MWR and DORIS– Supporting Envisat’s Radar AltimetryMission

J. Guijarro (MWR)Envisat Project Division, ESA Directorate of Application Programmes. ESTEC, Noordwijk, The Netherlands

A. Auriol, M. Costes, C. Jayles & P. Vincent (DORIS)CNES, Toulouse, France

MWRThe missionThe Microwave Radiometer (MWR) is a two-channel passive radiometer operating at 23.8and 36.5 GHz based on the Dicke principle. Byreceiving and analysing the Earth-generatedand Earth-reflected radiation at these twofrequencies, the instrument will measure theamount of water vapour and liquid water in theatmosphere, within a 20 km-diameter field ofview immediately beneath Envisat’s track. Thisinformation will provide the tropospheric pathcorrection for the Radar Altimeter. The MWRmeasurements can also be used for thedetermination of surface emissivity and soilmoisture over land, and in support of studieson surface energy budget and atmosphericand ice characterisation.

Instrument operationThe nadir-pointing antenna receives radiation at 23.8 and 36.5 GHz in linear polarisation. The antenna subsystem includes a 60-cmaluminium reflector with a focal length of 350 mm and an offset angle of 47 deg. Twofeeds are used such that the 23.8 GHz channelis pointing in the forward direction and the 36.5 GHz channel in the backward direction,with a footprint of about 20 km diameter foreach beam. These frequencies are separatelyrouted into the RF front-end, where a two-pointcalibration scheme is adopted, with hot andcold references.

The deep cold-space measurements will beaccomplished via the sky-horn feed, while theon-board calibration reference load, maintainedby the thermal control system at theinstrument’s physical temperature, provides thehot reference.

Following on from the great success of its ERS-1 and ERS-2 satellitemissions, which have contributed to a much better understanding ofthe role that oceans and ice play in determining the global climate,ESA is currently preparing to launch Envisat, the largest Europeansatellite to be built to date.

The Envisat altimetric mission objectives are addressed by the RadarAltimeter instrument (RA-2), complemented by the MicrowaveRadiometer (MWR), used to correct the error introduced by the Earth’stroposphere, and by the Doppler Orbitography and Radio-positioningIntegrated by Satellite (DORIS) instrument. DORIS has been developedby CNES, and is already operational on several satellites. It willmeasure Envisat’s orbit to an unprecedented accuracy, therebyserving as a major source of the improved performance that the RA-2system will be able to achieve.

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Figure 1. The Microwave Radiometer (MWR)

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Figure 3. The MWR radio-frequency (RF) front-end

(courtesy of EMS)

Figure 2. The 36.5 GHzDielectric Resonator

Oscillator (DRO)(courtesy of COMDEV)

The signals are down-converted in a mixer-amplifier subassembly, using the 23.8 and 36.5 GHz signals generated by DielectricResonator Oscillators (DROs). Both the RFfront-end and the DRO’s design andtechnology have been space-qualified forMWR. The intermediate-frequency (IF) and theanalogue boards are used to process the down-converted radiometric signals; both modules arelocated within the Central Electronics Unit(CEU). The IF module consists of an input filterto define the bandwidth, followed by an RFamplifier chain and a square-law detector. Theinput signal is 500 MHz bandwidth noise,square-modulated by the Dicke frequency(1276 Hz). The analogue switches performingthe detection can be opened by telecommandto avoid disturbances produced by theASAR/RA-2 instruments. The signal is digitisedin 64 bits every 150 msec.

The following calibration cycle is executed every38.4 s: Hot Load Calibration (two measurements),

Sky Horn Calibration (two measurements), OffsetCalibration (two measurements), Main AntennaSignal (250 measurements). Alternativecalibration periods of 76.8, 153.6 and 307.2 scan be selected by command.

The instrument is controlled by the common(MWR-DORIS) Instrument Control Unit, whichhandles the On-Board Data Handling (OBDH)interface protocol, exchanging macrocommands and telemetry data. The instrumenthas independent thermal-control elements(heaters and thermostats) to give its electroniccircuits optimum performance.

Absolute calibration has been performed at theRemote Sensing Instrumentation MeteorologicalOffice, in Farnborough (UK). The absoluteuncertainties in the brightness temperatures ofthe targets used were + 0.10/-0.00 K atminimum and +0.05/-0.09 K at maximumtemperatures (an improvement with respect towhat could be achieved for ERS-1 and 2). Adetailed model of the instrument has beendeveloped and has been validated during thecalibration campaign, at instrument radiatortemperatures of 0, 10, 20, 30 and 40°C. TheFixed- and Variable-Temperature Targets wereused in the ranges of 85 K and 85-300 K,respectively

The instrument performance figures arepresented in Table 1, which shows that theresults are better than the specifications.

