Sentinel-3

The ESA Sentinels will be the first series of

noperational satellites to meet the Earthlobservation needs of the European Union-

ESA Global Monitoring for Environment andSecurity (GMES) programme. The pair ofSentinel-3 satellites will provide global,frequent and near-realtime ocean, ice and landmonitoring. It continues Envisat’s altimetry, themultispectral, medium-resolution visible andinfrared ocean and land-surface observations ofERS, Envisat and Spot, and includes enhance-ments to meet the operational revisit require-ments and to facilitate new products andevolution of services. The first launch isexpected in 2011/2012.

What Users NeedThe pair of Sentinel-3 satellites willroutinely monitor ocean and landsurfaces to generate valuable informa-tion for the European Union (EU) andits Member States as part of the GlobalMonitoring for Environment andSecurity (GMES) programme (www.gmes.info). The EU Marine Core Service(MCS) and the Land Monitoring CoreService (LMCS), together with the ESAGMES Service Element (GSE), have

Miguel Aguirre, Bruno Berruti, Jean-Loup Bezy,Mark Drinkwater, Florence Heliere, Ulf Klein,Constantinos Mavrocordatos & Pierluigi Silvestrin Directorate of Earth Observation Programmes,ESTEC, Noordwijk, The Netherlands

Bruno Greco & Jerome BenvenisteDirectorate of Earth Observation Programmes,ESRIN, Frascati, Italy

esa bulletin 131 - august 2007 25

Sentinel-3 The Ocean and Medium-Resolution Land

Mission for GMES Operational Services

Sentinel-3 will continue the images providedby Envisat’s MERIS instrument

Aguirre 8/1/07 5:01 PM Page 24

Sentinel-3

The ESA Sentinels will be the first series of

noperational satellites to meet the Earthlobservation needs of the European Union-

ESA Global Monitoring for Environment andSecurity (GMES) programme. The pair ofSentinel-3 satellites will provide global,frequent and near-realtime ocean, ice and landmonitoring. It continues Envisat’s altimetry, themultispectral, medium-resolution visible andinfrared ocean and land-surface observations ofERS, Envisat and Spot, and includes enhance-ments to meet the operational revisit require-ments and to facilitate new products andevolution of services. The first launch isexpected in 2011/2012.

What Users NeedThe pair of Sentinel-3 satellites willroutinely monitor ocean and landsurfaces to generate valuable informa-tion for the European Union (EU) andits Member States as part of the GlobalMonitoring for Environment andSecurity (GMES) programme (www.gmes.info). The EU Marine Core Service(MCS) and the Land Monitoring CoreService (LMCS), together with the ESAGMES Service Element (GSE), have

Miguel Aguirre, Bruno Berruti, Jean-Loup Bezy,Mark Drinkwater, Florence Heliere, Ulf Klein,Constantinos Mavrocordatos & Pierluigi Silvestrin Directorate of Earth Observation Programmes,ESTEC, Noordwijk, The Netherlands

Bruno Greco & Jerome BenvenisteDirectorate of Earth Observation Programmes,ESRIN, Frascati, Italy

esa bulletin 131 - august 2007 25

Sentinel-3 The Ocean and Medium-Resolution Land

Mission for GMES Operational Services

Sentinel-3 will continue the images providedby Envisat’s MERIS instrument

Aguirre 8/1/07 5:01 PM Page 24

been consolidating those services wherecontinuity and success depends onoperational data flowing from theSentinels.

For MCS, operational oceanographyservices are being developed within theMERSEA 6th Framework Programme(w3.mersea.eu.org) and through the ESAGSE Marcoast (gmes-marcoast.com)and Polarview (www.polarview.org)projects. The MCS will deliver regular,

general suite of high-level geophysicalproducts, with priority on:– ocean, ice, land and inland water

surface topography;– sea-surface temperature;– ocean colour;– land surface biophysical properties;– land surface temperature.

These primary goals have driven thedesign towards a mission concept thatcan routinely and continuously (that is,operationally) deliver robust productswith well-characterised accuracy andconfidence limits. In turn, this will leadto improvements over previous missionsand instruments, which, through theirtechnology and data-processing, haveguided the design of the Sentinel-3system.

