r bulletin 103 — august 2000

MERIS – A New Generation of Ocean-Colour Sensor onboard Envisat

J.-L. Bézy, S. Delwart & M. RastESA Directorate of Applications Programmes, ESTEC, Noordwijk, The Netherlands

IntroductionIn mid-2001, ESA will launch the MediumResolution Imaging Spectrometer (MERIS)onboard its polar-orbiting Envisat Earth-observation satellite. MERIS, initially primarilydedicated to ocean-colour observations, hashad the scope of its objectives broadenedduring development to include atmosphericand land-surface related studies. This has beenpossible due to the great flexibility that sensorand ground segment provide.

to land-surface parameters, in particularvegetation processes.

With all of the above-mentioned features,MERIS is considered to be a remote-sensingtool with great potential to contribute to climatestudies and global-change observations byaddressing environmental features in amultidisciplinary way. This article introduces theMERIS end-to-end mission concept (Fig. 1),based upon the scientific requirements andmission objectives.

In advance of launch, the ground segment isbeing designed and algorithms are beingdeveloped for the interpretation of MERIS’sobservations, and dedicated studies areestablishing the means for validating its dataproducts. This effort is being undertaken inclose co-operation with the European ExpertSupport Laboratories, whose scientists are themain providers of the retrieval algorithms.Wherever possible, the underlying physicalmodels are being validated based onexperience acquired before Envisat’s launch,using data provided by airborne or shipbornecampaigns and in-situ measurements atspecially equipped campaign sites.

Scientific rationaleWith the advent of large-scale optical imagingspaceborne sensors in the seventies, a toolwas found that enabled the observation andmonitoring of the Earth’s surface in aqualitative, but synoptic fashion. The biggestasset of these sensors, of which the AdvancedVery High Resolution Radiometer (AVHRR) isthe most prominent example, was their largecoverage swath and high repeat rate, enablingthe timely observation of changing large-scalephenomena. With the end-of-life of the USCoastal-Zone Color Scanner mission (CZCS) in1986, the scientific oceanographic communitydemanded a new spaceborne ocean-colourobserving system that would allow moreaccurate determination of oceanic constituents,such as chlorophyll, suspended matter anddecayed organic material, thereby providing

ESA has developed a multidisciplinary Earth-observation instrumentthat focuses on biological ocean observations. This MediumResolution Imaging Spectrometer (MERIS) will provide a remote-sensing capability for observing, inter alia, oceanic biology and marinewater quality through observations of water colour. MERIS, which willbe the first spaceborne wide-field imaging spectrometer, is a nadir-looking sensor operated in a push-broom mode. The instrumentmeasures the solar reflected radiation in fifteen spectral bands in thevisible and near-infrared parts of the spectrum. Of particular interestis the ability to programme the position and width of the spectralbands in flight. This allows a large set of applications, making theinstrument concept also attractive for future Earth Watch missions.ESA will produce a set of validated data products, such as chlorophyllconcentration, water vapour and global vegetation index, which willbe available at two spatial resolutions (300 m and 1200 m).

MERIS will have a high spectral and radiometricresolution and dual spatial resolution, within aglobal mission covering open ocean andcoastal-zone waters and a regional missioncovering land surfaces. One of the instrument’smost outstanding features is the program-mability of its spectral bands in terms of widthand position.

MERIS’s global mission will make a majorcontribution to scientific projects that seek tounderstand the role of the oceans and oceanproductivity in the climate system, through itsobservations of water colour, and will furtherincrease our ability to forecast change throughmodelling. Secondary objectives of the missionwill be directed to the understanding ofatmospheric parameters associated withclouds, water vapour and aerosols, in addition

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Figure 1. The MERIS flightmodel during integration atAlcatel Space Industries inCannes (F)

Atmospheric missionThe radiation balance of the Earth/atmospheresystem is dominated by water vapour, carbondioxide and clouds, as well as being verydependent on the presence of aerosols.However, the global monitoring of cloudproperties and their processes is not yetsufficiently accurate. MERIS is intended to helpredress this balance by providing data oncloud-top height and optical thickness, water-vapour column content, as well as aerosolproperties.

Land missionQuestions related to global change include therole of terrestrial surfaces in climate dynamicsand bio-geochemical cycles. Spatial andtemporal models of the biosphere are currentlybeing developed to study the mechanics ofsuch complex systems in order to predict theirbehaviour under changing environmentalconditions. These models are based onphysical and biophysical relationships, whichneed to be validated on a regular basis usingdata from spaceborne sensors. Repetitive,accurate physical measurements arenecessary to quantify surface processes and toimprove the understanding of vegetationseasonal dynamics and responses toenvironmental stress.

