directorate of earth observation programmes, of earth · benveniste 11/9/06 4:25 pm page 42. the...

Radar Altimetry

Radar altimetry is about to enter a newera. It is becoming an indispensable toolfor oceanography as a new generation of

radar altimeters providing higher resolution andprecision is poised to begin service. Remarkableprogress has been made since the launch of thepioneering ERS-1 in 1991.

IntroductionThe ERS-1 European Remote Sensingsatellite, launched in 1991, was ESA’sfirst Earth-observation research satellite.Its comprehensive payload included animaging synthetic-aperture radar, aradar altimeter and other powerfulinstruments to measure the sea-surfacetemperatures and wind characteristics.ERS-2 followed in 1995 and, remarkably,is still operating. At the time, the ERStwins were the most sophisticated Earth-observation satellites ever developedand launched by Europe. They havecollected a wealth of valuable data onEarth’s land surfaces, oceans and polarcaps, and have been called upon tomonitor natural disasters such as severeflooding or earthquakes in remote partsof the world.

Jérôme Benveniste Science and Applications Department,Directorate of Earth Observation Programmes,ESRIN, Frascati, Italy

Yves MénardCentre National d’Études Spatiales, Toulouse,France

esa bulletin 128 - november 2006 43

Taking the Measureof EarthTaking the Measureof EarthFifteen Years of Progress in Radar Altimetry

Wave heights measured by ERS-2 during the northern summer.Red/pink shows rougher seas during the southern winter

benveniste 11/9/06 4:25 PM Page 42

The Principle of Radar AltimetryRadar altimetry measures the distancebetween a satellite and the surface belowusing radar echoes bounced back from thesurface, whether ocean, ice cap, sea-ice,desert, lake or river. The characteristics of theechoes contain further information on theroughness of the surface, wave heights orwind speeds over the ocean.

Altimetry measurements become scientific-ally more useful when the satellite’s position isaccurately known. Many satellites, includingEnvisat and CryoSat, carry the DORIS(Doppler Orbitography and RadiopositioningIntegrated by Satellite) radio receiver forprecise orbit determination. DORIS calculatesthe orbit to an accuracy of a few centimetresby measuring the Doppler shift on signalsbroadcast from a network of more than 50beacons spread around the world.

By 1992, the unique results from ERSand the CNES/NASA Topex-Poseidonhad provided a strong foundation forthe future of satellite altimetry and themissions then in development, such asJason (CNES and NASA/Jet PropulsionLaboratory), Envisat and GeosatFollow-On (US Navy). Countries withdifferent cultures, especially Europe andthe USA, learned to work towards

common goals, and the differentgeodesy, geophysics and oceanographyscientific communities had to learn towork closely together. Cryosphereresearchers benefited by adapting thetechnology and data-processingprogress made by oceanographers, tomonitor the ice caps and sea-ice.

Over several decades, new technologieswith improved accuracy were developed.

In the Indian and, particularly, Pacificoceans, the trends in both sea level andtemperature are still dominated by thelarge changes associated with the largeEl Niño Southern Oscillation of1997–1998. Fresh water brought by rain,snow, melting sea-ice, ice sheets andglaciers complicate our understandingof the rise in sea levels. It can beexpected that the next decade ofaltimetry will provide fundamental newinsights into these important features.

TsunamisThe Sumatran tsunami of December2004 was the first to be observed byaltimeters in space, which has allowedscientists to improve our understandingof how tsunamis propagate. This isinvaluable for helping to avoid disasters,in addition to being of great interest forscience. One of the main components ofbuilding propagation models is theunderwater equivalent to altimetry:bathymetry. This is the measurement ofthe depth contours of the soil, rock orsand at the bottom of a body of watersuch as an ocean or a lake. Deriving sea-floor topography from radar altimetry isimproving the accuracy of these modelsbecause the nature of the floor influenceshow tsunamis move and build.