The MWR instrument was the first Envisatinstrument to be delivered, back in September1997. It has since been integrated into theMWR/DORIS composite and mounted on thespacecraft. Thereafter It has performedsuccessfully in all of the satellite tests that havebeen conducted.

The ground segmentThe received MWR data are packaged intoLevel-0 products and ingested (together withthe RA-2 Level-0 and DORIS products) into theRA-2/MWR processor. No separate higher levelMWR products are to be generated. The MWRLevel-1b (brightness temperature) and theLevel-2 information (wet tropospheric pathdelay) is embedded as a Measurement DataSet (MDS) within the RA-2/MWR Level-1b andLevel-2 products (see article titled ‘The EnvisatRadar Altimeter System’ in ESA Bulletin No. 98).

The path correction due to the wettropospheric component is estimated on the basis of the two brightness-temperaturemeasurements (at 23.8 and at 36.5 GHz) fromthe MWR and from the σ0 information comingfrom the Radar Altimeter. This gives a residual

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– a meteorological package providingtemperature, pressure and humiditymeasurements, used to correct fortropospheric effects.

The beacons transmit a narrow-band ultra-stable signal plus auxiliary data: beaconidentifier, housekeeping data, meteorologicaldata, and time-tagging reference data.Presently, two master beacons located inToulouse (F) and Kourou (Fr. Guiana) areconnected to the control centre to allow datauploads to the onboard package. They are alsolinked to an atomic clock to allowsynchronisation of the DORIS system withinternational reference time.

The DORIS onboard package for Envisatincludes: a receiver performing Dopplermeasurements and receiving auxiliary datafrom the beacons, a dual-frequency omni-directional antenna, an ultra-stable oscillator,and a two-channel receiver (with DIODEnavigator capability as part of the onboardsoftware). The dual-frequency receiver allowsionospheric corrections to be made.

The DORIS control and processing centre, alsolocated in Toulouse (F), is responsible forbeacon network monitoring, onboard packagemonitoring and programming, sciencetelemetry acquisition and pre-processing,technological archiving, precise orbitdetermination, and beacon positioning. Thiscentre is included in the SSALTO (Orbitographyand Altimetry Multi-mission Centre) CNESground segment. The interfaces betweenSSALTO and the Envisat Flight OperationsSegment (FOS) and Payload Data Segment(PDS) have been defined to meet all of theEnvisat mission requirements.

inaccuracy of 1÷2 cm (comparable to thatachievable by a three-frequency radiometer).

AcknowledgementThe MWR instrument was developed under theleadership of Aleniaspazio, with equipmentprovided by Austrian Aerospace, ComDev,Contraves Italiana, EMS, Millitech and Schrack..DORISThe systemThe DORIS system (Doppler Orbitography andRadiopositioning Integrated by Satellite) wasdeveloped by CNES (Centre National d’EtudesSpatiales), IGN (Institut Géographique National)and GRGS (Groupe de Recherche en GéodésieSpatiale) to meet scientific and operational userrequirements for very precise orbit determination.Beyond its initial mission objectives, the DORISsystem can also fulfil other needs, such asprecise ground-beacon position determination(e.g. for measuring tectonic movements),provision of Earth-rotation parameters,measurement of Earth-centre position,improvement of Earth-environment models(e.g. gravity field, global ionosphere mapping),and real-time orbit determination.

The DORIS system was designed andoptimised to provide high-precision orbitdetermination and beacon positioning. It wasdeveloped within the framework of theTopex/Poseidon oceanographic altimetrymission and has been operational since 1990,when the Spot-2 satellite was launched withthe first DORIS receiver onboard.

DORIS is an up-link radio system based on theDoppler principle. It measures the relativevelocity between the orbiting satellite and adense, permanent network of orbit-determination beacons. The core of the systemis the beacon network distributed homo-geneously over the Earth. The dual-frequencysignals at 400 MHz and 2 GHz emitted by thebeacons are used by the receivers onboard the various satellites to perform Dopplermeasurements. The DORIS permanentnetwork includes 54 beacons (Fig. 4) hosted byinstitutes from more than 30 countries. Morethan 20 beacons are co-located with otherprecise-positioning systems to allow cross-calibration.

