French space programmes related to global change

Adv. Space Res. Vol. 11, No.3, pp. (3)213—(3)226, 1991

02734177/91 $0.00 + .50

Copyright © 1991 COSPAR

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FRENCH SPACE PROGRAMMES RELATED TO GLOBAL CHANGE J. L. Fellous and A. Ratier Centre National d’Etudes Spatiales, 2, Place Maurjce-Quentin, 75001 Paris, France

ABSTRACT The guidelines of the national and cooperative environmental programmes conducted by CNES are complementarity with third agencies’programmes, synergism between scientific and application projects, and promotion of innovative concepts likely to meet the requirements of the World Climate Research and International Geosphere-Biosphere Programmes. While the on-going SPOT series is to provide imageiy ofland surfaces until 2000, the TOPEX/POSEIDON altimetric mission is being developed by NASA and CNES for launch in mid-1992 in phase with the WOCE intensive field experiments. The design study of the AVISO ocean data system, and the development on behalf of ESA ofthe ERS-1-dedicated CERSAT facility are consolidating the French effort in space oceanography. Two other research space missions are studied by CNES and Frenchiaboratories. BEST, whose phase A study is nearing completion, is a low-altitude, .low-inclination, GEWEXdedicated mission for the investigation of the water and energy cycle in the tropics, with a target launch date in the late 1990’s. The phase A study of GLOBSAT, a more IGBP-oriented mission concept, has just been initiated. The first objective of this mission in polar orbit considered for launch around 1997, is to collect comprehensive data sets needed to document key processes related to cloud/radiation interaction, stratospheric/tropospheric chemistry and dynamics of continental and marine ecosystems. The second objective is to start monitoring long-term trends of parameters required to close global budgets of carbon and ozone. The analysis of this space mission concept is conducted in parallel with the development of instruments ofopportunity to be flown onboard foreign satellites, and of airborne sensors, either precursor to space instruments or designed for process studies and validation of space data. INTRODUCTION Satellite systems are an essential component of any strategy to monitor and document global change, because of their unique performances in terms of global coverage, field ofview, sampling, continuity, availability and their measurement capabilities. They harmoniously supplement In situ or ground-based systems, which have complementary measuring and sampling capabilities, but also numerical models designed to integrate all available observations. However the complexity of non-linear processes calls for the measurement of many variables, leading to conflicting sampling strategies. The bulk of requirements, when confronted with the limited possibilities of a single satellite on a given orbit and with the cost of space systems, imposes an internationally coordinated effort. The space agencies have therefore to harmonize their plans, to merge their efforts into cooperative initiatives and avoid duplication for the benefit ofthe scientific community. CNES is developing a number of space projects in line with this approach. TOPEX/POSEIDON is a space oceanography project jointly conducted with NASA and quite complementary to the ESA’s ERS-l and -2. The combination and overlap of TOPEX/POSEIDON and the ERS should indeed produce measurements of surface wind and ocean topography at different scales in the 1991-1997 timeframe, thereby ensuring the feasibility of the World Ocean Circulation Experiment. Similarly, CNES proposes to international cooperation the BEST project, a space mission in low-inclination (3)2 l3

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orbit dedicated to the investigation of the tropical energy budget. More recently CNES has initiated the study of a new mission concept called GLOBSAT aimed at providing observations of key parameters, and at understanding key processes involved in the expected global change of our environment, as well as initiating global scale measurements in the perspective of a continuous monitoring of.the Earth from space. GLOBSAT is seen as a precursor to the so-called international polarorbiting platforms planned by ESA, NASA and NASDA for flight in the late 1990’s. Fmally a number of space and airborne instruments of opportunity are also developed by CNES either in internationalcooperation or for flight on-board third satellites. In order to maximize scientific return of space systems, scientific projects are defined in close interactionwith the user community, who has to be involved in the mission specifications in order to best prepare for the interpretation and use of the data. CNES makes every effort to maintain this interaction as tight as possible through various processes. First, space science prospective seminars are organised every four years, which gather the French scientific community at large. This gives a unique opportunity to update priorities in CNES scientific programmes in the light of the evolution ofthird programmes and new scientific issues and to define sharply focussed projects accordingly. TOPEX-POSEIDON, BEST and GLOBSAT were defined through this process and their relevance to the WOCE and GEWEX experiments of the WCRP and to the IGBP was confirmed later on by the international community. Second, the investigators are closely associated to the definition of major missions. M an example in the case of TOPEX/POSEIDON, the Science Definition Team jointly established by NASA and CNES made recommendations as to the need for a single orbit cycle throughout the mission, characteristics of which were adopted in late 1989 by the same team. Synergism between application and scientific programmes is also necessary, when the monitoring of the Earth environment is at stake. Operational sateffite programmes have the advantage of continuity which becomes crucial as far as long term trends are concerned. The international atmospheric research community takes great advantage of the current meteorological satellites and has fruitful interactions with the operational community. Given the growing importance of the continental biosphere the mutual interest of the land research and application communities to cooperate in the long run is recognised. This prospect prompted CNES to discuss with the scientific community the need and specifications of awide swath, medium resolution, IGBP-relevant vegetation monitoring sensor, additional to the high resolution imager, considered for ifight on SPOT-4. Conversely the early ifight of innovative lidars on-board a research mission like BEST should help the meteorological community assign priorities to the various atmospheric sounding instruments underconsideration for future operational systems. The promotion of innovative concepts and technology is also a major objective of any long term strategy on space programmes, because the availability of technology ensures flexibility to face new requirements. Research and technology activity, as well as proof-of-concept experiments often based on preliminary airborne systems, are contributing to this approach. All those aspects are emphasized throughout the following sections, which briefly outline the main CNES programmes in the field of environment and climate research.