Marine meteorologyRadar altimeters are indispensable forobserving the sea state in a variety ofapplications. Wave climatology, a well-established altimeter application, iscontinuously enriched by new data asthey become available; the longer therecord, the more consistent and reliablethe results. Correlation of wave-heightvariations with climatological phenom-ena such as the North AtlanticOscillation has been observed – and hasopened a whole new area of science.Wave modelling, a traditional applica-tion, has greatly improved recentlythanks to the assimilation of altimeterdata in near-realtime, and is nowgenerating accurate sea-state predictions,to the enormous benefit of the shippingindustry. In addition, sea-surfacetopography measured by radar

altimeters is also used in near-realtimejointly with sea-surface temperature andmodels to investigate how the upper-ocean thermal structure is involved instrengthening hurricanes.

Understanding the Cryosphere Glaciologists had to wait until theadvent of ERS-1 to see a polar altimeterflying over sea-ice and the ice sheets. Theinstrument proved to be a very powerfultool for glaciology, with three majorscientific objectives: ice-sheet modellingand dynamics, ice-sheet mass balanceand sea-ice thickness. There are alsonumerous secondary uses.

Ice sheetsThe mass changes in ice sheets has beenstudied from space, ranging from thelargest sheets in Antarctica andGreenland, to smaller ones such as theAustfonna (in Svalbard, the world’sthird largest icecap and the largestglacier in Europe). Overall, theGreenland and Antarctica sheets arefound to be almost in equilibrium,losing as much ice as they gain, but localdata suggest a loss that could acceleratein the near future.

Radar altimeters are not only able tomap the global trend but can also catchlocal imbalances. However, discrepanciesbetween some studies may be explainedby the behaviour of radar wavesinteracting with packed snow. This iswhy Envisat’s dual-frequency altimeteris helping to improve our knowledge ofhow radar waves penetrate the snow.

Altimetry over Land and Inland WaterThe potential of satellite radar altimeterdata for applications over land andinland water is now well known, as datafrom Topex-Poseidon, Jason, ERS-1,ERS-2 and Envisat have extensivelyshown. Initially developed to makeprecise measurements of sea surfaces,radar altimeters rapidly proved able toprovide information over the continents.Their development to monitorcontinental water surfaces provides apowerful tool for studying regionalhydrological systems.

The accuracy of global geodeticmeasurements has increased from a fewhundred metres at the beginning of thesatellite era to a few centimetres now.Though a highly complex problem, itrecently became possible to exploitradar altimetry to monitor inland waterlevels; the accuracy over these difficultterrains is improving rapidly.

After many years of development anddata exploitation, radar altimetry isbecoming operational in oceanographicapplications. A new generation of high-resolution and high-precision instrumentsis entering service using techniques suchas ‘delay-Doppler’ and interferometry.We now know much more about ourEarth, ocean dynamics and the cryo-sphere than we would without altimetry,and we have laid the foundations forfully operational 3-D oceanography.

Understanding the Ocean Radar altimetry has made an importantcontribution to oceanography byinvestigating the high-frequencyvariability in sea-surface height fromglobal to basin scales, and its impact onthe oceans’ general circulation. Ithighlights the importance of eddies inshaping and controlling the flows ofmajor current systems such as theAntarctic circumpolar current andwestern boundary currents (GulfStream and Kuroshio), and theinfluence of eddies on the verticalmixing in the ocean. The results from 15years of altimeter data on eddyvariability in the oceans are outstandingand are certainly a major accomplish-ment. Altimetry has proved unique indramatically improving our tide models,in observing internal tides andunderstanding the genesis of climaticevents such as El Niño and the NorthAtlantic or North Pacific Oscillations.Now it is the interaction betweenphenomena such as planetary waves,eddies, tides and mean flow, and theirimpact on coastal regions, that must beinvestigated. These new studies willbenefit greatly from the higherresolution sampling provided by theupcoming altimeter missions.

Earth Observation

esa bulletin 128 - november 2006esa bulletin 128 - november 2006 www.esa.intwww.esa.int 4544

Radar Altimetry

The kinetic energy of eddies in the North Atlantic calculated from sea-level data along the Topex-Poseidon and ERS groundtracks. Redhighlights the greatest energy. (Le Traon & Dibarboure)

The variability of sea-surface height from Topex-Poseidon and ERS data in the southern Pacific Ocean. Box 1 highlights the variability ofthe South Equatorial Counter Current. Box 3 shows the variability of the South Tropical Counter Current. (Qiu &Chen)

The rise in sea level during the 20th century appears to beaccelerating. Satellite measurements for the last 15 years areshown in green. (Cazenave et al.)