Each site is equipped with a beacon packagethat includes:– a dual-frequency 400 MHz and 2 GHz

transmitter (including an ultra-stableoscillator)

– an omni-directional bi-dual-frequencyantenna, with a battery pack to provideautonomy of supply

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Table 1. MWR performance summary

Performance Requirement Achievement

Radiometric Sensitivity < 0.6 K 0.4 K

Radiometric Stability < 0.6 K 0.4 K

Radiometric Accuracy < 3 K at Ta = 300 K 1 K at Ta = 300 K

(after calibration) < 3 K at Ta = 85 ÷ 330 K

Dynamic Range 3 K to 300 K 3 K to 330 K

Non-Linearity < 0.5 K 0.35 K

Centre Frequency < 0.75 MHz / °C < 0.2 MHz / °C

Stability

Antenna Radiation > 93 % 97 %

Efficiency worst case

Antenna Main Beam > 89 % 94 %

Efficiency worst case

Antenna 3 dB Beamwidth < 1.7 ° 1.5°

Instrument Mass < 30 kg 24 kg

Operational Power < 50 W 18 W

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Figure 4. The DORIS groundnetwork

Instrument performanceFor Envisat, the accuracy of the real-time orbitprovided by the DORIS/DIODE onboardsoftware has been specified as 1 m (threeaxes). The performance of the DIODE softwarealready flying on Spot-4 and the improvementsthat have been tested on the ground indicatethat this level of accuracy should be achievedwithout any difficulty. Indeed, the performanceof the onboard DIODE real-time navigator hasalready been estimated at 40 cm from varioussimulations of the radial component of Envisat’sorbit. The 30 cm-level is expected to bereached using an upgraded version of thesoftware (see below).

The accuracy for the radial component of theoffline precise orbit has been specified as 10 cm,with the even more challenging figure of 3 cmoften quoted as a goal. Experience gained withthe Topex/Poseidon and Spot satellites andappropriate simulations indicate that the 10 cmspecification can be achieved without majordifficulty, whereas the 3 cm goal is a challengethat the Envisat Precise Orbit Determinationteam will actively pursue.

The DORIS/DIODE onboard capabilityThe version of DIODE that will fly on Envisat isimproved with respect to the Spot-4 version inthat it takes into account: the Earth’s gravityfield up to 40°x40°, the Sun’s and the Moon’sattractions with a simplified ephemeris model,the solar radiation pressure using a simplemodel of the spacecraft, and empirical andadjustable once per revolution accelerations toabsorb residual errors.

In addition, several new functions have beendesigned and already extensively tested on theground:

– Self-initialisation: Without any orbitalinformation, the DORIS receiver can performmeasurements by simply scanning aroundan average frequency. DIODE will be able toestimate the spacecraft’s position withoutneeding initial conditions sent from theground (‘lost in space’ scenario). The non-linear behaviour of the equations of motion issolved by using two separate filters, whichprocess the measurements from fourpassages. The two filters are based on twodifferent (one crude and one more accurate)models. The resulting orbit (generally with anaccuracy of a few metres) is then provided tothe standard filter for the final convergence.

– Self-programming: Normally, DIODE uses itsestimation of the orbit to inform the DORISreceiver about the next visible station and itsDoppler shift every 10 s. The accuracy issuch that these predictions can be used bythe receiver itself to self-programme the nextstation to be received. A selection algorithmis added for the cases in which severalbeacons may be visible simultaneously.

Also, a time-determination function now existsfor all versions of DIODE that is accurate towithin a few microseconds and can thereforebe used on the ground and/or by thespacecraft’s payloads and central flightsoftware. The Envisat ground segment willtherefore use DIODE outputs for accurate real-time product generation.

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ENVISAT Elevation: 12 deg, Altitude: 800 km

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Figure 5. The El Niño/La Niña 1997-2000 eventsas seen by altimetry

the observation. Focusing on ocean dynamics,it is clear that the corresponding signal has awide spectrum in both space and time:

– Mesoscale eddy features, with a typicalamplitude of the order of 5 to 20 cm, aspatial scale of the order of 100 to 300 km,and an associated temporal scale from a fewdays up to months or years.

– Seasonal signals of the order of 10 to 15 cm,varying mainly on a hemisphere basis.

– Inter-annual signals such as the El Niñophenomenon, with a typical amplitude of 20 cm and time scales ranging from severalweeks to months (Fig. 5).

– Very long time scale variations in mean sea-level, with magnitudes of some 1.5 mm/yr.

Satellite orbit error has been the bane ofoceanographers, who analyse altimetry dataquantitatively. To overcome this difficulty,altimeter users have pinned their hopes on veryefficient error-reduction methods, particularlyfor ocean mesocale recovery. For long-wavelength ocean signals, however, even themost sophisticated orbit error-reductionmethods are not satisfactory, and will neverreplace very precise orbit measurements, inwhich DORIS can play a major role.

Climate-change studiesWithin the climate-change research framework,

The ground networkThe next generation of DORIS beacons (thirdgeneration) will have the ability to transmit theirsignals on slightly shifted frequencies withrespect to the nominal system frequencies.This will avoid the risk of ‘Doppler collisions’when the DORIS system is used from high-altitude orbit, and will allow more DORISbeacons to be used in a given region.