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SPACE MISSIONS SPOT, TOPEX/POSEIDON, BEST and GLOBSAT are the flight projects managed by CNES on a national or cooperative basis. The relevance of these projects to WOCE, GEWEX and IGBP, illustrated in Figure 1, is discussed in more detail hereafter. SPOT and its Relevance to IGBP The biosphere, the IGBP and space observations. The ambitious approach of the Earth as a system proposed by the International Geosphere-Biosphere Programme (IGBP) highlights the need to concentrate efforts on the investigation of the role of the marine and continental biosphere. The un. derstanding and simulation of the interactions ofthe continental biosphere with the atmosphere is a first important objective, which calls for the development ofquantitative models of the behaviour of the various ecosystems, applicable on the global scale and suitable for integration into more comprehensive numerical climate models. Such parameterizations of bioprocesses need to be constrained or calibrated by space observations, as the unique source of global data. They constitute a basic ingredient ofany attempt to assess the contribution of the biosphere to the carbon dioxide budget, which remains a key uncertainty in the greenhouse effect issue. Estimates of evapotranspiration, biomass production and natural emissions of source gases like methane and nitrogen peroxide into the troposphere would be other potential key outcomes of such models, coupling the biosphere to the atmospheric dynamics and chemistry. These models would conversely document the response of various ecosystems to climate and anthropogeneous forcing, another major field ofinterest for the IGBP. Observations from space are expected to characterize the nature of the various ecosystems by their spectral signatures in the visible, infrared and microwave bands, their hydric, thermal (and to some extent, their phenological) state and some elements of the atmospheric forcing. They will thereby provide some of the parameters convoluted in the models but also state parameters for their validation. Global continuous observations from space are also an unvaluable tool for monitoring local or global changes in ecosystems due to climate forcing, agriculture, deforestation or other man-made disturbances. The SPOT system. SPOT (Système Pour l’Observation de Ia Terre) is a national application programme (with limited participation by Belgium and Sweden) based on an integrated system including the SPOT satellite and ground segment and SPOT-IMAGE, a commercial entity for data distribution and user interface. SPOT-i, launched in 1986, will stop operating by end 1990, after SPOT-2 was successfully launched early in the year. SPOT-3, and more recently SPOT-4, have been approved, which should ensure the continuity of homogeneous high resolution (10 m) reflectance imagery until the end of the century. The upgraded HRVIR (High Resolution Visible and InfraRed) instrument to be flown on-board SPOT-4will be compatible with the HRV instruments flown on the previous SPOT-I ,-2 and -3 spacecrafts. The association of the SPOT orbit cycle, the twin instruments on each spacecraft and their independant tilt capability allow cross-track stereo viewing, and global observation capability. HRV channels (0.50-0.59, 0.61-0.68, 0.79-0.89 micron in multispectral mode, 0.51-0.73 micron in panchromatic mode) allow vegetation indexing and other characterization of cover or soils. Many other environmental applications are possible, including the detailed analysis of the extension ofglaciers, a local climate index. SPOT-4 improvements and their relevance to IGBP. SPOT-4 will embark HRVIR instruments instead of HRV. HRVIR is an upgraded version of HRV which has the same resolution but incorporates an additional middle infrared (1.5-1.7 micron) channel expected to be more sensitive to the vegetation water content. The on-board storage capacity and the lifetime of the mission are respectively extended to 40 minutes and 5 years and panchromatic and spectral bands will be coregistered. Besides HRVIR, a wide field of view vegetation monitoring instrument is being considered and discussed with the application and scientific communities. In its current state of definition, it would have the same spectral channels as HRVIR but a resolution of about 1.2 km, kept constant throughout the 2200 km-wide swath by using telecentric lenses. An additional blue channel appli-

