ASAR – Envisat’s Advanced SyntheticAperture RadarBuilding on ERS Achievements towards Future Earth Watch Missions

Y-L. DesnosEarth Observation Applications Department, ESA Directorate of ApplicationProgrammes, ESRIN, Frascati, Italy

C. Buck, J. Guijarro, J-L. Suchail and R. TorresEnvisat System and Payload Division, ESA Directorate of Application Programmes,ESTEC, Noordwijk, The Netherlands

E. AttemaEarth Sciences Division, ESA Directorate of Application Programmes,ESTEC, Noordwijk, The Netherlands

The missionThe Envisat mission is an important element inproviding the long-term, continuous data setsthat are so crucial for addressing environmentaland climate issues. At the same time, it willfurther promote the transfer of applications ofremote-sensing data from experimental to pre-operational and operational exploitation.

– detecting large-scale vegetation changes– monitoring natural and man-made pollution

over the oceans.

ASAR is set to make a major contribution to theregional mission by providing continuous andreliable data sets for applications such as: – offshore operations in sea ice– snow and ice mapping– coastal protection and pollution monitoring– ship traffic monitoring – agriculture and forest monitoring– soil-moisture monitoring– geological exploration– topographic mapping– predicting, tracking and responding to natural

hazards– surface deformation.

Some of the regional objectives (sea-iceapplications, marine pollution, maritime traffic,hazard monitoring, etc.) require near-real-timedata products (within a few hours from sensing)generated according to user requests. Someothers (e.g. agriculture, soil moisture, etc.)require fast turn-around data services (within afew days). The remainder can be satisfied withoffline data delivery. As well as ASAR satisfyingspecific operational and commercial requirements,there will be major systematic data-collectionprogrammes to build up archives for scientificresearch purposes.

LandAs a result of observing the land surface withthe ERS SARs, a large number of landapplications have emerged, several based on

Following on from the highly successful ERS-1/2 SARs, which havecontributed to major scientific achievements and initiated pre-operational and commercial applications of SAR data, ESA is nowready to launch Europe’s largest remote-sensing satellite to date,carrying an Advanced Synthetic Aperture Radar (ASAR).

The ASAR is an all-weather, day-and-night high-resolution radar-imaging instrument. Compared with the ERS SAR, it features extendedobservational capabilities, three new modes of operation andimproved performances. The ASAR system has been designed toprovide continuity with ERS SAR, but also to extend the range ofmeasurements through the exploitation of its various operating modesand the development of new algorithms and data products. TheEnvisat ground segment will allow the generation of near-real-timeand off-line precision images to satisfy the needs of the scientific,institutional and commercial data users.

asar

The mission has both ‘global’ and ‘regional’objectives, with the corresponding need toprovide data to scientific and applications userson various time scales. Important contributionsby ASAR to the global mission include:– measuring sea-state conditions at various

scales– mapping ice-sheet characteristics and

dynamics– mapping sea-ice distribution and dynamics

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Figure 1. ERS-SARinterferometry estimated

displacement over POMONAusing the permanent-

scatterers technique: June1992 to January 1999

(courtesy of Politecnico diMilano, Italy)

important developments in the field of SARinterferometry. SAR data are being used foragricultural monitoring, forest mapping,geological exploration and flood mapping,while INSAR measurements of topography andsmall topographic changes are making majorcontributions to environmental risk assessmentinvolving earthquakes and land subsidence.

ASAR has extended observational capabilitiesin comparison to the ERS SAR, providing SARmission continuity as well as benefiting from theresults from ERS and other SAR missions.Operating in concert with other Envisat-1instruments MERIS and AATSR, ASAR provides

essential surface-roughness and land-coverinformation for the determination of land-surface processes and air/sea interaction forclimate studies.