Sea levelsAltimeter observations from satellitesshow that the global mean sea level hasrisen over the past decade at a rate ofabout 3 mm each year, well above therate of 1.8 mm per year over theprevious 100 years. This is despite largegeographical variations, including broadareas of falling sea level. Consistentincreases in both sea level and sea-surface temperatures have been found inmost parts of the Atlantic Ocean overthe past 15 years.


The Principle of Radar AltimetryRadar altimetry measures the distancebetween a satellite and the surface belowusing radar echoes bounced back from thesurface, whether ocean, ice cap, sea-ice,desert, lake or river. The characteristics of theechoes contain further information on theroughness of the surface, wave heights orwind speeds over the ocean.

Altimetry measurements become scientific-ally more useful when the satellite’s position isaccurately known. Many satellites, includingEnvisat and CryoSat, carry the DORIS(Doppler Orbitography and RadiopositioningIntegrated by Satellite) radio receiver forprecise orbit determination. DORIS calculatesthe orbit to an accuracy of a few centimetresby measuring the Doppler shift on signalsbroadcast from a network of more than 50beacons spread around the world.

By 1992, the unique results from ERSand the CNES/NASA Topex-Poseidonhad provided a strong foundation forthe future of satellite altimetry and themissions then in development, such asJason (CNES and NASA/Jet PropulsionLaboratory), Envisat and GeosatFollow-On (US Navy). Countries withdifferent cultures, especially Europe andthe USA, learned to work towards

common goals, and the differentgeodesy, geophysics and oceanographyscientific communities had to learn towork closely together. Cryosphereresearchers benefited by adapting thetechnology and data-processingprogress made by oceanographers, tomonitor the ice caps and sea-ice.

Over several decades, new technologieswith improved accuracy were developed.

In the Indian and, particularly, Pacificoceans, the trends in both sea level andtemperature are still dominated by thelarge changes associated with the largeEl Niño Southern Oscillation of1997–1998. Fresh water brought by rain,snow, melting sea-ice, ice sheets andglaciers complicate our understandingof the rise in sea levels. It can beexpected that the next decade ofaltimetry will provide fundamental newinsights into these important features.

TsunamisThe Sumatran tsunami of December2004 was the first to be observed byaltimeters in space, which has allowedscientists to improve our understandingof how tsunamis propagate. This isinvaluable for helping to avoid disasters,in addition to being of great interest forscience. One of the main components ofbuilding propagation models is theunderwater equivalent to altimetry:bathymetry. This is the measurement ofthe depth contours of the soil, rock orsand at the bottom of a body of watersuch as an ocean or a lake. Deriving sea-floor topography from radar altimetry isimproving the accuracy of these modelsbecause the nature of the floor influenceshow tsunamis move and build.

Marine meteorologyRadar altimeters are indispensable forobserving the sea state in a variety ofapplications. Wave climatology, a well-established altimeter application, iscontinuously enriched by new data asthey become available; the longer therecord, the more consistent and reliablethe results. Correlation of wave-heightvariations with climatological phenom-ena such as the North AtlanticOscillation has been observed – and hasopened a whole new area of science.Wave modelling, a traditional applica-tion, has greatly improved recentlythanks to the assimilation of altimeterdata in near-realtime, and is nowgenerating accurate sea-state predictions,to the enormous benefit of the shippingindustry. In addition, sea-surfacetopography measured by radar

altimeters is also used in near-realtimejointly with sea-surface temperature andmodels to investigate how the upper-ocean thermal structure is involved instrengthening hurricanes.

Understanding the Cryosphere Glaciologists had to wait until theadvent of ERS-1 to see a polar altimeterflying over sea-ice and the ice sheets. Theinstrument proved to be a very powerfultool for glaciology, with three majorscientific objectives: ice-sheet modellingand dynamics, ice-sheet mass balanceand sea-ice thickness. There are alsonumerous secondary uses.