Mission DefinitionThe sea-surface topography mission hasthe primary objective of providingaccurate, closely spaced altimetrymeasurements from a high-inclinationorbit with a long repeat cycle, tocomplement the Jason ocean altimeterseries. Monitoring mid-scale circulationand sea levels requires measurement ofocean topography. Measuring waveheights is essential for operational waveforecasting. Sea-ice measurementssimilar to those from CryoSat-2 (thoughfrom a slightly different orbit) will bemade. The single-antenna radar altimeter,with aperture synthesis processing forincreased along-track resolution,balances the needs for continuity andimproved performance. It will extendobservations to inland waters andcoastal zones.

Accompanying the altimeter will be aPrecise Orbit Determination (POD)system and microwave radiometer(MWR) for removing the errors addedas the signals are delayed by watervapour in the atmosphere. The altimeterwill track over a variety of surfaces:open ocean, coastal sea zones, sea iceand inland waters. The chosen mode willdepend on the surface below, withchanges pre-programmed in the satelliteto avoid data loss.

systematic products on sea-surfacetopography and sea-state and ecosystemcharacteristics over the oceans and theEuropean regional and shelf seas. Theinformation from Sentinel-3 will beassimilated into models in near-realtimeto provide routinely the best-availableestimate of the state of the oceans,together with forecasts (from days toweeks) and reanalyses (‘hindcasts’ ofocean state over long time-series in the

The Ocean and Land Colour Instru-ment (OLCI), based on Envisat’sMERIS instrument, fulfils ocean colourand land-cover objectives. The Sea andLand Surface Temperature Radiometer(SLSTR), based on Envisat’s AdvancedAlong Track Scanning Radiometer(AATSR), is designed for ocean andland-surface temperature observations.Unlike AATSR, SLSTR has a double-scanning mechanism, yielding a muchwider swath stretching almost fromhorizon to horizon. The OLCI andSLSTR swaths will broadly overlap,yielding extra information.

The Sun-synchronous orbit chosen isat 814 km altitude (14+7/27 revolutionsper day) with a local equatorial crossingtime of 10:00 a.m., as a compromise

between the needs of the optical instru-ment and altimeter. The baseline of twosatellites supports full imaging of theoceans within 2 days (even allowing forimages spoiled by the Sun glinting offthe water), while delivering global landcoverage in just over a day at the equatorand improving with latitude.

The continuous acquisitions ofSentinel-3 allow routine operations. Anaverage of 103 Gbit of data will bedownlinked once per orbit at 300 Mbit/sX-band to a single ground station withno blind orbits.

Four categories of products will bedelivered: ocean colour, surface topo-graphy, surface temperature (land andsea) and land. The surface topographyproducts will be delivered with threetimeliness levels: NRT (Near-Real Time,3 hours), STC (Standard Time Critical,1–2 days) and NTC (Non-Time Critical,1 month). Slower products allow moreaccurate processing and better quality.NRT products are ingested intonumerical weather prediction and sea-state prediction models for quick, short-term forecasts. STC products are ingestedinto ocean models for accurate presentstate estimates and forecasts. NTCproducts are used in all high-precisionclimatological applications, such as sea-level estimates. The resulting analyses

past). The fast-track component of theMCS focuses on ocean dynamics andprimary ecosystem characteristics, butalso covers sea-ice monitoring and oil-spill detection.

The Land Monitoring Core Service isbeing developed through the Geoland6th Framework Programme (www.gmes-geoland.info) and ESA GSE projectssuch as Forest Monitoring (www.gmes-forest.info), Global Monitoring forFood Security (www.gmfs.info), and thedisaster mitigation and humanitarianrelief RESPOND (www.respond-int.org)service.

LMCS focuses on exhaustive high-resolution continental-scale land-cover/land-use mapping complemented byland-monitoring based on daily land-cover mapping, vegetation character-istics and fire monitoring at continentaland global scales. The dynamics ofvegetation characteristics require anupdate frequency of days to weeks, andcomprehensive global observations withthe best revisit frequency possible(especially to minimise the effects ofclouds and high levels of aerosols).

