Rough ocean surface showing currents. Credit: Peter Gueth

BRDF correction of S3 OLCI water reflectance products


Rough ocean surface showing currents. Credit: Peter Gueth
Rough ocean surface showing currents. Credit: Peter Gueth

Implementation of a BRDF-correction module for Copernicus Sentinel-3 OLCI over clear and optically complex waters.

Last Updated

13 March 2023

Published on

16 May 2022

The Bidirectional Reflectance Distribution Function (BRDF) describes angular reflection properties. If the illuminated area was opaque, such as dry land, the BRDF effect would only be driven by the optical characteristics of the reflecting surface. In the ocean, the BRDF also depends on the optical properties of the illuminated volume. The BRDF correction aims to minimise the dependence of the measured water reflectance on the solar and viewing geometry, improving the quality of ocean colour data products.

The development of BRDF correction methods has been a matter of various investigations, yet the ocean colour community recognises the need for improvements. The underlying difficulty is the lack of an exact radiative transfer solution that fully accounts for environmental conditions, and allows correcting BRDF effects in a remote sensing image on a pixel basis. Hence, the need to formulate an approximated BRDF correction as a trade-off between accuracy requirements and operational constraints.

This study has analysed reference BRDF correction schemes presented in the literature, and evaluated their performance with satellite and in situ data, considering both clear and optically complex waters. The main outcomes are:


The study's objective is to analyse the performance of reference BRDF correction methods presented in the literature — including potential enhancements of such methods — and select the most appropriate for operational S3 OLCI data processing over clear and optically complex waters.


1. Study framework

The BRDF effect of interest in ocean colour remote sensing is changes in the water reflectance as a function of the sun zenith angle, the viewing zenith angle, and the relative azimuth angle between the sun and the observer. The scope of the BRDF correction is to minimise these variations of the water reflectance. Morel and Gentili (1996) gave the following definition of the fully (or exact) normalised water reflectance: the reflectance that a nadir-viewing instrument would measure if the sun were at the zenith, in the absence of any atmospheric loss and when the Earth is at its mean distance from the sun (Figure 1).

Measurement and illumination geometry of the water reflectance and the exact water reflectance in the left and right panel, respectively
Figure 1: Measurement and illumination geometry of the water reflectance and the exact water reflectance in the left and right panel, respectively

The BRDF correction schemes investigated in this study for the processing of OLCI data are those proposed by:

  • Morel et al. (2002), termed M02
  • Park and Ruddick (2005), termed P05
  • Lee et al. (2011), termed L11
  • He et al. (2017), termed H17
  • Twardowski and Tonizzo (2018), termed T18—the original study denotes this scheme as ZTT for Zaneveld-Twardowski-Tonizzo (Zaneveld, 1995)


All methods were implemented in the OLCI operational Instrument Processing Facility (IPF) as a sub-module, except the H17 method, due to limited spectral applicability.

The validation of the M02, P05 and L11 BRDF corrections was based on:

  • Measurements acquired with Optical Floating System (OFS, Talone et al., 2018) in the Mediterranean and the Black Sea (Figure 2), as well as measurements performed with two TriOS RAMSES systems at the NIOZ Jetty Station (NJS) in the Dutch Wadden Sea.
  • EUMETSAT Matchup Data Dase (MDB) with S3 OLCI and coincident field measurements at the AERONET-OC (Zibordi et al., 2021) and MOBY sites (Clark et al., 2002; Voss et al., 2018).
  • Overlapping OLCI A and B images acquired during and outside the tandem phase (the former as a benchmark and the latter to account for different viewing geometries).
Table 1: Summary of BRDF assessment results. These results are not entirely consistent at this stage because match-up in-situ data were corrected by M02.
  No BRDF M02 P05 L11
Field measurements Third option Second option Second option Best option
Match-up data Best option Third option Second option Second option
OLCI-A and B images Third option Second option Second option Best option

Assessment results are summarised in Table 1. The study recommendations are to:

  • rely on L11 BRDF correction implementation in the IPF for the operational processing of OLCI data (Figure 3);
  • enable access to OLCI data that are not BRDF corrected as a user’s post-processing option.
Spectral percent corrections ε^* determined from OFS measurements
Figure 2: Spectral percent corrections ε* determined from OFS measurements (black) or obtained with the M02, P05, and L11 BRDF correction schemes in Case-1 (left), Case-2a (middle), and Case-2b (right) waters—panels (a), (b), and (c), respectively. Values closer to the measured “Meas” are indicative of better performance. The corresponding differences Δ* between theoretical and experimental corrections are also displayed—panels (d), (e), and (f), respectively. In this case, values closer to 0 indicate better performance. Error bars represent the standard deviations. Dashed areas indicate the estimated precision of the mean. N is the number of data points available for each independent comparison (see Talone et al., 2018 for details).
: OLCI images in the North Adriatic Sea, example of BRDF correction obtained with the L11 method.
Figure 3: OLCI images in the North Adriatic Sea, example of BRDF correction obtained with the L11 method. The left and center panels show uncorrected and corrected water reflectance maps at 510nm, respectively. The panel in the right column shows percent differences between the two maps, displaying larger differences in coastal areas.

The assessments based on in situ OFS data are reported in Figure 2. An application example of the L11 BRDF correction to OLCI data is presented in Figure 3.

The study also implemented novel methods (Figure 4) to:

  • Identify the BRDF correction validity based on the projection of input IOPs into a 2D vector space expressing the relative contribution of 1) molecules versus particle scattering and 2) single versus multiple scattering.
  • Estimate the accuracy of the BRDF corrections based on a repeatability analysis.
Novel methods
Figure 4: The region out of the L11 applicability range is marked in a shade of violet (left). Accuracy estimates based on replicability analysis of BRDF correction results (right).

Future developments

The following guidelines have been identified for future developments:

  • Adopt the L11 main design.
  • Use Fournier-Forand scattering phase functions (Fournier, 2007; Fournier and Forand, 1994) for both phytoplankton and non-algal scattering.
  • Extend the IOPs variability in comparison to L11 (Lee et al., 2002, 2011), for instance, by referring to the Coastcolour data (Nechad et al., 2015).
  • Revise the empirical steps of the QAA to retrieve the IOPs from the water reflectance and provide a performance assessment.
  • Account for Raman scattering (but also verify the performance when Raman verify the performance when Raman is excluded).
  • Complement BRDF correction results with the accuracy estimates.