The higher frequency of coverage provided byASAR will improve greatly the value of SARdata for hazard monitoring, because locallyinfrequent events such as earthquakes,volcanic eruptions, floods and fires, requireintensive observation over short periods. Thebeam steering mode will also permit (at least)3-day repeat observations of certain localisedevents at high spatial resolution. Table 1 showsthe average revisit frequency per 35-day orbit

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Figure 2. Dual-polarisationSAR images of Thetford

Forest (UK), from the SIR-Cmission. Left image:

HH (green) and VV (red).Right image: HH (green)

and HV (red)(courtesy of GEC-Marconi,

UK)

Table 2. ASAR modes and land-application themes (lighter colour indicates primaryapplications)

Table 3. ASAR modes and ocean-application themes (lighter colour indicates primaryapplications)

Table 1. Average revisit cycle per 35-day orbit as a function of latitude andincidence-angle variation, illustrating ASAR revisit time capability (descending pathonly)

motion, each target element remains within theillumination beam for a period known as the‘integration time’. As part of the on-groundprocessing, the complex echo signals receivedduring this integration time are addedcoherently. This process is equivalent to a longantenna – so-called ‘synthetic aperture’ –illuminating the target. This synthetic aperture isequal to the distance the satellite has travelledduring the integration time.

The along-track (equivalent to the azimuth inthe ground processing) resolution obtainablewith the SAR principle is half the physicalantenna length. The resolution achieved can be

cycle as function of latitude and incidenceangle (descending tracks only).

The potential value of the Global Monitoringmode is indicated by previous work carried outover land, using the ERS wind scatterometer at25 km spatial resolution. For local land-covermapping, ASAR high-resolution products willcontinue the role already established for ERSSAR in complementing conventional opticalimages from other satellites, particularly underpoor solar-illumination conditions or in cloudyareas. The new features of ASAR include imageacquisitions at multiple incidence angles andwith dual polarisation, which will open up newpossibilities in land-cover classification fromSAR.

Ocean and iceThe original focus of the ERS missions wasocean and ice monitoring, and there has beenan impressive range of scientific investigationsin oceanography, polar science, glaciology andclimate research which will be supported byASAR. These include measurements of oceansurface features (currents, fronts, eddies,internal waves), directional ocean-wave spectra,sea-floor topography, snow cover and ice-sheet dynamics. Operational systems havebeen developed for mapping sea ice, oil-slickmonitoring and ship detection.

Major features of the interaction between theocean and the atmosphere are the creation ofwaves and ocean currents by surface winds.Wind and wave data are needed forclimatological research, as inputs to meteoro-logical models and for sea-state forecasting insupport of marine operations. ASAR willprovide observations of surface waves andwinds over the ocean.

The ASAR Wave mode combined with a newalgorithm called ‘inter-look cross-spectralprocessing’, whereby information on the wavepropagation direction is computed from pairs ofsingle-look images separated in time bytypically a fraction of the dominant wave period,will provide meteorological users with wavedirectional and geophysical parameters andwind parameters.

Recently, investigations have shown that it ispossible to derive wind-speed estimates fromSAR data (see Fig. 3). The ASAR Wide Swathmode is of interest in this respect because of itslarge coverage area and the short revisit period.

Instrument operationMeasurement principleThe radar antenna beam illuminates the groundto the side of the satellite. Due to the satellite’s

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LatitudeIncidenceAngle

0¡ 45¡ 60¡ 70¡NoConstraintsr 5 7 11 16± 5¡ 3 4 6 9

± 2¡ 1 1.4 2 3Exact Repeat 1 1 1 1

ASAR LAND

APPLICATIONS

Image Alternating

Polarisation

Wide

Swath

Global

Monitoring

HH orVV HH/VV or

VV/VH orHH/HV

HH or VV HH orVV

Vegetat ion / AgricultureForestryLand cover classification

CartographyTopography GeomorphologyHydrology / Soil moistureSnow cover

IceGlacierTectonicsLand transformation processClimate s tudies

ASAR OCEANAPPLICATIONS

Image Alternat ingPolarisat ion

WideSwath

GlobalMonitoring

Wave

HH or VV HH/VV orVV/VH orHH/HV

HH or VV HH or VV HH or VV

Weather forecast ing

Sea state forecastingCurrent modellingInternal wave modellingOffshore activitiesShip routing

Ship detectionFisheriesSea iceOil pollutionCoastal zone

Bathymetry

traded off against other imagequality parameters (such as theradiometric resolution).