LMCS will evolve in Europe toinclude vegetation monitoring linked toCommon Agricultural Policy require-ments such as reviewing and monitoringEU policies (water framework directive,biodiversity strategy, common agricul-tural, regional policies) and reportingobligations under international treatiessuch as the Kyoto Protocol.

Sentinel-3 will frequently revisit allsites. This is useful, for example, for cropyield monitoring and food security, andforest cover mapping and changemonitoring. Parameters include landcover, leaf area index, fraction ofabsorbed photosynthetically activeradiation, burnt areas, and land surfaceand active fire temperature. These globalvegetation data, together with atmo-spheric corrections, such as aerosoloptical depth, will provide critical inputdata for numerical weather forecasting,global climate models and in climateand greenhouse gas monitoring.

To meet user needs, Sentinel-3 willsupport the routine generation of a

Earth Observation

esa bulletin 131 - august 2007esa bulletin 131 - august 2007 www.esa.intwww.esa.int 2726

Sentinel-3

Marine and Coastal EnvironmentSea-surface topographyMesoscale circulationWater qualitySea-surface temperatureWave height and wind speedSediment load and transportEutrophication (excessive plant growth in

water bodies)

Polar Environment Sea-ice thickness Ice surface temperature

Maritime SecurityOcean-current forecastingWater transparency Wind and wave height

Global Change OceanGlobal sea-level riseGlobal ocean warmingOcean carbon dioxide flux

Land Cover & Land-Use ChangeLand-use mapping Vegetation indices

Forest MonitoringForest cover mapping

Food Security Early WarningRegional land-cover mappingDrought monitoring

Humanitarian AidLand-use mapping

Air Pollution (local to regional scales)Aerosol concentration

Risk Management (flood and fires)Burned scar mappingFire detection

Global Change LandForest-cover change mappingSoil degradation mapping

GMES/Sentinel-3 Initial Services Sentinel-3 will deliver routine operational services to policy-makers and marine and landservice users:

Sentinel-3 in launch configuration

OLCI is pointed eastwards, away from the Sun, to minimise Sun-glint off the sea. SLSTR has a symmetric double-scanning geometry,producing two swaths. The wide swath totally covers the OLCI swath for product synergy. The narrow-swath data, corrected withinformation from the wide swath, provides nadir-centred high-accuracy sea-surface temperature

MWR

SLSTR swath with single view

SLSTR swath with single view

Longitude (deg)

Lo

ng

itu

de

(d

eg

)

SLSTR

sampling geometry

(in blue)

OLCI swath

OLCI

sampling geometry

OLCI

SLSTR

LRR

RA

Aguirre 8/1/07 5:01 PM Page 26

been consolidating those services wherecontinuity and success depends onoperational data flowing from theSentinels.

For MCS, operational oceanographyservices are being developed within theMERSEA 6th Framework Programme(w3.mersea.eu.org) and through the ESAGSE Marcoast (gmes-marcoast.com)and Polarview (www.polarview.org)projects. The MCS will deliver regular,

general suite of high-level geophysicalproducts, with priority on:– ocean, ice, land and inland water

surface topography;– sea-surface temperature;– ocean colour;– land surface biophysical properties;– land surface temperature.

These primary goals have driven thedesign towards a mission concept thatcan routinely and continuously (that is,operationally) deliver robust productswith well-characterised accuracy andconfidence limits. In turn, this will leadto improvements over previous missionsand instruments, which, through theirtechnology and data-processing, haveguided the design of the Sentinel-3system.

Mission DefinitionThe sea-surface topography mission hasthe primary objective of providingaccurate, closely spaced altimetrymeasurements from a high-inclinationorbit with a long repeat cycle, tocomplement the Jason ocean altimeterseries. Monitoring mid-scale circulationand sea levels requires measurement ofocean topography. Measuring waveheights is essential for operational waveforecasting. Sea-ice measurementssimilar to those from CryoSat-2 (thoughfrom a slightly different orbit) will bemade. The single-antenna radar altimeter,with aperture synthesis processing forincreased along-track resolution,balances the needs for continuity andimproved performance. It will extendobservations to inland waters andcoastal zones.