Ice sheetsThe mass changes in ice sheets has beenstudied from space, ranging from thelargest sheets in Antarctica andGreenland, to smaller ones such as theAustfonna (in Svalbard, the world’sthird largest icecap and the largestglacier in Europe). Overall, theGreenland and Antarctica sheets arefound to be almost in equilibrium,losing as much ice as they gain, but localdata suggest a loss that could acceleratein the near future.

Radar altimeters are not only able tomap the global trend but can also catchlocal imbalances. However, discrepanciesbetween some studies may be explainedby the behaviour of radar wavesinteracting with packed snow. This iswhy Envisat’s dual-frequency altimeteris helping to improve our knowledge ofhow radar waves penetrate the snow.

Altimetry over Land and Inland WaterThe potential of satellite radar altimeterdata for applications over land andinland water is now well known, as datafrom Topex-Poseidon, Jason, ERS-1,ERS-2 and Envisat have extensivelyshown. Initially developed to makeprecise measurements of sea surfaces,radar altimeters rapidly proved able toprovide information over the continents.Their development to monitorcontinental water surfaces provides apowerful tool for studying regionalhydrological systems.

The accuracy of global geodeticmeasurements has increased from a fewhundred metres at the beginning of thesatellite era to a few centimetres now.Though a highly complex problem, itrecently became possible to exploitradar altimetry to monitor inland waterlevels; the accuracy over these difficultterrains is improving rapidly.

After many years of development anddata exploitation, radar altimetry isbecoming operational in oceanographicapplications. A new generation of high-resolution and high-precision instrumentsis entering service using techniques suchas ‘delay-Doppler’ and interferometry.We now know much more about ourEarth, ocean dynamics and the cryo-sphere than we would without altimetry,and we have laid the foundations forfully operational 3-D oceanography.

Understanding the Ocean Radar altimetry has made an importantcontribution to oceanography byinvestigating the high-frequencyvariability in sea-surface height fromglobal to basin scales, and its impact onthe oceans’ general circulation. Ithighlights the importance of eddies inshaping and controlling the flows ofmajor current systems such as theAntarctic circumpolar current andwestern boundary currents (GulfStream and Kuroshio), and theinfluence of eddies on the verticalmixing in the ocean. The results from 15years of altimeter data on eddyvariability in the oceans are outstandingand are certainly a major accomplish-ment. Altimetry has proved unique indramatically improving our tide models,in observing internal tides andunderstanding the genesis of climaticevents such as El Niño and the NorthAtlantic or North Pacific Oscillations.Now it is the interaction betweenphenomena such as planetary waves,eddies, tides and mean flow, and theirimpact on coastal regions, that must beinvestigated. These new studies willbenefit greatly from the higherresolution sampling provided by theupcoming altimeter missions.

Earth Observation


Radar Altimetry

The kinetic energy of eddies in the North Atlantic calculated from sea-level data along the Topex-Poseidon and ERS groundtracks. Redhighlights the greatest energy. (Le Traon & Dibarboure)

The variability of sea-surface height from Topex-Poseidon and ERS data in the southern Pacific Ocean. Box 1 highlights the variability ofthe South Equatorial Counter Current. Box 3 shows the variability of the South Tropical Counter Current. (Qiu &Chen)

The rise in sea level during the 20th century appears to beaccelerating. Satellite measurements for the last 15 years areshown in green. (Cazenave et al.)

Sea levelsAltimeter observations from satellitesshow that the global mean sea level hasrisen over the past decade at a rate ofabout 3 mm each year, well above therate of 1.8 mm per year over theprevious 100 years. This is despite largegeographical variations, including broadareas of falling sea level. Consistentincreases in both sea level and sea-surface temperatures have been found inmost parts of the Atlantic Ocean overthe past 15 years.

benveniste 11/9/06 4:25 PM 4

Earth Observation


Radar Altimetry

Satellite altimetry is improving the Global Digital Elevation Model andour knowledge of sea floor topography. (Berry et al.)


Although monitoring inland waters israpidly developing, the potential ofmulti-mission altimetry is only nowbeing realised. One such mission isWatER, proposed both to ESA in 2005as a response to the second call forEarth Explorer core missions and toNASA as a possible partnership mission

gravity models. Subtraction of this newmodel from an altimeter-derived seasurface reveals the dynamic oceantopography, at a resolution nearlysufficient to resolve the westernboundary currents.

