ALTIKA : A MICRO-SATELLITE Ka-BAND ALTIMETRY MISSION

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2.2. Summary of some characterisitics of the KabandaltimeterFrom the above requirements, we derived altimeterinstrument specifications that are compared with thePOSEIDON-2 specifications in the table below :AltiKa POSEIDON2Altitude 500 to 800 1340 kmkmInclination ~98° 66°Lifetime 2 years 5 years(objective :3 years)Frequency 35.75 GHz 13.65 GHzBandwidth 480 MHz 320 MHzPRF 4000 Hz ~2000 HzSignal to noise > 10dB > 10dBratioSpectral analysis 128 gates 128 gatesIntegration 25 to 50 ms 50 msdurationMass
2.4. An integrated Altimeter + RadiometerinstrumentOne of the initial requirements to design the AltiKapayload was that it should be possible to embark iton a microsatellite and that it could also be providedas a whole to become a passenger on an apportunityplatform.This has lead us to define an integrated instrumentthat allows for interface optimization and reductionof the number of units (boxes) . Indeed, theintegrated instrument is composed of :• One microwave unit that gathers all microwavefunctions of the altimeter and the radiometer,including the calibration functions of theradiometer and the sources of the antenna.• One processing unit that gathers all functionsdedicated to the altimeter and radiometerprocessing, as well as a global management unit.3. Performances and impact on somescience objectives3.1. Minimizing the range noise of the altimetermeasurementThe Ka-band (35 GHz) is more interesting than theKu-band for the altimeter since it improves the linkbudget and allows larger bandwidth (up to 500MHz) and pulse repetition frequency (4 kHz).The selected 480 MHz bandwidth provides a 0.3 mvertical resolution.Due to the smaller antenna beamwidth, the Brownecho has a sharper shape in Ka-band than that whatis obtained with conventional altimeters in Ku/Cband(e.g. POSEIDON 2).The proposed architecture for the Ka-band altimeteris based on the classical deramp technique for pulsecompression and it takes benefits of Frenchexperiences from the realisations of POSEIDON 1 &2. A wide band chirp (~480 MHz) of about 107 µsduration is emitted. The digital processing includesthe real time FFT, echo integration and radiometerprocessing (power averaging). The main limitationof Ka-band is the non-operationality for rain ratehigher than several mm/h (5 to 10% of time forAltiKa depending on the geographic area).Range noise comparison with POSEIDON 2altimeter shows that AltiKa will provide excellentperformances (see Figure 1 where the Ku/C and Kucurves correspond to the dual and Ku-band-onlyfrequency estimates of POSEIDON 2 range noise).Figure 1: Comparison of AltiKa (at 800 km) andPOSEIDON 2 (at 1340 km) performanceSince no ionospheric correction is required forAltiKa (DORIS might indeed provide a correction inextreme, rare ionospheric conditions), the expectedimprovement is close to a factor of 3 (for example,1-s range noise at 2-m SWH would be less than 0.8cm for AltiKa vs 2.2 cm for POSEIDON 2).3.2. Space resolution improvement wrt classicalKu-band altimetryThis very good range performance will be associatedwith reduced footprints and better coastalperformance as well.Performance for coastal applications depends :(i) on waveform / footprint relationship(ii) on antenna diagram / footprint relationship(to obtain attenuation of land contributionto echo(iii) on tracking performance for complex(coastal or in-land) echoes.AltiKa characteristics have been optimized for these3 issues :- AltiKa is close to a beam limited altimeter :there is no ‘plateau’ in the echo, since it stronglyattenuates shortly after leading edge due to thesmall antenna aperture. This will greatly reducethe pollution of ‘land gates’ into ‘ocean gates’.- Tracking loops were designed at the same timeas SIRAL/CRYOSAT altimeter for whichcontinuous tracking in ocean/land/ice conditionsis required. Thus, AltiKa is supposed to providedata in most coastal zones and in-land waterareas. Moreover, in case of loss of track,acquisition/locking phase is required to beshorter than 500 ms, allowing a quick return to
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ALTIKA : A MICRO-SATELLITE Ka-BAND ALTIMETRY MISSION

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