Accompanying the altimeter will be aPrecise Orbit Determination (POD)system and microwave radiometer(MWR) for removing the errors addedas the signals are delayed by watervapour in the atmosphere. The altimeterwill track over a variety of surfaces:open ocean, coastal sea zones, sea iceand inland waters. The chosen mode willdepend on the surface below, withchanges pre-programmed in the satelliteto avoid data loss.

systematic products on sea-surfacetopography and sea-state and ecosystemcharacteristics over the oceans and theEuropean regional and shelf seas. Theinformation from Sentinel-3 will beassimilated into models in near-realtimeto provide routinely the best-availableestimate of the state of the oceans,together with forecasts (from days toweeks) and reanalyses (‘hindcasts’ ofocean state over long time-series in the

The Ocean and Land Colour Instru-ment (OLCI), based on Envisat’sMERIS instrument, fulfils ocean colourand land-cover objectives. The Sea andLand Surface Temperature Radiometer(SLSTR), based on Envisat’s AdvancedAlong Track Scanning Radiometer(AATSR), is designed for ocean andland-surface temperature observations.Unlike AATSR, SLSTR has a double-scanning mechanism, yielding a muchwider swath stretching almost fromhorizon to horizon. The OLCI andSLSTR swaths will broadly overlap,yielding extra information.

The Sun-synchronous orbit chosen isat 814 km altitude (14+7/27 revolutionsper day) with a local equatorial crossingtime of 10:00 a.m., as a compromise

between the needs of the optical instru-ment and altimeter. The baseline of twosatellites supports full imaging of theoceans within 2 days (even allowing forimages spoiled by the Sun glinting offthe water), while delivering global landcoverage in just over a day at the equatorand improving with latitude.

The continuous acquisitions ofSentinel-3 allow routine operations. Anaverage of 103 Gbit of data will bedownlinked once per orbit at 300 Mbit/sX-band to a single ground station withno blind orbits.

Four categories of products will bedelivered: ocean colour, surface topo-graphy, surface temperature (land andsea) and land. The surface topographyproducts will be delivered with threetimeliness levels: NRT (Near-Real Time,3 hours), STC (Standard Time Critical,1–2 days) and NTC (Non-Time Critical,1 month). Slower products allow moreaccurate processing and better quality.NRT products are ingested intonumerical weather prediction and sea-state prediction models for quick, short-term forecasts. STC products are ingestedinto ocean models for accurate presentstate estimates and forecasts. NTCproducts are used in all high-precisionclimatological applications, such as sea-level estimates. The resulting analyses

past). The fast-track component of theMCS focuses on ocean dynamics andprimary ecosystem characteristics, butalso covers sea-ice monitoring and oil-spill detection.

The Land Monitoring Core Service isbeing developed through the Geoland6th Framework Programme (www.gmes-geoland.info) and ESA GSE projectssuch as Forest Monitoring (www.gmes-forest.info), Global Monitoring forFood Security (www.gmfs.info), and thedisaster mitigation and humanitarianrelief RESPOND (www.respond-int.org)service.

LMCS focuses on exhaustive high-resolution continental-scale land-cover/land-use mapping complemented byland-monitoring based on daily land-cover mapping, vegetation character-istics and fire monitoring at continentaland global scales. The dynamics ofvegetation characteristics require anupdate frequency of days to weeks, andcomprehensive global observations withthe best revisit frequency possible(especially to minimise the effects ofclouds and high levels of aerosols).

LMCS will evolve in Europe toinclude vegetation monitoring linked toCommon Agricultural Policy require-ments such as reviewing and monitoringEU policies (water framework directive,biodiversity strategy, common agricul-tural, regional policies) and reportingobligations under international treatiessuch as the Kyoto Protocol.

Sentinel-3 will frequently revisit allsites. This is useful, for example, for cropyield monitoring and food security, andforest cover mapping and changemonitoring. Parameters include landcover, leaf area index, fraction ofabsorbed photosynthetically activeradiation, burnt areas, and land surfaceand active fire temperature. These globalvegetation data, together with atmo-spheric corrections, such as aerosoloptical depth, will provide critical inputdata for numerical weather forecasting,global climate models and in climateand greenhouse gas monitoring.