These models are an improvementover the EGM96 combined model andthus will provide an improved referencefor higher-resolution marine gravitymodels derived from altimetry. The newglobal-scale geoid models also serve as aglobal vertical reference system.Scientists are preparing for the GOCEmission, which will provide improvedspatial resolution (about 100 km),sufficient to resolve western boundarycurrents such as the Gulf Stream fully.

Marine gravityAlthough no new non-repeating orbitradar altimeter data have been availablesince the ERS-1 geodetic phase in1994–1995, reprocessing the rawaltimeter waveforms has produced anear-40% improvement in the accuracyof the gravity field. Comparisons withshipborne gravity measurements overthe deep ocean show the accuracy is now3–5 mGal and the shortest half-wavelength resolved is approaching thealtimeter track spacing of 8 km fromERS-1.

The Cryosat launch failure was amajor setback for the marine gravityand geophysics communities becausethat mission would have provided a newglobal altimeter dataset with dense trackspacing and, more importantly, it wouldhave demonstrated the technology forthe next generation of marine gravity

with ESA. There are different scientificand technical challenges to be overcomein such missions. One is the need to mapriver basins in two dimensions, inferringthe slopes as well as the levels of rivers.

While WatER would need severalyears to be developed, if selected,scientists are meanwhile focusing on

measurements by altimetry. Scientistsare delighted that ESA is rebuilding thesatellite.

The laser altimeter aboard NASA’sICESat has provided new gravityinformation in those parts of the ArcticOcean where permanent sea-ice closelyconforms to the shape of the geoid.Further improvements in the accuracyand resolution of marine gravity wouldprovide important contributions in bothscientific and practical studies such aslocating 50 000 uncharted seamounts inthe deep oceans and exploring theoffshore sedimentary basins for oil.Other applications include mapping thedetails of plate tectonics, planningshipboard surveys in remote areas andimproved inertial navigation of aircraftand ships.

BathymetryOcean bathymetry is currently bestmeasured by sonars aboard ships, butonly a small fraction of the global oceanbasins have been surveyed; it isestimated that it will take 125 ship-yearsto survey all of the deep oceans. Theneed for improved global bathymetry iscritical because it forms the basic datafor many fields, including tsunamipropagation, hydrodynamic tide models/tidal friction, ocean circulation, seafloortectonics, identification of volcanicchains, defining the 2500 m isobath forthe law of the sea, and fisheriesmanagement.

For example, the Pacific-AntarcticRise and Louisville Ridge wereunknown features 30 years ago. ThePacific-Antarctic Rise, covering an area

about equal to North America, is abroad part of the ocean floor lifted upbetween two major tectonic plates. TheLouisville Ridge lies to the west of this,and is a chain of large underwatervolcanoes discovered in 1972 using depthsoundings collected along random shipcrossings of the South Pacific. Six yearslater, the full extent of this chain wasrevealed by a radar altimeter aboardNASA’s Seasat. Recent data collected byGeosat and ERS-1 show the Pacific-Antarctic Rise and the Louisville Ridgein unprecedented detail.

Progress in Ocean Integrated SystemsIn 15 years there has been a wide rangeof activities: synergies between remotesensing and in situ data, development ofoperational oceanography systems,model validation and studies of theimpact of the integrated approach onresearch and applications. Theinternational Global Ocean DataAssimilation Experiment (GODAE) is apractical demonstration of near-realtime global ocean data assimilationthat supports operational oceanography,seasonal-to-decadal climate forecastsand oceanographic research.

The main integrated systemsdeveloped as part of GODAE coverhigh-resolution (eddy-resolving) systemsthat focus on the forecasting of oceanmesoscale conditions, and lower resolu-tion systems for climate applications.Such applications require a precise anddynamically consistent description ofthe ocean state. Use of advanced dataassimilation techniques allows diagnosticstudies such as heat balance. A major

exploiting today’s missions (ERS-2,Envisat, Jason-1, Geosat Follow-On)and on the future CryoSat, Jason-2 andSentinel-3 missions for studying inlandwaters.