Scientific requirementsIn order to achieve these mission goals, thedifferent radiometric and geometricrequirements imposed by the variousobjectives have to be satisfied. With the help ofthe ESA Science Advisory Group for MERIS,these requirements have been refined, takinginto account the constraints imposed by a

vital information about the water’s quality andits productivity.

Driven by the concerns about our environmentand the pressing need for a new ocean-colourobserving system, ESA, supported by itsinternational scientific advisors, began thedevelopment of a spaceborne large-scaleoptical system with the primary objective ofproviding quantitative ocean-colour measure-ments, but having enough flexibility to alsoserve applications in atmospheric and land-surface science.

Mission objectivesBased on the above rationale, missionobjectives were derived driving the developmentof the MERIS instrument and its mission.

Ocean missionThe principal contributions of MERIS data tothe study of the upper layers of the ocean will be:

– the measurement of photosyntheticpotential by detection of phytoplankton(algae)

– the detection of yellow matter (dissolvedorganic material)

– the detection of suspended matter (re-suspended or river-borne sediments).

Apart from the above three major observablefeatures, it should also be possible to detectspecial plankton blooms, for example red tidesthrough their absorption feature near 520 nm.In addition, investigations of water quality, themonitoring of extended pollution areas andtopographic observations (such as coastalerosion) should also be possible.

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Table 1. The 15 spectralbands of MERIS

polar-orbiting platform and the technicalcapabilities of an imaging spectrometer.

Geometric requirementsThe MERIS data are of interest both for globalobservations and for detailed examinations forregional applications, and operation at twospatial resolutions has therefore been selected.Full-resolution (FR) data with 300 m on-groundresolution at the sub-satellite point will bemainly required in coastal zones and over land. Reduced-resolution (RR) data with 1200 mon-ground resolution at the sub-satellite point are intended for large-scale studies.Oceanographic and atmospheric investigationsrequire global Earth coverage within three days.

Spectral requirementsMERIS is designed to acquire 15 spectralbands in the 390 - 1040 nm region of theelectromagnetic spectrum. The band position,bandwidth and gain are in-flight programmable.The spectral bandwidth can be varied between1.25 and 30 nm, depending on the width of aspectral feature to be observed and theamount of energy needed within a given bandto perform an adequate observation. Overopen ocean, an average bandwidth of 10 nm isrequired for bands located in the visible part ofthe spectrum. Driven by the need to resolveother spectral features such as the oxygenabsorption band occurring at 760 nm, aspectral bandwidth lower than 10 nm isrequired. In accordance with the mission goalsand priorities for this instrument, a set of 15spectral bands has been derived foroceanographic and interdisciplinary applications(Table 1). These spectral bands are used as thereference set for prototyping the ground-segment algorithms. Preliminary results seemto advocate a fine tuning of some of the bandsto optimise product quality. If confirmed, thisfine-tuning can be performed using theinstrument’s programmability.

The spatial, spectral and radiometric

programmability of MERIS is justified by thedifferent scales of the various targets to beobserved and the diversity of their spectral andradiometric properties. The advantage of theprogrammability lies not only in being able toselect the width and position of a particularspectral band, but also in being able to tune thedynamic range and adapt it to different targetobservations, which may become of higherpriority during the lifetime of the MERIS mission.

Radiometric requirementsThe radiometric performance is one of the mostcrucial requirement for MERIS because thesignals coming from the ocean are weak andthus most difficult to detect and quantify. Eventhough the radiometrically most challengingtarget to be observed is the open ocean,MERIS also has to have a large dynamic rangeto cover these low-level signals as well assignals emanating from bright targets such asclouds and land surfaces, throughout itsspectral range. This imposes demandingradiometric performance requirements.

- Open oceanIn the upper layer of the open ocean, thechlorophyll concentration varies from less than0.03 mg.m-3 in oligotrophic waters, up to about30 mg.m-3 in eutrophic situations. Oceancolour responds to this variation, which spansover three orders of magnitude, in a non-linearmanner. The goal with MERIS is to discriminate30 classes of pigment concentrations withinthe three orders of magnitude. These classesshould be of equal logarithmic width. Thisrequirement translates into a radiometricsensitivity of 2 x 10-4 for NE∆R (noise equivalentspectral reflectance at sea level) being set forMERIS.