The across-track or rangeresolution is a function of thetransmitted radar bandwidth.Pulse compression techniques areused to improve the performancetaking into account the instrument’speak power capability. The factthat the end-to-end system workscoherently means that both theamplitude and the phaserelationships between the complextrans-mitted and received signalsare maintained throughout theinstruments and the processingchain.

Operating modesThe ASAR instrument is designedto provide a large degree ofoperational flexibility. The maininstrument parameters can beselected by ground command foreach of the five operational modes:

– The Image mode generateshigh-spatial-resolution dataproducts (30 m for precisionimages) selected from the totalof seven available swathslocated over a range of incidenceangles spanning 15 to 45 deg.

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Figure 3. Wind-fieldestimate from ERS-2 SARdata (courtesy of Norut,Norway)

Figure 4. The ASARoperating modes

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– The Wave mode generates vignettes of 5 kmby 5 km, spaced 100 km along-track. Theposition of the vignette can be selected toalternate between any two of the sevenswaths.

– The Wide Swath and Global Monitoringmodes are based on the ScanSAR techniqueusing five sub-swaths, and they generatewide-swath products (400 km) with spatialresolutions of 150 and 1000 m, respectively.

These four modes may be operated in one oftwo polarisations, either HH or VV (the letterfirst indicates the polarisation of the transmitsignal – H for horizontal, V for vertical – and thesecond the polarisation of the receive signal).

– The Alternating Polarisation mode providestwo simultaneous images from the samearea in HH and VV polarisations, HH and HVor VV and VH, with the same imaging geometryas the Image mode and similarly high spatialresolution.

The ASAR instrumentThe ASAR instrument consists of two mainelements: the Central Electronics Sub-Assembly (CESA) and the Antenna Sub-Assembly (ASA).

The active antenna contains 20 tiles with 16sub-arrays, each equipped with a transmit/receive (T/R) module. The instrument is drivenby the Control Sub-System (CSS), whichprovides the command and control interface to

Figure 5. ASAR CESA flightmodel during thermaltesting (courtesy of Alenia,Italy)

generation is the inherent flexibility of such adesign, which allows for chirp versatility interms of pulse duration and bandwidth, thusaccommodating efficiently the variousrequirements associated with the high numberof available operational modes and swaths ofthe instrument.

At reception, the echo signal is first filtered anddown-converted in the RF Sub-System, thendemodulated into the I & Q components of thecarrier. These two signals are then bothdigitised into 8-bit samples. If required, it is thenpossible to perform digital decimation of the

the spacecraft, manages the distribution of theoperational parameters (such as transmit pulsecharacteristics and antenna beam-set), andgenerates the instrument operation time line.

The transmit pulse characteristics are set in theData Sub-System (DSS), the output of which isan up-chirp pulse centred on the IF carrier (124 MHz). In the RF Sub-System (RF S/S) thepulse is up-converted to the RF frequency(5.331 GHz) and amplified. The signal is thenpassed to the Tile Sub-System (TSS) through awaveguide distribution network (RFPF) andsubsequently, within the tile, to each individualT/R module using a microstrip corporate feed.The T/R modules apply phase and gaincharacteristics according to the pre-selectedbeam settings transferred from the ControlSub-System and stored in the Tile ControlInterface Unit (TCIU).

In receive, the RF-echo signal follows thereciprocal path down to the Data Sub-System,where the raw science data are generated andprovided to the spacecraft interface.

Central Electronics Sub-AssemblyThe CESA is in charge of generating thetransmitted chirp, converting the echo signalinto measurement data, as well as controllingand monitoring the whole instrument.Compared to ERS-1 and ERS-2, which useSurface Acoustic Wave (SAW) devices foranalogue chirp generation and on-board rangecompression, ASAR uses digital technologiesfor on-board chirp generation and datareduction for temporary storage associatedwith on-ground range compression. Afundamental advantage of using digital chirp

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Figure 6. ASAR functionaldiagram

Figure 7. Flight model of theASAR antenna (courtesy ofMatra Marconi Space, UK)

Figure 8. The ASAR T/Rmodule (courtesy of Alcatel,

France)

samples, in order to reduce the data stream,such as in Global Monitoring mode where thetransmit bandwidth is low. Following thisdecimation, a Flexible Block Adaptive Quantiser(FBAQ) compression scheme is applied to theecho samples.