Gravity and Marine Geoid ModellingGeodesyAfter years of slow progress, twomissions – CHAMP (2000) andGRACE (2002) – are providing excitingresults in global geodesy, and GOCE(2007) will soon join them. Geodesyprimarily concerns positioning and thegravity field and geometrical aspects oftheir variations, and it can include thestudy of Earth’s magnetic field. Gravityanomalies reflect mass variations insidethe Earth, offering a rare window on theinterior. The geoid is the shape of anideal global ocean at rest, and it is usedas the reference surface for mapping alltopographic features, whether they areon land, ice or ocean. The geoid’s shapedepends solely on Earth’s gravity field,so its accuracy benefits from improvedgravity mapping. Measuring sea-levelchanges, ocean circulation and icemovements, for example, need anaccurate geoid as a starting point. Heatand mass transport by oceans areimportant elements of climate change,but they are still poorly known andawait measurement of ocean surfacecirculation.

The new EIGEN04 gravity modelderived from these missions andterrestrial gravity data eliminates muchof the ‘meridional striping’ seen in

Earth Observation


Radar Altimetry

The ground track of Envisat’s radar altimeter over the Amazon basin, using a 35-day repeatcycle. The height readings over the rivers were used to produce the accompanying graph ofriver levels since 2002 (Berry et al.)

Envisat altimeter measurements (circles) of the river levels in the Amazon Basin compare well within situ readings from gauges (line). (Berry et al.)

The WatER interferometric radar altimeter concept uses twoantennas to create a 2-D image of surface height. Themission was proposed for ESA’s Earth Explorer programme(Mognard et al.)

Improvement in our knowledge of sea floor topography from Seasat (1978,right) to the Geosat and ERS-1 geodetic missions. (Smith & Sandwell)

Louisville Ridge

Pacific Antarctic Rise


For the GMES core marine services,the operational Sentinel-3 mission willdeliver key information on sea-surfacetopography, sea-surface temperatureand water quality, for example. Theoperational phase of Sentinel-3 isplanned for around 2011–2015.

For the near future, the mainoperational mission will be Jason-2,developed by NASA and CNES to beoperated by the US National Oceanic &Atmospheric Administration (NOAA)and Eumetsat. It extends the Topex-Poseidon and Jason-1 series andenhances the current altimetry servicesfor climate monitoring and operationaloceanography. In the longer term,Eumetsat is offering its capability as aleading European operational organisa-tion to run some proposed futuremissions, such as GMES Sentinel-3 and,possibly, the Jason-2 follow-on.

Scientific developments have seen arecent tendency towards Ka-bandaltimetry. In particular, CNES is nowproposing its AltiKa mission for alaunch around mid-2009, aiming at

filling the possible service gap afterEnvisat and complementing Jason-2 forthe resolution of ocean mesoscalevariability. It will increase accuracy andsampling capabilities in coastal regionsand improve continental ice-sheetmonitoring, though with the possiblereduction of observing capability underexceptional rain and cloud conditions.

Considerable scientific progress isexpected from wide-swath interfero-metric altimetry, not only by resolvingsmaller-scale ocean variability, but alsoby providing a truly 2-D sampling ofhydrological systems. In August 2005, aconsortium with over 150 participantsfrom the wider hydrological communitysubmitted the WatER mission proposalto ESA’s Earth Explorer programme. Tobe flown after 2010, WatER wouldcontribute to a fundamental under-standing of the global water cycle byproviding global measurements ofterrestrial surface water storage changesand discharge. The main instrument isthe KaRin wide-swath Ka-bandinterferometric altimeter, which could

map rivers, lakes and wetlands at aspatial scale over 100 m with a heightaccuracy of 5–10 cm.

Finally, higher resolution is needednot only for progress in mapping oceanmesoscale and coastal variability andhydrological systems, but also to makethe next advances in geodetic andbathymetric signals using spacealtimetry. Studies have shown that theseadvances could be realised in a highlycost-effective manner with a high-resolution radar altimeter (as carried byCryoSat) aboard a microsatellite.