To meet user needs, Sentinel-3 willsupport the routine generation of a

Earth Observation

esa bulletin 131 - august 2007esa bulletin 131 - august 2007 www.esa.intwww.esa.int 2726

Sentinel-3

Marine and Coastal EnvironmentSea-surface topographyMesoscale circulationWater qualitySea-surface temperatureWave height and wind speedSediment load and transportEutrophication (excessive plant growth in

water bodies)

Polar Environment Sea-ice thickness Ice surface temperature

Maritime SecurityOcean-current forecastingWater transparency Wind and wave height

Global Change OceanGlobal sea-level riseGlobal ocean warmingOcean carbon dioxide flux

Land Cover & Land-Use ChangeLand-use mapping Vegetation indices

Forest MonitoringForest cover mapping

Food Security Early WarningRegional land-cover mappingDrought monitoring

Humanitarian AidLand-use mapping

Air Pollution (local to regional scales)Aerosol concentration

Risk Management (flood and fires)Burned scar mappingFire detection

Global Change LandForest-cover change mappingSoil degradation mapping

GMES/Sentinel-3 Initial Services Sentinel-3 will deliver routine operational services to policy-makers and marine and landservice users:

Sentinel-3 in launch configuration

OLCI is pointed eastwards, away from the Sun, to minimise Sun-glint off the sea. SLSTR has a symmetric double-scanning geometry,producing two swaths. The wide swath totally covers the OLCI swath for product synergy. The narrow-swath data, corrected withinformation from the wide swath, provides nadir-centred high-accuracy sea-surface temperature

MWR

SLSTR swath with single view

SLSTR swath with single view

Longitude (deg)

Lo

ng

itu

de

(d

eg

)

SLSTR

sampling geometry

(in blue)

OLCI swath

OLCI

sampling geometry

OLCI

SLSTR

LRR

RA

Aguirre 8/1/07 5:01 PM Page 26

and forecast products and predictionsfrom ocean and atmosphere adding datafrom other missions and in situobservations, are the key productsdelivered to users. They provide a robust basis for downstream value-addedproducts and specialised user services.

Satellite Configuration Though carrying five instruments,Sentinel-3 is of moderate size – about1.3 t, 3.9 m high and compatible withsmall launchers like Vega (baseline) andDnepr-ST3 (back-up). It is designed fora 7-year lifetime, with 90 kg of hydrazinepropellant for 12 years of operations,including deorbiting at the end.

The layout is driven by the need toprovide a large anti-Sun side to cool theoptical instruments.

Optical instrumentsThe two optical instruments, OLCI andSLSTR, will provide a common quasi-simultaneous view of the Earth to helpdevelop synergetic products.

The 150 kg OLCI benefits fromMERIS heritage. The field-of-view isdivided between five cameras on acommon structure with the calibrationassembly. Each camera has an opticalgrating to provide the minimum baselineof 16 spectral bands required by themission together with the potential foroptional bands for improved atmo-spheric corrections.

OLCI is an autonomous instrumentwith simple interfaces to the satellite,thus allowing easy integration andminimising development risks.

topography like ice margins. In thismode, SRAL’s tracking window iscontrolled based on a priori knowledgeof the surface height, from existinghigh-resolution global digital elevationmodels, combined with knowledge ofthe location of the satellite from thesatnav receiver. The main advantage isthat the measurements are continuous,avoiding the data gaps typical of closed-

The 90 kg visible/infrared SLSTRmeasures sea- and land-surface temp-eratures, following the AATSR concept.Its rotary scan mirror mechanismproduces the wide swath of 750 km. Itfeatures ~1 km resolution at nadir forthermal-infrared channels and 500 mfor visible and shortwave infraredchannels. Like AATSR, it offers dualviews – inclined forward and near-vertical nadir – to provide robustatmospheric correction over the swath.The nadir and forward views aregenerated by separate scanners, allowinga wider swath than possible with thesingle conical scan of the original ATSRdesign.

The channel selection (1.6, 3.7, 10.8and 12 μm in the infrared and 0.55, 0.66and 0.85 μm in the visible) include theEnvisat/AATSR and ERS/ATSR-2channels for continuity. Additionalchannels at 1.378 and 2.25 μm improvecloud detection, besides being used fornew products.

