; observed changes for OLCI-B are within 0.2 nm for all cameras but camera 3 have stabilised.

Level 1 products performance

The geometric performance is monitored using the ESA GeoCal tool CFI. It is currently fully compliant for OLCI-A and OLCI-B to the 0.5 pixel RMS requirement. However, a significant along-track drift of OLCI-B cameras has been assessed, requiring frequent geometric re-calibration.

The OLCI-A and OLCI-B Radiometric Gain Models (gain at reference date + time drift) are used to calibrate Earth Observation data at any date. Their current performance is better than 0.1% RMS.

Absolute and inter-band calibration performance is monitored by indirect methods over natural targets. Three methods are used within S3-MPC: the "Rayleigh" method (molecular atmospheric backscattering over clear sky off-glint open ocean) provides absolute calibration in the blue-to-red spectral domain; the "Glint" method (spectral dependency of the Sun specular reflection over ocean) provides inter-band calibration; and the PICS method (Pseudo-Invariant Calibration Sites, temporally stable desert areas) provides absolute calibration over the whole spectral domain as well as cross-mission comparisons for sensors with comparable channels.

Two of these methods, Rayleigh and Glint, are undertaken by two different implementations providing very consistent results.

All methods point out an excess of brightness for OLCI-A radiances. Results are in pretty close agreement around 2-3% between 560 and 900 nm (Oa06 to Oa19). Rayleigh gives higher biases in the blue-green (about 6 % while PICS remains around 2%) but this method is suspected to overestimate the simulated signal at those wavelengths so PICS are considered more reliable. Channel Oa21 (1020 nm) is only addressed by the Glint interband method and the results are much worse: 3 to 7%, depending on the reference band.

Radiometric validation for OLCI-B indicates performance within the 2% requirement for all bands from 560 nm (Oa05) to 940 nm (Oa20). As for OLCI-A, the PICS method shows compliance also in the blue region (Oa1 to Oa4, 400 to 510 nm) while the Rayleigh method shows biases of about 3 to 5%, depending on implementation. The OLCI-B 1020 nm (Oa21) has a similar performance that its OLCI-A counterpart.

Level 2 products performance

Integrated Water Vapour

Integrated Water Vapour has been validated against available in-situ data, according to the surface type: GNSS and AERONET networks over Land, AERONET (coastal stations), AERONET-OC and AERONET Maritime networks over water.

Validation demonstrates that the product is of high quality (bias corrected RMS difference of ~ 0.8 to 1.5 kg/m2) for retrievals above land surfaces, but there is a systematic overestimation of 9% to 13%. Validation for OLCI-B gives similar results.

The comparison with GNSS stations close to water shows a larger wet bias for the ocean retrievals (up to 25%), and in particular in transition zones between glint and off glint.

Land Products

OLCI Global Vegetation Index (O-GVI, a.k.a. FAPAR)

Quantitative validation against in-situ data is not possible so far, as no in-situ station provides directly comparable products. Several specific campaigns have been conducted however, and significant efforts are undertaken to generate adequate in-situ data. In the meantime, OLCI FAPAR is regularly compared to MERIS 10-years climatology.

There is a fairly good agreement, accounting for the methodology limitations, with high correlation, > 0.9 (when sufficient dynamics are present) and good RMSD (<0.1).

OLCI Terrestrial Chlorophyll Index (O-TCI)

For the same reason as for O-GVI, no quantitative validation against in-situ data is available and comparison with MERIS TCI (M-TCI) climatology has been done over a number of sites, showing high correlation, > 0.9 (when sufficient dynamics are present) and good RMSD (<0.1).

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