- Coastal watersFor the detection of several water substances,commonly used techniques like simple colourratios which are applied successfully for openoceans, are not sufficient. The similarity of thespectral scattering and absorption coefficientsfor all optically active water substances posesproblems in finding an adequate procedure fortheir detection. Here the Sun-stimulatedchlorophyll fluorescence at a wavelength of681.25 nm can improve the detection ofpigment concentration. The fluorescence signalis small, but is detectable from space. Thespectral resolution should ideally be better than 10 nm and the radiometric resolution betterthan 0.03 W (m-2 sr-1 nm-1) for the discriminationof a 1 mg.m-3 pigment concentration.

Outstanding radiometric accuracy is imperativefor the atmospheric correction, which is ofcritical importance since, over ocean, typically

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satellite. Envisat also has a Solid-StateRecorder (SSR) which can be used to recordFR data when outside DRS coverage orground-station visibility. The combined usageof DRS, ground station and SSR will ensurethat every part of the globe can be accessedwith FR data.

Instrument design and performanceFigure 3 shows the mechanical layout of MERIS.The instrument consists of five identicalcameras sharing the large field of view of 68.5°(Fig. 3). These cameras are arranged in a fan-shaped configuration in which their fields ofview overlap slightly. The modular designensures high optical image quality over thelarge field of view. The output of each camerais processed separately in an analogue anddigital processing unit (Fig. 4).

Optical sub-assemblyThe optics of each camera has an externalwindow, a folding mirror, an off-axiscatadioptric ground imager and aspectrometer. The window scrambles theincident light coming from the Earth, makingthe instrument less sensitive to changes inpolarisation. The dispersive element of thespectrometer is a low-groove-density holo-graphic grating. The aperture stop is located onthe grating. The entrance pupil is located infront of the camera optics, making it a suitableposition for the scrambling window.

Focal-plane sub-assemblyThe camera’s detectors are CCD arraysspecifically developed for MERIS (Fig. 5).Thinned back-side-illuminated CCDs havebeen selected, which offer the required highresponsivity in the blue part of the spectralrange. The CCDs are operated in frame transfermode. At the end of a fixed period (integrationtime), the charges are rapidly transferred to thestore region allowing the acquisition of the next frame to start. Charges in the store regionare subsequently transferred to the shiftregister.

90% of the signal reaching the sensororiginates from the atmosphere. MERIS isdesigned to achieve this by having spectralbands positioned such that quantification of theatmosphere’s influence is possible. Moreover,MERIS is required to have a sensitivity to thepolarisation of the incoming light scattered fromthe atmosphere lower than 1%.

Instrument operationMeasurement principleMERIS is an imaging spectrometer thatmeasures reflected solar radiation in the visibleand near-infrared parts of the spectrum. It is anadir-looking sensor operated in a pushbroommode. Image acquisition is performed with aline of detectors scanning the two-dimensionalscene. The motion of the satellite provides thealong-track dimension. This imaging modepermits a much longer dwell time compared toclassical scanners. The spectrum of everysingle ground pixel is recorded thanks to theuse of a two-dimensional detector. The swathwidth is imaged over the detector line, thespectrum over the detector column. Theinstrument has the ability to transmit 15spectral bands. As a unique feature, thespectral bands can be changed in width andposition by ground command. The global Earthcoverage within three days required byoceanographic and atmospheric investigationsis ensured thanks to the large instrument swathof 1150 km.

Operating conceptMERIS is designed to acquire data wheneverillumination conditions are suitable. Instrumentoperation is restricted to the day zone of theorbit where the Sun’s incidence angle is lessthan 80° at the subsatellite point (Fig. 2).Calibration will be carried out, on average, onceevery two weeks, when the spacecraft fliesover the south orbital pole and the Sunilluminates the instrument’s onboard calibrationdevice. For the rest of the orbit, MERIS will bein non-observation mode.

The MERIS instrument is designed to serveboth global and regional applications. It willacquire reduced-resolution (RR – 1200 m) datacontinuously over the Sun-illuminated part ofthe orbit, with data recorded on-board anddumped once per orbit (every 100 min) via X- orKa-band links to high-latitude ground stations.This data will be processed systematically for allorbits.