The FBAQ allows the data rate to be maintainedwithin data-transmission requirements withoutdegrading the image quality. This is achieved byusing a compression algorithm optimised to thestatistics of the radar signal. The FBAQ ASICthat has been developed can be operated inthree ways: compression according to theFBAQ algorithm (8 to 4, 3 or 2 bits), bypass ornoise (fixed exponent), depending on the typeof data to be processed.

In order to optimise raw data transfer, the dataequipment also contains science memory,where the echo samples are temporarily storedbefore their transmission to the on-boardrecorders.

Active phased-array antennaThe ASAR active antenna is a 1.3 m x 10 mphased array. The antenna consists of five 1.3 m x 2 m panels which are folded for launch.Each panel is formed by four 0.65 m x 1 m tiles

mounted together. The Antenna Sub-Assemblyis divided into three sub-systems: the AntennaServices Sub-System (ASS), the Tile Sub-System (TSS) and the Antenna PowerSwitching and Monitoring Sub-System (APSM).

The antenna is based on a mechanicalstructure consisting of five rigid Carbon-Fibre-Reinforced Plastic (CFRP) frames and two RFdistribution networks of CFRP waveguidesrunning in parallel along the five panels. Inlaunch configuration, the five panels are stowedfolded over the fixed central one, and heldtogether by eight hold-down and releasemechanisms (HRMs). Each HRM consists of aretractable telescopic tube levered by asecondary mechanism based on non-pyrotechnic technology (kevlar cable andthermal knife).

After release, the panels deploy sequentiallyaround four hinge lines by using steppermotors. Latching is performed by the eightbuilt-in latches to achieve the final antennaplanarity of ±4 mm in orbit. Each of the 20 tilesis a self-contained, full-operating sub-systemwhich includes four Power Supply Units(PSUs), a Tile Control Interface Unit (TCIU), two microstrip RF distribution corporate feedsand 16 sub-arrays of 24 dual-polarised low-loss dispersion-free radiating elements. Eachsub-array is connected to a T/R module, withindependent connections for the twopolarisations. The 16 sub-arrays are mountedtogether, although thermally and mechanicallydecoupled, on a radiating panel, which providesboth structural and thermal integrity to the tile.The TCIU provides the control functions withinthe Tile. It performs the local control of the T/Rmodules, transfers data and interfaces to theControl Sub-System.

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Figure 9. An Image modemedium-resolution product,showing the WalgreenCoast, Antarctica (from ERSraw data)

Table 4. ASAR performancesummary

worst-case scenario and demonstrate that theEnvisat ASAR mission objectives can indeed befulfilled.

Instrument calibrationASAR, unlike the ERS AMI-SAR, is an activeantenna and any instabilities in gain and phasecharacteristics will therefore distort theelevation beam patterns and can contribute

Each of the 320 T/R modules consists of two(H & V) transmit chains and one commonreceive chain. For calibration purposes, acoupler (-24 dB) has been implemented at theoutput of the module to the antenna. For anactive antenna, the amplitude and phasecharacteristics of the T/R modules varyprincipally as a function of temperature. Tohandle this, the instrument includes a schemeto compensate for drifts over the temperaturerange. To this end, the temperature of each T/Rmodule is monitored and utilised by the TCIU tocompensate the amplitude and phase settings.This scheme provides the antenna with a highdegree of stability.

Qualification of new technologiesIn order to guarantee the required operationalflexibility and performance, a number of newtechnologies, processes and componentsneeded to be developed and qualified (from thehybrid line for RF components, to GaAsfoundry, parallel-gap welding, etc.).

