ALTIKA : A MICRO-SATELLITE Ka-BAND ALTIMETRY MISSION

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nominal tracking conditions (about 3 km alongtrack).These values will be associated with small footprintsfor the radiometer as well since altimeter andradiometer will share the same antenna. In additionto the altitude reduction, which is the maincontributor to the radiometerfootprint reduction, it may also be noticed that theabsence of an additional 19-GHz channel to theradiometer is also an advantage since this lowfrequency would give the largest footprint.POSEI--DON2(Ku-band)POSEI--DON2 (Cband)AltiKa (800km)AltiKa (500km)Leadingedge(2-mSWH)128-gateecho(2-mSWH)Footprintdiameter at3-dBattenuation5.3 19.1 19.55.3 19.1 49.54.1 12.7 7.23.4 10.3 4.6Table : Footprint diameter for different criteria(values given for a 75-cm antenna for AltiKa)This is why it is expected that AltiKa will give usefuldata as close as 5 km from a majority of coastalareas. Associated with a shorter data cycle (probably500 ms instead of 1 s), it will probably represent amajor improvement in mesoscale and coastalapplications of radar altimetry.3.3. Minimizing penetration effects overcontinental iceAn empirical analysis of the temporal variability ofthe satellite radar altimetric observationdemonstrates that the Ku-band radar penetrationabove the dry snowpack of Antarctica is between 5m in the interior to 14 m at a lower altitude, beforedecreasing due to wetness near the coast. Theabsorption coefficient of dry snow being 0.05 m -1 , itinduces a scattering coefficient comprised between0.05 and 0.16 m -1 , depending on ice the grain sizewhich varies from 0.2 to 3 mm.In Ka-band, Mie scattering can be assumed, then thescattering coefficient is inversely proportional to theradar wavelength at a power 4. From Ku to Kabands, the scattering coefficient will increase by afactor 55: the volume scattering will then be clearlydominant over the surface scattering. Inversely, theradar wave extinction will also increase to valuescomprised between 2.75 and 8.75 m -1 , leading to apenetration depth over snow surface between 0.3 and0.1 m. The altimetric observation and heightrestitution will thus correspond to a thin subsurfacelayer. The accuracy will then be considerablyimproved.Moreover, ice grain size, that is one of the pertinentclimatological snow parameters, could be directlyderived from Ka-band scattering coefficient, whichis not possible from present day Ku measurementsthat also highly depend on surface roughness.However, the major benefit will lie in the reductionof the induced long term bias due to the temporalchange in the snowpack surface. Indeed, a strongtemporal variability of the altimetric observationsover Antarctica ice sheet has been recently exhibitedwith the help of the ERS-1 three day orbit mission.These temporal variations are linked tometeorological events and consequently play a roleon a large band of the temporal spectrum, from fewhours to few decades. They are due to changes insurface roughness induced by change in wind, whichmodifies the echo surface part and then, both thewaveform shape and the elevation recovery. Evenwith a dedicated correction, the residual correctionyields to a raw noise estimate of 10 cm.3.4. Propagation losses3.4.1. Attenuation effectsIt is known that propagation of EHF waves throughthe troposphere may suffer from severe attenuationdue to the interaction of the electromagnetic wavewith the atmospheric contents. As far as the Ka-bandaltimeter is concerned, current requirement is thatthe instrument provides nominal performance 90%of time, with an objective as high as 95%. Anextensive study has been made to analyze thecontributions from :(i) gazes : attenuation is calculated for oxygen andwater vapour. Even if the allocated frequency rangeis nearly ideally chosen with respect to the waterpropagation spectral window, strong attenuation mayresult from moderate to high humidity contents.Values may vary from 0.4 to 2.1 dB when humidityvaries from 7.5 to 50 g/m3.(ii) clouds : 7 types of clouds are taken into accountfor the propagation models. Attenuation is estimatedfrom their liquid vapour contents and depths. Asexpected, cumulus and cumulonimbus are generally
