ALTIKA : A MICRO-SATELLITE Ka-BAND ALTIMETRY MISSION

More documents

Recommendations

Info

Abstract.Ka-band altimetry has been proposed by Verron etal. (2001) to complement the altimetry referencemissions of the Jason class. The goal of this paper isto provide information on the so-called “AltiKa”proposal in terms of science requirements,responsive technical specifications, and a prioriperformances. Besides the fact that the feasibility ofembarking an AltiKa payload on a microsatellitehas already been assessed, there is also theinformation that a small launcher may have thecapability of launching several microsatellitessimultaneously (up to three) in a number ofconfigurations that may be adapted to the spacetime requirements of high resolution altimetry.IntroductionThe development of altimetry has been steady andthe benefits outstanding. There has been a quasicontinuousseries of missions, starting withGEOSAT (1985), and then ERS-1/2 (1991 and1995), and TOPEX-POSEIDON (1992). These willcontinue from Year 2000 with Jason-1 andENVISAT, that are respectively scheduled forlaunch in 2001.More generally, the heritage of past and currentoceanographic space missions has paved the way forthe incremental implementation of the spacecomponent of the Global Ocean Observing System.Because they have already demonstrated their “preoperational”maturity, in terms of end-to-endperformances, value to users and affordability,altimeter and scatterometer missions are the firstimmediate candidates for operational transition.Short-term decisions are therefore required to securecontinuity and enhancement of the current altimeterand scatterometer systems in the GODAE (GlobalOcean Data Assimilation Experiment) timeframe andbeyond.In the short term, the future looks secure with a twosatellitealtimeter system being maintained for thenext five years with Jason-1 and ENVISAT. It isforeseen that the combined use of their data withmeasurements from independent gravity missionssuch as CHAMP (2000), GRACE (2001) andpossibly GOCE (2004) will be another breakthrough.It should enable absolute estimates of ocean currents,to be obtained by subtracting an independentestimate of the geoid from individual altimeterheight measurements.A first firm decision is however needed in the veryshort term to secure continuity through Jason-2measurements in the GODAE window. Furtherdecisions will be needed to maintain a multi-satellitesystem providing the required accuracy, coverageand sampling beyond the GODAE window, after theend of life of ENVISAT. Such a system shouldprovide an affordable service equivalent to oneJason-class mission and one or two medium accuracymissions of the ENVISAT class. Several optionsneed to be evaluated, taking into account results of R& D and demonstration missions aimed at validatingnew candidate observing techniques.The goal of this paper is then to provide the readerwith the status of Ka-band altimetry, considering theembarkment of the system on a microsatelliteplatform.1. The AltiKa proposal : about scienceobjectivesThe science objectives of the AltiKa mission aredescribed in Verron et al. (2001) and in Remy et al.(1999) in very details.Hereafter, we only recall those of the objectives thatmost impact the design of the altimeter instrumentand the payload.1.1 Ocean Mesoscale variabilityBased on the need to resolve the mesoscalevariations and to study the effect of thesefluctuations on the energetics of the meancirculation, the spatial and temporal resolutionrequired is set by the need to directly observe themesoscale variability to a degree that allows, inconjunction with assimilative numerical models, aproper description of the oceanic eddy field and theinteractions of this field with the mean field.In 1999, G. Mitchum et al. summarized some of therequirements for a well adapted sea-surface heightsystem: The section below partly derives fromMitchum et al.’ statements concerning spacealtimetry. The constraint concerning the spatial andtemporal resolution of a sea-surface height systemmeans that we must be able to resolve signals thatoccur at spatial scales on the order of several oceanicRossby radii, and on time scales of days to weeks,which cannot presently be done with a singlealtimeter. The important question is how manyaltimeters must be available at any given time, andwhat orbits should these satellites occupy? Jacobs etal. (1999) have estimated that three altimeters areneeded to directly observe the mesoscale variationswithout additional statistical or dynamical modelinput. If assimilative models are used, however, twosatellites may be adequate. In a different set ofsimulations, Le Traon and Dibarboure (1999)