PerformancesThe inherent principle of the Synthetic ApertureRadar impedes the direct measurement ofASAR instrument performances on the ground.The selected alternative is computation of theinstrument performance characteristics frommeasurable lower level parameters throughoutthe various stages of instrument testing.

In the case of ASAR, due to the large numberof operating modes and measurementcapabilities, the verifications of instrumentperformances have resulted in an extensive testprogramme. It has also allowed the determinationof an optimal combination of parameters inorder to achieve the best overall performanceacross all operating configurations.

The predicted end-of-life performances for theASAR instrument are summarised in Table 4.These figures have been derived assuming a

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Parameter Unit Image AlternatingPolarisation

WideSwath

GlobalMonitoring

Wave

Polarisation VV or HH VV/HH, HH/HV,or VV/VH

VV or HH VV or HH VV or HH

Spatial resolut ion (az. x ra.)* except swath IS1

mm 28 x 28* 29 x 30*

150 x 150 950 x 980

28 x 30 *

Radiometric Resolution dB 1.5 2.5 1.5 to 1.7 1.4 1.5

Point Target Ambiguity Ratio- azimuth- range

dBdB

26 to 3032 to 46

19 to 2826 to 41

22 to 2926 to 34

27 to 2925 to 32

27 to 3031 to 46

Distributed Target Ambiguity Ratio- azimuth- range

dBdB

23 to 2517 to 39

18 to 2517 to 39

20 to 2517 to 31

25 to 2817 to 31

23 to 2521 to 48

Radiometric Stability dB 0.32 to 0.40 0.50 to 0.55 0.32 to 0.42 0.46 to 0.53 0.55 to 0.60

Noise Equivalent σ0 dB -20 to -22 -19 to -22 -21 to -26 -32 to —35 -20 to -22

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Figure 10. An Image modeprecision product, showing

Bathrust Island, Canada(from ERS raw data)

to radiometric errors in the SAR image. For this reason, a sophisticated scheme for theASAR’s radiometric calibration has beenselected, composed of three elements: internalcalibration, external calibration and externalcharacterisation.

Internal calibrationThe objectives of the ASAR instrument internalcalibration are to derive the instrument’sinternal path transfer function and to performnoise calibration.

During normal operation in any of the ASARmeasurement modes, a sequence of calibrationpulses is interleaved with normal radar pulses.These pulses characterise the active array inboth transmit and receive, on a row-by-rowbasis.

As well as providing internal calibration duringthe measurement modes, the ASAR includes aspecial ‘module stepping’ mode, in whichindividual T/R module characteristics can bemeasured. It can be used to investigate T/Rmodule failures and ageing effects. In thismode, only one module is activated at a time ineither transmit or receive.

The internal calibration scheme also includesmeasurements of the instrument noise level.They are taken during the initial calibrationsequences at the beginning of a mode. In themodes that have natural gaps in their imagingsequence (i.e. wide-swath and globalmonitoring modes), noise measurements arealso made during nominal operation throughoutthe mode.

External calibrationThe objective of the external calibration is toderive the overall scaling factor. The successful

methodology developed for ERS will be re-used for ASAR.

To this end, four high-precision transpondershave been developed, characterised by a 0.08 dB stability and 0.13 dB accuracy. Thesetransponders will be deployed in Flevoland (NL)across the ASAR swath and observed severaltimes during each orbital cycle of 35 days.

Images acquired over suitable regions of theAmazonian rain forest will also be used toderive the in-flight elevation antenna patterns.Absolute calibration factors derived fromtransponder measurements and across-swathcorrections derived from the radar equation willbe used to calibrate the final image product.

For the wide-swath mode using the ScanSARtechnique, the external calibration approachwill be similar to the one used for the narrow-swath mode.

External characterisationThe ASAR has a dedicated ExternalCharacterisation mode to monitor all thoseelements that are outside the internalcalibration loop, as well as the calibration loopitself.

In this mode, planned to be operated every 6months, a sequence of pulses sent by eachantenna row in turn is detected by the antennacalibration loop and simultaneously recordedon the ground by a special receiver built intothe ASAR calibration transponder. The datarecorded in the transponder and that down-linked from the instrument are compared in theground processor to reveal the relative phaseand amplitude of the pulse from each row.These relative amplitudes and phases are usedto characterise the row of radiating sub-arraysand the calibration path of the row.