AcknowledgementsThis article is based on a report of theSymposium ‘15 Years of Progress inRadar Altimetry’ held on 13–18 March2006 in Venice (I). All abstracts, oralpresentations and posters can be viewedat http://earth.esa.int/venice06. Thepapers are available in the proceedingsSP-614 from ESA Publications Divisionat http://www.esa.int e

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Radar Altimetry

issue for effective data assimilation is toestimate the model ‘error covariance’and there has been a significant advancein accounting better for these errors.

A major contributing project toGODAE is Argo, an array of more than2000 free-floating floats that providetemperatures and salinity measurementsat various depths across the oceans.Argo results are scientifically valuable intheir own right but can be combinedwith altimetry data for enhancingenvironmental and climate knowledge.Studies on the impact of altimetry andArgo on seasonal forecasts show theycan significantly improve data-assimilation systems. Thanks to thesestudies, Argo and altimetry are nowused in the operational seasonalforecasting systems of the EuropeanCentre for Medium range WeatherForecasting.

Physics and biology can be coupledthrough the joint analysis of altimetry,sea-surface temperature, ocean colourand model data. There are now studiesinto the different mechanisms that couldexplain the observation of planetarywaves in altimeter, sea-surface tempera-ture and ocean colour data. Horizontaladvection is an important mechanismbut vertical and biological effects cannotbe ruled out. Other studies have shownthe importance of ocean physics on thedevelopment of phytoplankton bloomsin the wake of islands.

The main conclusion is that majoradvances over the past 5 years havehelped to develop an ‘integrated’approach to describe and forecast oceanconditions. Integrated descriptions ofthe ocean state are now available and areused to characterise and understandocean climate variations better. This iscrucial for the long-term sustainabilityof the global ocean observing system.The use of Argo and altimetry data isessential for developing an improvedunderstanding of variations in the oceanclimate. The strong synergies betweenArgo and altimetry will become more ormore obvious as Argo is expanded.

A New Challenge: Coastal MonitoringAltimetry may contribute in many waysto the study of coastal phenomena,especially tides, currents and sea state,that directly affect, for example, offshoreoil exploration, fishing, marine aqua-culture and coastal planning anddevelopment. Altimetry can supplydirect measurements of sea level and seastate, and vital information about‘forcing’ from areas just outside thecoastal domain. These include theinfluence of offshore ocean circulationand the inflow of fresh water from landmasses, closely tied to river and lakelevels and to ice extent, all of which canbe observed by altimetry satellites.However, coastal monitoring has verydemanding requirements. The phenomena

are often small-scale, rapidly changingand highly turbulent events calling forcombined satellite and in situ data (suchas from tide gauges and buoys) to ensureadequate resolution and coverage, asclose as possible to the shore-line.Future altimetry systems will also haveto meet these requirements, either byemploying constellations of satellites orby developing new wide-swath radarconcepts.

The Future of AltimetryThe European Commission-fundedGAMBLE (Global Altimeter Measure-ments By Leading Europeans) projectbrought together European experts in2002–2003 to consider future develop-ments in satellite altimetry. The aim wasto provide recommendations forresearch and future altimeter missionsto support and build on current work inoperational oceanography and tomaintain ocean-monitoring programmes.

GAMBLE recommended in 2003 thatcoverage by a single satellite is notsufficient to meet both operational andscientific user needs. Rather, aconstellation of at least three nadir-viewing altimeters is needed to providethe sampling required for manypractical purposes. GAMBLE stressedthe demonstration of new technologysuch as wide-swath altimeters and largerconstellations of altimeters aboardmicrosatellites. The latter could prove tobe very effective in the timelydeliverance of sea-state information andin warning of natural hazards.

An important topic for the future ofaltimetry is the ongoing transitiontowards operational services. In Europe, aleading initiative is the GlobalMonitoring for Environment andSecurity (GMES) programme to developa coordinated operational environmentalinformation service, partly based ontoday’s space infrastructures. TheMERSEA ocean science component ofGMES involves 50 European partnersaiming to develop and sustain anintegrated, operational system to provideanalysis and forecasting over the globalocean and European seas.

Earth Observation

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Copies of are available at EUR 30 each. Justfill in the Order Form at the back of thisissue of the Bulletin and send it in by mailor fax. If you have any questions, pleasesend them to [email protected]


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