The instrument includes onboardradiometric sources for accurate andstable in-flight calibration. The infrareddetectors are cooled to 80K by activecoolers.

Topography packageThe topography payload consists of theSynthetic Aperture Radar Altimeter(SRAL), MWR and POD (a satnavreceiver supplemented by a laser retro-reflector). They will determine veryaccurately the height of the Earthsurface, and in particular the sea surface,relative to a precise reference frame.

loop tracking, which has problems intracking the rapid topographic changesat coastal margins and in mountainousregions.

The 26 kg MWR measures thethermal radiation emitted by Earth. Thereceived signal is proportional to theabundance of the atmospheric compo-nent emitting at the observed frequencyand the sea-surface reflectivity. Thisinformation reveals the delay added tothe altimeter pulses by moisture in thetroposphere.

Each of the three channels addresses adifferent geophysical parameter. The18.7 GHz channel, where the tropo-sphere is transparent, is influencedmainly by sea-surface reflectivity. Thisallows separation of the atmosphericsignal from the sea-surface contribu-tion in the other two channels. The23.8 GHz channel is used mainly todetermine the delay of the altimeterpulse by tropospheric water vapour. The36.5 GHz channel primarily addressesthe delay from non-precipitating clouds.The observed signals are calibrated bycomparison with a precisely known andvery stable electronic reference source.

The POD equipment provides thesatellite altitude to an accuracy of 2 cm.The 11 kg satnav receiver will operatewith Global Positioning System satell-ites for the Sentinel-3 first generation

and add Galileo for the followinggenerations. The receiver can track up to12 satellites at the same time.

The signals transmitted by the naviga-tion satellites are also disturbed by theionosphere. The effect is corrected bycomparing two signals at differentfrequencies within 1160–1590 MHz.

The receiver produces an onboardheight to within about 3 m, which isused to control SRAL’s open-looptracking and for Sentinel navigation.Ground processing yields the altitude toan accuracy of better than 8 cm within3 hours for operational applications and2 cm after some days of refinement.

Laser tracking stations will use the1 kg retro-reflector to measure thedistance to Sentinel-3 to within 2 cm.This will be done during thecommissioning phase and regularlyduring the mission to check the otherPOD results.

ConclusionGMES Sentinel-3 is a series ofoperational satellites that will guaranteeaccess to an uninterrupted flow of robustglobal data products. Together with theother Sentinels, this mission will fulfil themonitoring needs of the GMES marineand land services and climate researchcommunities for years to come. e

The radar altimeter determines thedistance to the surface by bouncingmicrowave pulses off the surface. Thetime taken is derived very precisely byground processing of the data. However,the propagation speed through theatmosphere is variable. The ionosphereand the troposphere add delaysdependent on the density of electrons inthe ionosphere, and the density of gasesand the moisture content in thetroposphere. The MWR determines theamount of water contained in the pathof the radar pulses. The altimetertransmits pulses alternatively at twofrequencies. Comparing their relativedelay, the frequency-dependent partintroduced by the ionosphere can thenbe found. The influence of gas density inthe troposphere is less variable and canbe determined sufficiently accuratelyusing meteorological data and models.

Sentinel’s own height will be measuredwith cm-accuracy by the geodetic-qualitysatnav receiver and laser retro-reflector.

The 60 kg SRAL is a dual-frequency,nadir-looking microwave radar employ-ing technologies from the CryoSat andJason altimeter missions. The mainrange measurements are performed at13.575 GHz Ku-band, while a secondfrequency at 5.41 GHz C-band allowscompensation for ionospheric effects.

A conventional pulse-limited, low-resolution mode employs autonomousclosed-loop echo tracking, and is theprimary operational mode for observinglevel homogeneous and smoothsurfaces, like open oceans or central ice-sheet plateaux.