Another major feature of these third-generationbeacons is that they broadcast the currentdate (year/month/day/hour/minute/ seconds) inTime Atomic International (TAI) format. It allowsthe in-flight DORIS instruments to perform theirinitialisation process – from equipment turn-onto satellite position, velocity and time estimation– fully autonomously, without any groundcommanding or uploading.

Beacon data transmission (synchronisationword, auxiliary data, uploading in case ofmaster beacons) is performed according to a 10 s sequencing. This sequencing issynchronised with respect to TAI to within ± 1 sto guarantee correct reception of these beacondata by the in-flight instruments.

DORIS’s impact on Envisat mission objectives Precise orbit determinationWhen designing an observing system, one hasfirst to identify the signals within the scope of

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today’s rate of global sea-level change is acrucial measurement, for which altimetry hasbeen widely used. From their analysis ofcollections of tide-gauge measurements,several researchers are already quoting figuresof about 1.5 mm/yr. In the framework ofgeodesy/altimetry, it is important to focus onhow such tide-gauge and space-altimetry datacan complement each other to arrive at areliable estimation of global sea-level change.DORIS will contribute significantly in thiscontext, providing a reliable terrestrial referenceframe over time.

One crucial advantage of altimetry from spaceis that observations are performed on a globalscale in a centre-of-mass fixed reference frame.The positions of the stations tracking thesatellite define the orbit reference frame, andconsequently the ability to precisely determinetheir locations within a co-ordinate systemwhose origin is located at the Earth’s centre ofmass is of considerable importance. It is widelyagreed that the international network of satellitelaser ranging systems is an importantcontributor to the reference-frame definition.The permanent orbitography network of DORISbeacons is the other major contributor. Indeed,since Topex/Poseidon’s launch, knowledge ofthe co-ordinates of the ground beacons hasgreatly improved, allowing the DORIS systemto be included in the IERS reference-framecomputations.

Because a primary goal of altimetry is tocontribute to a continuous ocean observingsystem on a long-term basis, it becomesextremely important to manage the evolution ofthe terrestrial reference frame. Use of the 2ndgeneration of DORIS instruments onboardEnvisat and Jason will allow the DORIS stationmotion analysis to be pursued with even betteraccuracy, since the instrument noise of theDORIS receiver will go down to the order of 0.1 mm/s, compared with 0.3 mm/s forTopex/Poseidon-like receivers.

Another subject of careful study has to be thestability of the reference frame in which sea-levels are computed. It is known that geo-centre variations are affected by the nature ofthe reference system adopted, and in particularits origin. Sea-surface heights are related to theEarth’s centre of mass, since the satellite orbitis defined in an inertial reference frame with thatat its centre. In practice, tracking data involvedin the orbit computation are collected bystations that are distributed over the Earth’ssurface, which contributes to the Earth-fixedreference frame definition, the so-called ITRF(International Terrestrial Reference Frame).Hence motions of the ITRF centre relative to the

centre of mass should be taken into account inresolving the global mean sea-level equations.

Sea-level monitoringObservation of the ocean is now thought of byoceanographers in terms of a global and an‘integrated’ system. Indeed, there is nowgeneral acceptance that space and in-situtechniques are complementary in terms of thecharacteristics of their sampling, precision andaccuracy, and that they must be exploitedjointly to provide the optimum observingsystem. The same concept is valid for the sea-level-change problem, and GPS and DORISgeodetic techniques have been used forseveral years together with altimetry and tide-gauges data to estimate the rate of sea-levelchange. Continuous enhancement of theDORIS ground network by increasing the co-location of DORIS beacons with tide gauges isvery attractive. Upgraded versions of theDORIS system, for instance with the multiplechannel capability, now offer the possibility todesign an efficient integrated tide gauge + GPS+ DORIS + altimetry + laser sea-level monitoringsystem.

ConclusionFrom its ‘probationary’ status on board Spot-2,the DORIS system has evolved to become amajor contributor to oceanographic andgeodetic science and applications. On Envisat,DORIS will contribute significantly to thefulfilment of the altimetry-mission objectives, aswell as more generally supporting theinstrument payload data processing, in near-real-time and off-line, with orbital information.

An International DORIS Service (IDS) iscurrently being created, to support the use ofthe DORIS system and products, to definestandards, to promote research anddevelopment activities to improve systemperformance, operationality and applications,as well as to interact with the user community.DORIS data and expertise from the Envisatmission will be of great value for the new IDS.

AcknowledgementWe are very grateful to the DORIS programmeand system teams, and particularly theDORIS/Envisat project team, for their help andtheir contributions to this article. r

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