WP500 – Options and approaches to the long-term vicarious adjustment of Sentinel- OLCI & MSI A/B/C and D instruments

The launch of CZCS in the 1970s has demonstrated the potential of ocean colour orbital sensors in oceanographic studies. The ocean science community had to wait over a decade before the launch of SeaWiFS but since then, we have been granted access to continuous ocean colour data with the successful operation of MODIS, MERIS and VIIRS sensors. The ocean colour earth observation is now entering a new era with the COPERNICUS space component and the Sentinel-3 programs. The Sentinel series will ensure continuous data flow from identical sensors, therefore opening the path to operational services. A fundamental interest of the ocean colour sensors for the scientific community is to provide worldwide information that can be analysed and interpreted in a context of global climate change. This unique data source is therefore a key element to construct Climate Data Records (CDRs) of Essential Climate Variables (ECVs).

The historical uncertainty requirement for remotely-sensed water leaving reflectance (ρw) was defined as 5% in the blue green part of the spectrum (Gordon & Clark, 1981, Gordon et al., 1983, Gordon, 1987, Gordon 1997).

However, it has been demonstrated (Gordon, 1998, Hooker et al 1992, Hooker and McClain 2000) that reaching a 5% accuracy on 𝝆𝒘 requires an absolute accuracy on 𝝆𝑻𝑶𝑨 between 0.25% and 1%, which cannot be insured through purely instrumental calibration and characterisation. For this  reason,  we  rely  on  vicarious  adjustment,   that  is  on  a complementary calibration using ground-truth measurements (Franz et al. 2001, Franz et al. 2007; Bailey et al. 2008), to meet the 5% requirement accuracy.

Historical methods to derive operational vicarious adjustment gains have been derived from a two steps procedure:

  • Gain computation over NIR bands of the
  • Gain computation over visible bands of the

The first one, just like the basis for atmospheric correction relies on the black pixel assumption that is the sea water is black in the NIR infrared bands. For this first step, we can make use of the clearest regions of the world ocean like the South Pacific Gyre. This step does not rely on actual in situ measurements and we can therefore rely on statistically representative computation of NIR gains.

The latest rely on the availability of highly accurate in situ reference measurements preferably collected in clear oligo or mesotrophic water. Owing to the necessity to produce statistically reliable gains and over a sufficiently long period of time, the fundamental sources of such data are coming from long term deployment of radiometric buoys, namely BOUSSOLE and MOBY.

With the perspective of the Sentinel-2 and Sentinel-3 series of COPERNICUS space program and the objectives in operational oceanography, it appears necessary to rethink and secure a permanent flow of FRM for orbital sensors vicarious adjustment.

D-230 Prepare and host a 2-3 day international workshop


The workshop is organized in ESRIN, Italy.


The workshop organized to fulfil this task shall address two fundamental aspects of the implementation of a vicarious adjustment:

  • Protocols and requirements for the acquisition of FRM for vicarious adjustment
  • Procedures and methodologies to derive vicarious gains

We organise the workshop on 5 days with three half day sessions of four presentations (12 speakers)  and two round tables. Key presentations should detail the instrumentation, structure, data processing and data quality control of existing long- term deployments (namely AERONET-OC, BOUSSOLE and MOBY). A critical analysis of these systems should be presented to demonstrate the benefits and drawbacks of each system with regards to vicarious adjustment.

Invited guests might be, among others, David Antoine (LOV, BOUSSOLE’s PI) Giuseppe Zibordi (JRC, AERONET-OC’s PI), Kenneth Voss (Uni of Miami, MOBY’s PI), Jean-Paul Huot (ESTEC) and Brian Franz (NASA). Experience from GOCI PIs (Younge-Je Park would also be valuable for the debate as they performed a vicarious adjustment on an imager that does not sample the usual oligotrophic waters used for vicarious adjustment.

The call for abstract will obviously be sent to the S3VT community.

Protocols and requirements for the acquisition of FRM for vicarious adjustment

To address this point, the first step will consist, in preparation of the workshop, to document the lessons learned from international teams. Based on the lessons learned and prior the workshop, an open-forum shall provide a critical analysis of existing systems (notably BOUSSOLE, AERONET-OC and MOBY) and point out their strength and weaknesses in terms of:

  • concept (mooring versus stations ie above or below water sensors)
  • instrumentation (multi versus hyperspectral, characterisation, …)
  • data processing
    • conversion to water leaving reflectance (from under or above water measurements)
    • tilt correction
    • polarisation correction
    • wave focussing
    • instrument and structure shadow correction
    • bio-fooling
  • data quality control
  • error budgets
  • value for money

Then new emerging technologies shall be investigated to assess their potential in providing FRM for OCR (optical profiler, gliders, opportunity ships, medium term deployments of mooring …). These new technologies could provide new sources of FRM and possibly with a better “value for money”. The approach to investigate the potential of this data would be:

  • to analyse measurements and data processing protocols, quality control procedures to ensure their compliance with FRM protocols defined in task -2
  • to include such systems in task 4 round
  • to analyse their performances against contemporaneous vicariously adjusted satellite data

This debate and final workshop shall end-up with recommendations for the next generation of a System Vicarious Adjustment (SVA). These recommendations shall envisage not one but several potential SVA. The recommendation shall include the design of the systems with specifications about the instrumentation to be installed, how it should be operated and maintained and how the data should be processed and quality controlled. Most results of task 2 to 4 should highly contribute to these specifications.