ASAR ground processorThe development of the ASAR processor isbased on the following driving concepts:

– The need for users to have identical productsirrespective of the processing facility.

– The need to broaden the range of productswhilst ensuring the quality of the ERS SARhigh-resolution products.

– The capability to cope with the large amountof products to be generated.

– The ability to generate continuous mediumand low-resolution products along the orbit(stripline processing, without radiometric orgeometric discontinuity).

Following the above concepts, ESA hasdeveloped a single ASAR processor able to

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The ASAR processor will be used to ensure thesystematic processing in near-real-time of allreceived high-rate data to generate medium-resolution and browse products. All Wavemode or Global Monitoring mode data will alsobe systematically processed in near-real-time.

Furthermore, the ASAR processor will allowhigh-resolution products to be processed fromImage or Alternating Polarisation acquisitions(Precision Image, Single Look Complex orEllipsoid Geocoded products) in near-real-timeor off-line, depending on user requests. Theaccompanying figures show examples of aPrecision Image and a Medium ResolutionProduct.

The different products, their coverages andqualities are summarised in Table 5.

Data collection and user servicesThe ASAR instrument modes of operation canbe divided into two categories: – Low-data-rate modes (Global Monitoring

and Wave) with an operational capability ofup to 100% of the orbit. Both modes aresystematically recorded on-board and theon-board recorder is dumped every orbit whenvisible from an ESA station.

– High-data-rate modes (Image, AlternatingPolarisation and Wide Swath) with a maximumoperating time of 30 min per orbit (includingthe ability to operate for up to 10 min ineclipse).

Compared to ERS, the Envisat mission offersimproved data recording and transmissioncapabilities for the ASAR high-data-ratemodes:– 12 minutes of on-board recording– real-time transmission X-band in visibility of

ground stations

handle data from any of the ASAR modes innear-real-time and off-line. This processor willbe installed at the ESA Payload Data HandlingStations (Kiruna in Sweden and ESRIN,Frascati, in Italy), at the Envisat Processing andArchiving Centres (PACs), and at the nationalstations offering ESA ASAR services. The useof a single processor will ensure productconsistency for the users, independent of theESA processing centre selected (same formatand processing algorithm) and will simplifyproduct validation and future product upgradecycles.

One of the key new features of the ASARprocessor is the ability to generate medium-resolution (150 m) and low-resolution (1 km)products with their corresponding browseimages in stripline without geometric orradiometric discontinuity. The stripline imageproducts represent processed data from anentire acquisition segment of up to 10 minutesfor Image, Alternating Polarisation and WideSwath modes and up to a complete orbit forGlobal Monitoring mode. The user can selectany segment in the processed stripline.

The processor computes the replica of thetransmitted pulse from the calibration-pulsemeasurements, the row patterns ascharacterised on-ground and the externalcharacterisation data. The constructed replicatracks variations in the transmit and receivechains and is used to determine the rangereference function for range-compressionprocessing.

The ground processor includes a DopplerCentroid Estimator with a specified accuracy of50 Hz for Image and Wave modes as for ERSand 25 Hz in the ScanSAR modes in order tolimit radiometric errors in azimuth.

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Table 5. ASAR products,coverages and qualities

Mode and Product Name Nominal Resolution (m) Pixel spacing (m) Coverage (km) Product ENL