For more variable surfaces, SRAL hastwo extra features that can be usedindependently or in combination: theSAR mode, similar to that of theCryoSat SIRAL instrument, and theopen-loop tracking mode. In the SARmode, the horizontal resolution isincreased along the ground track bysending out signals some 10 times moreoften. This will be used mainly over sea-ice and ice-sheet edges and inland water.Open-loop tracking will be used mainlyover discontinuous surfaces (like land-sea transitions) or rapidly varying

Earth Observation

esa bulletin 131 - august 2007esa bulletin 131 - august 2007 www.esa.intwww.esa.int 2928

Sentinel-3

OLCI contains five cameras to continue the work of Envisat’sMERIS

SLSTR has a wide nadir view and a narrow oblique view

oblique vieoblique vie ww

nadir vienadir vie ww

radiaradia tt oror

baseplabasepla tt ee

EarEar th th aperaper turtur e e holehole

blackbodyblackbody

The measurement principle of the topography payload Locations of the SRAL and MWR elements (green) on the satellite. SRAL is in the lower part and MWR in theupper part

Radio Frequency Unit

Digital

Processing UnitAntenna Assembly

Aguirre 8/1/07 5:01 PM Page 28

and forecast products and predictionsfrom ocean and atmosphere adding datafrom other missions and in situobservations, are the key productsdelivered to users. They provide a robust basis for downstream value-addedproducts and specialised user services.

Satellite Configuration Though carrying five instruments,Sentinel-3 is of moderate size – about1.3 t, 3.9 m high and compatible withsmall launchers like Vega (baseline) andDnepr-ST3 (back-up). It is designed fora 7-year lifetime, with 90 kg of hydrazinepropellant for 12 years of operations,including deorbiting at the end.

The layout is driven by the need toprovide a large anti-Sun side to cool theoptical instruments.

Optical instrumentsThe two optical instruments, OLCI andSLSTR, will provide a common quasi-simultaneous view of the Earth to helpdevelop synergetic products.

The 150 kg OLCI benefits fromMERIS heritage. The field-of-view isdivided between five cameras on acommon structure with the calibrationassembly. Each camera has an opticalgrating to provide the minimum baselineof 16 spectral bands required by themission together with the potential foroptional bands for improved atmo-spheric corrections.

OLCI is an autonomous instrumentwith simple interfaces to the satellite,thus allowing easy integration andminimising development risks.

topography like ice margins. In thismode, SRAL’s tracking window iscontrolled based on a priori knowledgeof the surface height, from existinghigh-resolution global digital elevationmodels, combined with knowledge ofthe location of the satellite from thesatnav receiver. The main advantage isthat the measurements are continuous,avoiding the data gaps typical of closed-

The 90 kg visible/infrared SLSTRmeasures sea- and land-surface temp-eratures, following the AATSR concept.Its rotary scan mirror mechanismproduces the wide swath of 750 km. Itfeatures ~1 km resolution at nadir forthermal-infrared channels and 500 mfor visible and shortwave infraredchannels. Like AATSR, it offers dualviews – inclined forward and near-vertical nadir – to provide robustatmospheric correction over the swath.The nadir and forward views aregenerated by separate scanners, allowinga wider swath than possible with thesingle conical scan of the original ATSRdesign.

The channel selection (1.6, 3.7, 10.8and 12 μm in the infrared and 0.55, 0.66and 0.85 μm in the visible) include theEnvisat/AATSR and ERS/ATSR-2channels for continuity. Additionalchannels at 1.378 and 2.25 μm improvecloud detection, besides being used fornew products.

The instrument includes onboardradiometric sources for accurate andstable in-flight calibration. The infrareddetectors are cooled to 80K by activecoolers.

Topography packageThe topography payload consists of theSynthetic Aperture Radar Altimeter(SRAL), MWR and POD (a satnavreceiver supplemented by a laser retro-reflector). They will determine veryaccurately the height of the Earthsurface, and in particular the sea surface,relative to a precise reference frame.

loop tracking, which has problems intracking the rapid topographic changesat coastal margins and in mountainousregions.

The 26 kg MWR measures thethermal radiation emitted by Earth. Thereceived signal is proportional to theabundance of the atmospheric compo-nent emitting at the observed frequencyand the sea-surface reflectivity. Thisinformation reveals the delay added tothe altimeter pulses by moisture in thetroposphere.