In addition to the RR data, MERIS will deliverfull-resolution (FR – 300 m) data in the same 15spectral bands. This FR data will be transmittedvia a Data Relay Satellite (DRS) or an X-bandground receiving station when in view of the

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Figure 2. MERIS operationprofile

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Figure 3. The MERISmechanical layout, showingthe locations of the various

subsystems

The spectral range of MERIS is 390 to 1040 nm,with a spectral sampling interval of 1.25 nm.530 rows are required to cover this range,including a margin of 10 rows for spectralalignment. The camera swath is imaged over740 pixels along the CCD line. Included oneither side of the line are dark reference pixels,which are covered by the aluminium shield.These pixels are used for offset compensationto ensure the stringent signal stability requiredalong the orbit and between calibrations. TheCCDs are operated at –22.5°C to reduce thedark current.

A special coating has been developed for theCCD surface. This coating features a wedgealong the CCD column to reduce internal straylight to an acceptable level.

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Figure 4. MERIS functionalblock diagram

Figure 5. The MERIS CCD

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Table 2. MERIS performancesummary

the data need to be corrected for any non-uniformities and distortions introduced in theoverall measurement system, as well as beingconverted into radiance values. Four in-flightcalibration sequences are defined:– dark calibration– radiometric gain calibration– diffuser ageing characterisation– wavelength referencing.

During the dark calibration, the signal isrecorded with the Earth and Sun apertureclosed. In the gain calibration mode, a whitediffuser plate, Sun-illuminated, is inserted at thecross-over point of the fields of view of the fivecameras. The diffuser provides a reflectancestandard across the entire spectral range andfield of view.

The calibration diffuser will be exposed to theSun for a total cumulative period of about 1 hduring MERIS’s lifetime. Some limiteddegradation due to UV/VUV radiation andparticle exposure may be expected eventhough ground testing has demonstratedremarkable diffuser stability over the instrument’sspectral range. A second white diffuser istherefore implemented to evaluate changes inBRDF of the gain calibration diffuser. Thisdiffuser will be used infrequently and will thusnot degrade at the same rate as the firstdiffuser. Diffuser ageing will be monitored bycomparing the data acquired with both diffusers.

Wavelength calibration will be achieved byusing another diffuser featuring well-knownabsorption peaks. MERIS spectral bands willbe reprogrammed to sample the absorptionfeatures adequately. From this calibration, thespectral position of any spectral band can be

The CCDs play an important role in theprogramming of the spectral bands. Thanks toits large storage capacity, several lines can beaccumulated in the shift register beforeclocking out the pixels. The selected bandwidthis obtained by summing the correct number oflines. The CCD lines that fall outside the 15spectral bands are dumped at shift-registerlevel.

Processing chainSignals read out from the CCD pass throughseveral processing steps in order to achieve therequired image quality. Analogue electronicsperform pre-amplification of the signal,correlated double sampling and gainadjustment before digitisation of the videosignal on 12 bits. The signal amplification isdone by selecting one of the 12 fixed gainsdefined in the range 1 to 3.75. The amplificationgain is selected separately for each spectralband to minimise the noise contribution of theprocessing chain. Thus, the saturation level ofany band can be optimised for the purposes ofthat band. For instance, a spectral band usedonly for ocean applications can saturate overclouds, leaving the full 12-bit digitisation for theuseful dynamic range of the oceanic signal.

The digital output of the Video Electronic Unit is subsequently processed by the DigitalProcessing Unit in three major steps:– completion of spectral relaxation up to the

required bandwidth– subtraction of the offset and correction for

gain non-uniformity– reduction of the spatial resolution of the data

to 1200 m for the global mission summationof four adjacent pixels across-track over fourconsecutive frames.

The offset and gain corrections are based oncoefficients computed during the calibrationsequences. These coefficients are stored on-board as well as being sent to the ground. Theinstrument design offers the flexibility to havethese corrections applied either onboard or onthe ground. In the latter case, offset and gaincorrection are bypassed in the onboardprocessing flow and performed on the ground.

PerformancesVerification of the instrument’s performance isbased on an extensive testing programme atcamera and instrument level (Table 2). Ofparticular importance is the large signal-to-noise ratio in the blue part of the spectrum,which permits the required Noise EquivalentDifference in Reflectance at sea level to be met.

Instrument calibrationTo meet the stringent accuracy requirements,

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Figure 6. The MERIScalibration mechanism

supporting three diffusers -for gain calibration (1), fordiffuser ageing monitoring

(2) and wavelengthcalibration (3) - the Earthshutter (4) and the Earth

diaphragm (5)

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derived. Use of the solar Fraunhofer absorptionlines as an alternative when observing the Sun-illuminated white diffuser is also envisaged.

Following extensive environmental tests,SpectralonTM has been selected for the diffuserplates. Diffusers manufactured with thismaterial offer both the requisite uniformity overthe field of view and remarkable stability. Adiffuser plate doped with a rare Earth oxide isused for the wavelength calibration.