IM precision IMP 30 x 30 12.5 x 12.5 56-100 x 100 3.9

IM single look IMS 9slant x 6 natural 56-100 x 100 1

IM geocoded IMG 30 x 30 12.5 x 12.5 100 x 100 3.9

IM medium resolution IMM 150 x 150 75 x 75 56-100 x 100 40

IM browse IMB 900 x 900 225 x 225 56-100 x 100 80

AP precision APP 30 x 30 12.5 x 12.5 56-100 x 100 1.9

AP single look APS 9slant x 12 natural 56-100 x 100 1

AP geocoded APG 30 x 30 12.5 x 12.5 100 x 100 1.9

AP medium resolut ion APM 150 x 150 75 x 75 56-100 x 100 50

AP browse APB 900 x 900 225 x 225 56-100 x 100 75

WS medium resolution WSM 150 x 150 75 x 75 400 x 400 12

WS browse WSB 1800x1800 900 x 900 400 x 400 57-62

WV imagette &

cross spectra WVI

9slant x 6 natural 5x5 to 10x5 1

n/a

WV cross spectraWVS - - 5x5 to 10x5 n/a

GM image GM1 1000 x 1000 500 x 500 400 x 400 12

GM browse GMB 2000 x 2000 1000x1000 400 x 400 18-21

– real-time transmission or recorded data dumpin visibility of ESA X-band ground stations orvia ESA data-relay satellite Artemis using Ka-band links.

The instrument will be operated according tothe following strategy:– In response to user requests for Category 2

use (commercial applications).– In response to user requests for Category 1

use (scientific and demonstration projects).– For backgound acquisition, including

reference data archive build-up as well asroutinely planned data acquisitions (e.g. Wavemode over oceans).

All ASAR high-rate data acquired by ESAfacilities will be systematically processed innear-real-time to generate medium-resolutionproducts (~150 m) and browse products. Thebrowses will be available on line via the EnvisatUser Service. The Envisat Ground Segment willprovide a unified service to the users withaccess – once they are registered – to the fullrange of ESA services and products.

Users’ requests will be supported for inventorysearching on already acquired data, as well asfor data requiring future acquisitions. Userswho have ordered Envisat products will be keptinformed of the status and progress of theirorders: from acceptance, planning of instrumentoperation, data-taking, processing and throughto product delivery.

Data use for science and applicationsThe Envisat Announcement of Opportunity (AO)for data exploitation was issued in December1997, with a deadline for the submission ofproposals of end-May 1998. A total of 674proposals have been accepted, 376 of whichinvolve the use of ASAR (71% science, 24%commercial and 5% Cal/Val). Figure 11 showsthe distribution of the ASAR projects across thedifferent fields of application.

Most of the AO proposals involve theuse of multiple ASAR operating modes.There is no clear preference for specificswaths or specific polarisations, anddifferent polarisation combinations arerequested for all modes. Interferometryis a component of 38% of the projects.

ConclusionsThe ASAR instrument is characterisedby extensive flexibility, thanks to its fiveoperating modes, the ability to operatein Horizontal and Vertical polarisations,the wide range of elevation anglescovered, and the possibility to shapethe antenna beam in both transmit and

in receive by individually controlling theamplitude and phase of each of the 320transmit/receive modules.

In order to achieve the required performanceand operational flexibility, many new technologies,processes and components have been qualified.

All acquired data will be processed (either innear-real-time or off-line) within the ESAGround Segment by a single design ofprocessor, to ensure product consistencyirrespective of the processing centre. A largenumber of ASAR products will be routinelyproduced by ESA and made available to users.

The large number of applications that theinstrument will be capable of supporting,illustrated by the responses to theAnnouncement of Opportunity, qualifies ASARas a precursor of future Earth Watch missions.

The ASAR instrument test campaign wascompleted in early February 2000. CESA wassubsequently delivered to ESTEC in Noordwijk(NL) and mounted on the Envisat spacecraft.The ASAR antenna was delivered a month laterand integrated onto the spacecraft in earlyMarch.

AcknowledgementsMany European industries have participated inthe effort required to develop the largest singleinstrument (and associated ground processingchain) ever built in Europe for remote-sensingapplications. We limit ourselves here tomentioning only the major industrial contractors:Dornier (Mission Prime), MMS-UK (InstrumentPrime), Alcatel Space Industries (Tile S/S),Alenia Aerospazio (CESA) and, for the ASARGround Processor, MDA under the direction ofAlcatel Space Industries (PDS PrimeContractor). r

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Figure 11. Distribution ofASAR Announcement of

Opportunity projects acrossthe different application

fields