the main contributor (0.2 to 0.7 dB) to the total loss,and sometimes stratocumulus (0.05 to 0.2 dB).(iii) rain : attenuation by rain, and possibly snow, iscomputed from rain rate and rainfall thickness.Other possible contributors (ice, fog) have beenshown to have no significant influence (order of 0.1dB for the fog).As far as rain is concerned, maximum precipitationrates are known as less than 1.5 mm/hr everywhere(95% through time) or even 1.0 mm/hr (90% throughtime) over ocean and polar areas, except in a regionoff the Philippines islands for which previous figureshave to be doubled. Following accepted statisticaldistributions, one can associate some classicallayering distributions with the previous rates. Then,total attenuation due to rain may be computed.Results show that attenuation is lower than 1.8 dB(95%) or even 1.2 dB (90%). Given these figures,we can conclude that :- Case 1 : no rain or very low rain rates, then anominal functioning of AltiKa is ensured,- Case 2 : rain rates greater than 1.5 mm/h, thenaltimeter measurements will be degraded oreven not acquired for the largest rain rates,- Case3 : moderate rain rates less than 1.5 mm/h,then the echoes will have to be corrected using aspecific algorithm modeling the perturbation ofthe echoes by rain cells.To refine the previous statements, 7 years of TMRdata from TOPEX/POSEIDON have been used tocheck some seasonal effects and to analyze the effectof local time which may be an important parameterto select in the case of a sun-synchronous orbit forAltiKa. Translation of TMR measurements into rainattenuation has been performed using dedicatedalgorithms and results have been mapped versuslocation, local time, month in the year, and on amulti-year time scale. On the one hand, the studyhas confirmed that Ka-band data availability will begreater than 90% over the world ocean and evengreater than 95% for some areas such as the NorthAtlantic. On the other hand, it appeared that localtimes from 6 am to 12 am, and from 6 pm to 12 pmare the worst cases for rain, which may provide a« light » constraint for the optimization of the linkbudget of the instrument and select the mostadequate local time for a sun-synchronous orbit.3.4.2. Impact of rain cells on waveformanalysisFollowing considerations are derived fromTournadre (1999).Approximations made in previous studies to estimatethe effect of rain cells on Ku-band altimeterwaveforms are no longer valid ; a second newmethod has thus been developed based on the trueintegration of rain attenuation.It may be shown that the difference between the twomethods is significant even for low rain rates (fewtenths of mm/hr). The method can be used tocompute the altimeter waveform for any rain field. Itshows that a precise retrieval of altimetricparameters (significant wave height, wind speed andsea surface topography) will require to take intoaccount corrections for rain cell perturbations evenfor low rain rates.The sensitivity of Ka band measurements to rain mayalso lead to a way of estimating very light rainfallover the oceans, for which we dramatically lack ofinformation and which could lead to a greatimprovement of our knowledge of the oceanic rainclimatology.3.5. Anticipated radiometer performancesThe following figures present the major elements ofthe radiometer performance budget :-resolution : about 0.3 K for the 23.8 GHzfrequency, and 0.4 K for the 37 GHz frequency-sensitivity : better than 1K-accuracy : better than 3K (worst case estimate).4. Conclusion : Status of the Ka-bandaltimetry studiesThe Ka-band altimetry concept that is proposedexhibits enhanced performances in terms of verticalresolution, time decorrelation of echoes, spaceresolution, range noise. In addition, an adaptedtracker algorithm has already been designed toperform near-continuous altimetric tracking aboveall surfaces, which is especially important whenapproaching or leaving coasts. Also, it appears that,even if Ka-band measurements are more sensitive torain than Ku-band measurements, rain does notprevent from acquiring a fairly high percentage ofmeasurements (objective is up to 95%) except for
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ALTIKA : A MICRO-SATELLITE Ka-BAND ALTIMETRY MISSION

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