The calibration hardware is implemented on aselection disc (Fig. 6). A stepper motor allowsthe selection of five disc positions, as theinstrument mission requirements dictate. Thecalibration mechanism also supports thediaphragm introduced into the field of view tominimise stray light when observing the Earth.

Data processingThe MERIS products will be available withdifferent time scales – with both near-real-timeand consolidated processing – and withdifferent spatial scales depending on thegeographical location – global (1200 m) andregional (300 m) products – and with differentlevels of processing (1b & 2).

Near-real-time and consolidated processing The Near Real Time (NRT) services will providedata 3 h after acquisition. Consolidation is agradual process by which more accurateexternal (auxiliary) data become available overtime. The consolidated data products willtherefore typically be available within aboutthree weeks. The formats of the NRT andconsolidated data products are the same, butthe quality of the auxiliary data used in theprocessing of the consolidated product isimproved.

Global and regional products MERIS products will be available in FR and RRresolution. The RR data will be acquiredglobally for the entire daytime orbit andprocessed systematically for all orbits. The FRdata will be available mainly over land andcoastal zones, representing the regionalmission for which data will be processed onlyon user request.

Data product levels ESA has defined the following data productlevels for MERIS (Table 3): – Level-0 Product: This raw format data will

not generally be made available to the users. – Level-1b Products: These consist of

calibrated top-of-atmosphere radiances in15 VIS/NIR spectral bands, geolocated andresampled on a regular grid. Annotations willinclude surface identification (land, sea andclouds), localisation (latitude, longitude,altitude, or bathymetry), viewing and solargeometries, as well as meteorological data(ECMWF).

– Level-2 Products: These are called‘distributed’ products because thegeophysical quantities found in the data setsvary depending on the surface beingmeasured. They will contain both geophysicalparameters and surface radiances/reflectances, depending on the surfaceproducts group – ocean colour products,land products and cloud products.

– Browse Product: This will be processedsystematically to a 4.8 km resolution for theentire daytime orbital segment. It will consistof an RGB colour product, where the threeMERIS bands have been chosen for the bestvisualisation of the land, sea, ice and cloudfeatures. The browse will enable the user tochoose the position of a scene of interest.User tools will be provided to allow theselection and ordering of scenes of 1150 kmx 1150 km for RR products, and of 575 kmx 575 km or 296 km x 296 km for FRproducts located anywhere along- andacross-track in the acquired data sets. Userswill be able to order multiple scenes toacquire a complete (daytime) orbit.

Processing architectureThe architecture of the MERIS Level-2processing is shown schematically in Figure 7.Pixel identification is the process by which theunderlying surface type (cloud, land and ocean)is determined. This is achieved using radiometryto separate bright targets (cloud, snow and ice)from land or ocean. The darker targets (landand ocean) are further classified according totheir typical spectra and, finally, clouds aredistinguished from the other bright targetsusing pressure (altitude) information retrieved

EARTH

Sunshutter

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Figure 7. Level-2 productprocessing steps

Water vapour is computed over all surfacetypes. The same algorithm is used over brighttargets – land and Sun glint. Over the ocean,the aerosol contribution to the signal iscorrected for.

Finally, all intermediate results are formattedaccording to the Envisat product specificationsand the product confidence informationcomputed using all of the results of the internaltests performed during the computation.

The data productsThe variety of geophysical parameters that areto be measured by MERIS will support a host ofapplications in marine biology, land andatmospheric sciences, as outlined below.

Ocean productsOcean colour is the key objective of the MERISmission and therefore the ocean is the surfacefor which the largest number of products will begenerated. In the open ocean (97% of theWorld’s oceans), the concentration ofphytoplankton (algae) is a key parameter forunderstanding the processes involved in thecarbon cycle. In coastal areas, phytoplanktoncohabit with suspended particulate anddissolved detritus matter (yellow substance)which will be derived by inverting a model of theoptical properties of these complex water

from the bands situated in the spectralabsorption features of oxygen (760 nm).