conclude that most of the improvement inreproducing the mesoscale variability is obtained ingoing from a single altimeter to two flyingsimultaneously. Adding further altimeters improvesthe results, but to a lesser degree.It is clear that at least two altimeters are required,and that additional altimeter would still improve thescience return, but the optimal number is still understudy.Other ocean topics may also add spatial andtemporal requirements that will impact the number ofaltimeters needed to get high space-time resolution.1.2 Contribution in coastal altimetry andcontinental watersThe initial single satellite AltiKa proposal did nothave the goal to answer the space-time requirementscorresponding to a fully adapted sampling of thecoastal dynamical features. We’ll come to thepossibility of flying a multisatellite AltiKaconstellation in the conclusion.However, coastal altimetry has been a driver toderive new instrumental spcecifications to answernew performance requirements both in terms ofnoise and along-track space resolution.At the same time, we also considered how a newaltimetry system may answer requirements dealingwith the observation of continental waters (lakes,reservoirs, rivers) such as the requirements listed inthe HYDRASAT proposal. Once again, besides thetime sampling issue, we looked at the orbit patternthat may be useful to have in complement of Jason tosample the major continental water areas; we alsotried to focus on space resolution requirementsavoiding to design an instrument that would bespecific to the altimetry of lakes and rivers keepingin mind that the main driver is ocean altimetry.Some main technical features of the Ka-bandaltimeter that is part of the AltiKa payload aredirectly derived from the coastal and continentalwater requirements : they are described in moredetails in sections 2 and 3.1.3. Ice sheet ice monitoringAs the AltiKa payload has been foreseen to alsocontinue the ERS/ENVISAT series of altimetermeasurement of ice sheets, we also considered somebasic features of ice altimetry that may impact thetechnical specifications of AltiKa.The monitoring of the ice-sheet height overAntarctica and Greenland is of primary importancefor climate studies. However, one of the majorlimitations of the ice sheet mass balance study, ispresently the poor knowledge of the radar wavepenetration within the snowpack, in Ku-band for theERS1/2 altimeters. The induced volume echo is ofthe same magnitude as the surface echo, yielding to acritical situation. The induced effect is twofold: itdirectly acts on the surface elevation accuracy and,due to the temporal change in the snowcharacteristics from daily to yearly scales, itproduces a long term analysis error that can hide thelong term trend.Then, the objective was to try specifying thealtimeter so that penetration effects would beminimized with respect to Ku-band altimetry.2. Ka-band payload characteristics2.1. General requirements and proposed payloadThe above objectives were translated into some mainfeatures of the Ka-band payload :• get an altimeter instrument whose range noiseperformance may be so that the recovery of theocean short wavelength features is improved,• get an altimeter instrument with an improvedspace resolution along-track and a betterperformance when approaching or leavingcoastal boundaries,• get an altimeter instrument that will minimizethe penetration effects over media such ascontinental ice,• embark an orbitography system that will ensurea high level of accuracy in terms of orbitographyand that will ease the connection of historicalaltimetry series within a common well surveyedgeodetic reference frame,• embark a microwave radiometer that will helpcorrecting altimeter measurements for wettroposphere effects.To answer the previous requirements, it is proposedto compose an AltiKa payload with :• A single frequency Ka-band (35 GHz) altimeterinstrument (see Phalippou et al. (2000) fordetails)• A two-frequency radiometer,• A DORIS receiver,• A passive laser retroreflector array.
Page 1: IAF - 01 - B.2.10ALTIKA : A MICRO-S
Page 5 and 6: 2.4. An integrated Altimeter + Radi
Page 7 and 8: the main contributor (0.2 to 0.7 dB
Page 9: we would then get multi-frequency m

ALTIKA : A MICRO-SATELLITE Ka-BAND ALTIMETRY MISSION

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?