Each of the three channels addresses adifferent geophysical parameter. The18.7 GHz channel, where the tropo-sphere is transparent, is influencedmainly by sea-surface reflectivity. Thisallows separation of the atmosphericsignal from the sea-surface contribu-tion in the other two channels. The23.8 GHz channel is used mainly todetermine the delay of the altimeterpulse by tropospheric water vapour. The36.5 GHz channel primarily addressesthe delay from non-precipitating clouds.The observed signals are calibrated bycomparison with a precisely known andvery stable electronic reference source.

The POD equipment provides thesatellite altitude to an accuracy of 2 cm.The 11 kg satnav receiver will operatewith Global Positioning System satell-ites for the Sentinel-3 first generation

and add Galileo for the followinggenerations. The receiver can track up to12 satellites at the same time.

The signals transmitted by the naviga-tion satellites are also disturbed by theionosphere. The effect is corrected bycomparing two signals at differentfrequencies within 1160–1590 MHz.

The receiver produces an onboardheight to within about 3 m, which isused to control SRAL’s open-looptracking and for Sentinel navigation.Ground processing yields the altitude toan accuracy of better than 8 cm within3 hours for operational applications and2 cm after some days of refinement.

Laser tracking stations will use the1 kg retro-reflector to measure thedistance to Sentinel-3 to within 2 cm.This will be done during thecommissioning phase and regularlyduring the mission to check the otherPOD results.

ConclusionGMES Sentinel-3 is a series ofoperational satellites that will guaranteeaccess to an uninterrupted flow of robustglobal data products. Together with theother Sentinels, this mission will fulfil themonitoring needs of the GMES marineand land services and climate researchcommunities for years to come. e

The radar altimeter determines thedistance to the surface by bouncingmicrowave pulses off the surface. Thetime taken is derived very precisely byground processing of the data. However,the propagation speed through theatmosphere is variable. The ionosphereand the troposphere add delaysdependent on the density of electrons inthe ionosphere, and the density of gasesand the moisture content in thetroposphere. The MWR determines theamount of water contained in the pathof the radar pulses. The altimetertransmits pulses alternatively at twofrequencies. Comparing their relativedelay, the frequency-dependent partintroduced by the ionosphere can thenbe found. The influence of gas density inthe troposphere is less variable and canbe determined sufficiently accuratelyusing meteorological data and models.

Sentinel’s own height will be measuredwith cm-accuracy by the geodetic-qualitysatnav receiver and laser retro-reflector.

The 60 kg SRAL is a dual-frequency,nadir-looking microwave radar employ-ing technologies from the CryoSat andJason altimeter missions. The mainrange measurements are performed at13.575 GHz Ku-band, while a secondfrequency at 5.41 GHz C-band allowscompensation for ionospheric effects.

A conventional pulse-limited, low-resolution mode employs autonomousclosed-loop echo tracking, and is theprimary operational mode for observinglevel homogeneous and smoothsurfaces, like open oceans or central ice-sheet plateaux.

For more variable surfaces, SRAL hastwo extra features that can be usedindependently or in combination: theSAR mode, similar to that of theCryoSat SIRAL instrument, and theopen-loop tracking mode. In the SARmode, the horizontal resolution isincreased along the ground track bysending out signals some 10 times moreoften. This will be used mainly over sea-ice and ice-sheet edges and inland water.Open-loop tracking will be used mainlyover discontinuous surfaces (like land-sea transitions) or rapidly varying

Earth Observation

esa bulletin 131 - august 2007esa bulletin 131 - august 2007 www.esa.intwww.esa.int 2928

Sentinel-3

OLCI contains five cameras to continue the work of Envisat’sMERIS

SLSTR has a wide nadir view and a narrow oblique view

oblique vieoblique vie ww

nadir vienadir vie ww

radiaradia tt oror

baseplabasepla tt ee

EarEar th th aperaper turtur e e holehole

blackbodyblackbody

The measurement principle of the topography payload Locations of the SRAL and MWR elements (green) on the satellite. SRAL is in the lower part and MWR in theupper part

Radio Frequency Unit

Digital

Processing UnitAntenna Assembly

Aguirre 8/1/07 5:01 PM Page 28