Once the surface type has been identified,surface-specific computing can start. Overwater, computation starts with the identificationof those areas affected by Sun glint (directreflection of Sun on the water surface) and itspartial correction in order to increase thecoverage area of the ocean-colour products.This is followed by the identification ofwhitecaps and high inland lakes where theatmospheric correction quality will be reduced.Turbid waters are then screened and a special‘bright water’ correction is applied to removethe marine signal contribution in the near-infrared (NIR) prior to entering the atmosphericcorrection, which is based on the assumptionthat the marine signal in the NIR is nil. Theatmospheric-correction computations returnwater-leaving reflectances which are used forthe estimation of the ocean-colour products(see below) and the aerosol optical thicknessand type, which are then used in thecomputation of water-vapour content overoceans. Ocean-colour products are thencomputed using band ratios over the openocean, while for the more complex coastal-water products all visible bands are used asinputs to a neural network.

Over clouds, the cloud optical thicknessand albedo are derived directly usingtop-of-atmosphere radiances and theresults of the cloud-top pressurecomputed earlier; a simple cloud type is computed based on cloudcharacteristics established by theInternational Satellite Cloud ClimatologyProject (ISCCP).

Over land, the MERIS Global VegetationIndex (MGVI) is computed using top-of-atmosphere reflectances. Atmosphericcorrection here consists only of removingmolecular scattering and absorption,while the aerosol optical thickness andtype are computed only above thosetargets identified as Dense DarkVegetation (DDV), where it is assumedthat the target has a well-determinedreflectance.

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Table 3. MERIS products

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Table 4. Estimated productaccuracies

bodies using a neural network. ThePhotosynthetically Active Radiation (PAR) – theamount of radiation available to the oceanicflora – will be provided to support studies of chlorophyll fluorescence. Desert dust,continental and maritime aerosol types andtheir optical thicknesses will be generated as abyproduct of the atmospheric correction.

Cloud productsCloud-top pressure, optical thickness, albedoand type will be provided for climatologicalstudies of the Earth’s radiation budget.

Land productsThe MERIS Global Vegetation Index (MGVI) is ameasure of the presence of healthy live greenvegetation, estimated from the fraction ofabsorbed photosynthetically active radiation.Surface pressure will be provided as anexperimental product. Aerosol optical thicknessand type will be provided for users to improvethe atmospheric correction over land.

Water-vapour productsThe concentration of water vapour found in thetotal atmospheric column will be computedover all surfaces. It is of particular interest overland where the traditional measurement meansare limited and where MERIS can provideproducts with a resolution of up to 300 m.

Breadboarding for modelled test-cases hasenabled the accuracy of the products expectedfrom MERIS to be estimated (Table 4). Typicalimages of the sort that will be acquired byMERIS are illustrated in Figure 8. They havebeen generated by the MERIS processor usingreformatted MOS data.

Product validationESA has the responsibility to guarantee thequality of its data products. Consequently, acalibration/validation plan is being defined thatincludes in-flight calibration and campaigns insupport of vicarious calibration and productvalidation. The plan forms the basis for developingscientific exploitation and application pilotprojects related to MERIS, as already selectedthrough the first Announcement of Opportunityfor the exploitation of Envisat data.

ConclusionsThe Medium Resolution Imaging Spectrometer(MERIS) belongs to a new generation of ocean-colour sensors that will provide a majorimprovement in our knowledge of such crucialprocesses as the contribution that the oceansmake to the carbon cycle. The instrument has the unique capability of in-flightprogrammability of the position and width ofthe 15 spectral bands acquired in the visibleand near-infrared parts of the spectrum. Data

will be acquired with two spatialresolutions, 300 m and 1200 m, over aswath of 1150 km.

ESA will produce a set of validated dataproducts from MERIS that will beavailable at various spatial resolutions,processing levels and within different timeframes. These products and geophysicalparameters have been identified as themost important, globally attainableparameters to be derived from MERIS. Inaddition to the Level-2 data, ESA will alsoprovide the user with sufficientinformation to process the data to higherlevels. The inherent flexibility of theMERIS instrument and the phasedimplementation plan will result in thedevelopment of a ground segmentproviding the most up-to-date products

possible.

AcknowledgementsThe MERIS instrument has been developed byan international team led by Alcatel SpaceIndustries (F) under the Envisat primecontractorship of Dornier (D). The data-productalgorithms have been developed under theleadership of ACRI (F). r

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Figure 8. Images generatedby the MERIS processor

using reformatted data fromthe Modular Optoelectronic

Scanner (MOS) on India’sIRS-P3 satellite:

- TOA Radiance is a Level -1b product in band 2

acquired over the AdriaticSea, central Italy and the

Tyrenean Sea- Chlorophyll concentration

and MGVI illustrate theLevel-2 distributed product

concept over the same areaas TOA Radiance

- Cloud optical thicknessand water vapour

computed using an imageacquired over Germany

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