In addition, proposition for potential new locations shall be investigated with the objective to maximize the following requirements (Gordon 1998, Zibordi 2015):

  • Cloud free conditions and, clear maritime atmosphere dominated by maritime aerosols
  • Horizontal homogeneity of the ocean properties
  • Oligo or mesotrophic waters to maximize the signal in the blue part of the

All these points shall be assessed through careful analysis of global level 3 products of cloud cover as well as radiometry and chlorophyll climatology.

In addition to these three technical considerations should also be added a practical one: the “value for money” and therefore accessibility and/or the availability of a qualified group in the vicinity of the station to ensure the regular and high quality maintenance of the station.

Procedures and methodologies to derive vicarious gains

As mentioned in the above section, existing and historical vicarious adjustment gains have been derived by two step procedure (NIR then visible). A critical analysis of the literature will be performed to analyse the different procedures for vicarious adjustments. The different approaches will be reassessed with respect to existing in situ marine or atmospheric data but also with new algorithm development. Recent algorithm development in MERIS 4th reprocessing have indeed rendered useless the recourse to implement a NIR bands vicarious adjustment.

D-240  Write a Proceedings (PROC-1) of the FRM4SOC International Workshop

This report to be delivered at KO+12 will merge and analyse the outcome of the workshop and will describe the options for a future structure required for long-term vicarious adjustment in terms of

  • Potential location
  • Structure
  • Instrumentation
  • Data processing, quality control, archiving and
  • Cost

It is recalled here that the SoW mention that the proceeding “could take the form of a scientific Journal Special Issue”. The proceeding publication costs if needed shall be covered by the Limit Of Liability.

D-250  Write a Technical Report (TR-10)

This document to be delivered at KO+24 will provide, from the experience of the workshop and the overall results of the project, requirements and recommendations for the infrastructure required for the long-term vicarious adjustment of the Sentinel-3 OLCI and Sentinel-2 MSI A/B/C and D instruments.

It is recalled here that the SoW mention that the Technical report “could be a peer reviewed journal article or, better still, managed according to the processes defined by the IOCCG that would result in an IOCCG monograph if approved by the IOCCG”. The peer-reviewed journal or IOCCG monograph publication costs if needed shall be covered by the Limit Of Liability.

  • Bailey, S. W., B. H. Hooker, D. Antoine, B. A. Franz and P. J. Werdell, 2008. Sources and assumption for the vicarious calibration of ocean color satellite observations, Applied Optics Vol. 47, No. 12, 2035 – 2045.
  • Franz, B. A., E. J. Ainsworth and S. W. Bailey, 2001. SeaWiFS, vicarious calibration: an alternative approach utilizing simultaneous in situ observations of oceanic and atmospheric optical, properties, NASA Tech. Memo. 209982, National Aeronautics, and Space Administration, Goddard Space Flight Center, Greenbelt, MD.
  • Franz, B. A., S. W. Bailey, J. Werdell, Ch. McClain, 2007. Sensor-independent approach to the vicarious calibration. of satellite ocean color radiometryc. Applied Optics, Vol. 46, No. 22, 5068 – 5082.
  • Gordon, H. R. (1987). Calibration requirements and methodology for remote sensors viewing the ocean in the visible. Remote Sensing of Environment,22,103–126.
  • Gordon, H. R., 1997. Atmospheric correction of ocean color imagery in the Earth observing system era, J. Geophys. Res. 102D, 17081 – 17106.
  • Gordon, H. R., & Clark, D. K. (1981). Clear water radiances for atmospheric correction of coastal zone color scanner imagery. Applied Optic,20, 4175–4180.
  • Gordon, H. R., Clark, D. K., Brown, J. W., Brown, O. B., Evans, R. H., & Broenkow, W. W.(1983). Phytoplankton pigment concentrations in the Middle Atlantic Bight: Comparison of ship determinations and CZCS estimates.Applied Optics,22,20–36.
  • Gordon, H. R. (1998).In orbit calibration strategy for ocean color sensors.Remote Sensingof Environment,63,265–278.
  • Hooker, S. B. and C. R. McClain, 2000. The calibration and valida-tion of SeaWiFS data, Prog. Oceanogr. 45, 427 – 465.
  • Hooker, S. B., W. E. Esaias, G. C. Feldman, W. W. Gregg and C. R. McClain, 1992. An overview of SeaWiFS and ocean color, NASA Tech. Memo. 104566, National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, MD.
  • Lerebourg C., Mazeran C., Huot J.P., Antoine D., 2011. MERIS ATBD 2.24 -Vicarious adjustment of the MERIS Ocean Colour Radiometry.