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cable to ocean colour imagery is also envisaged. The combination of HRVIR and this instrument on-board the same spacecraft would be a unique tool to investigate the Issue of heterogeneous vegetation cover and scale integration, central to any attempt to parameterize the behaviour of vegetation at the scale ofthe grid ofclimate models. TOPEX/POSEIDON and WOCE The World Ocean Circulation Exoeriment. The general objectives of the World Climate Research Programme /1/ are to determine to what extent climate can be predicted and to assess human influence on its variability. The associated strategy consists in observing, so as to document climate variability and understand the physical mechanisms involved, while developing models capable of simulating climate and assessing their ability to predict climate changes on time scales from months to decades. Substantial progress along these lines should ultimately enable the investigation of the climate sensitivity to natural and man-made perturbations and the prediction of changes likely to result from specific disturbances. The WCRP is split into three “streams”, each being centred on a given scale of climate variability and corresponding to a particular “climate system” including the minimum set of all the coupled components of the Earth system which contribute to the variability at the scale ofinterest. Stream 1 is focussed on time scales of 1-2 months, stream 2 on scales from several months to several years and stream 3 on decades. At longer scales the “climate system” tends to encompass an increasing number of components including those with very low frequency variability and slow response, like the deep ocean or the cryosphere. In particular the substantial quantity of heat slowly conveyed by the deep ocean makes the three-dimensional ocean circulation a major element to take into account, when addressing time scales of the order of decades, within the third stream. Given the shortcomings of the available datasets, documenting the variability at such scales of the atmosphere, ocean, land surfaces and cryosphere and their relation to climate appears a first priority prior to the modelling of their full coupling. The World Ocean Circulation Experiment has been designed in thn spirit, in recognition that the observational and modelling ingredients of a global comprehensive description of the three-dimensional ocean circulation are still missing or unsatisfactory. This situation results from the difficulties to sample and observe the global ocean with classical In situ measurement techniques, given the variety of interacting scales present in the ocean and the importance of eddies with relatively small scales ofthe order of 15 to 150 km. WOCE is again organized in three “core projects”. Core project 1 is dedicated to the global description, core project 2 to the southern ocean and core project 3 to the investigation of the dynamics of gyres and to other process studies. The WOCE observational strategy is essentially multisystem, for the three-dimensional ocean circulation is underdetermined by any individual observing system. Active and passive spaceborne sensors are the only ones likely to afford global dense coverage, and therefore essential to this strategy, although their observations are unfortunately twodimensional, representative ofthe sole surface of the ocean, since signals measurable from space do not penetrate into nor originate from the inner ocean. Moreover, the feasibility of WOCE relies on the implementation of new satellite systems like the ERS and TOPEX/POSEIDON, as the unique potential source of global observations of the surface wind forcing by scatterometry, and surface currents by topographic altimetry, but the overall observational strategy is based on the combination of In situ observations, remote sensing and numerical modelling. In situ measurements from ships or buoys, surface or subsurface floats, inverted echosounders, tide gauges, tomographic arrays are clearly needed to document the vertical structure of the ocean unaccessible from space. The finishing touch is the development or upgrading of numerical models with data assimilation capabilities needed to quality-control, filter and merge the bulk of data into a three-dimensional dynamically consistent picture. The potential of topographic altimetry. Under the geostrophic approximation, valid at mid latitudes, the low-frequency component of the surface current along a given axis produces a contribution to the slope ofthe ocean topography in the orthogonal direction, according to equation(l): v(x)~~ .r’ fax

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More generally, under the geostrophic and hydrostatic approximations, the integral of the horizontal gradient of the density over a layer of the ocean is proportional and orthogonal to the difference between the two currentvectors at the boundaries of this layer: v(x,z)

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Classical mid-latitude oceanography used extensively this relationship to infer current profiles from density measurements, assuming a zero current at a “reference” level where the density is known. The existence of such a reference level is unfortunately disputable. Provided the current-induced slope of ocean topography can be derived from altimetry, this technique does allow the accurate calculation of the velocity profile by integrating equation (2) from the surface to any vertical level, rather than from a hypothetical “reference” level. Besides, the vertical structure of the barodlinic ocean tends to be expandable into some basic vertical functions called modes. The prominent mode function in a given area of the deep ocean can be to a large extent determined by the typical size of the eddies /2/, observable by eddy resolving altimetry. As a result altimetric estimates of the surface stream function can be roughly extrapolated into vertical profiles by projection on the diagnosed dominant vertical mode and further assimilated by a numerical ocean model in order to produce a final three-dimensional picture of a portion of the ocean /3/. This example illustrates the capabilities of satellite altimetry to recover features of the inner ocean, through the analysis of their signatures at the surface. Closer to the equator, where the Coriolis parameter vanishes, the geostrophic component of the current can be inferred from altimetry with a surprisingly high accuracy (Picaut, personal communication) using the along-track derivative of the geostrophic motion equation, while the displacements of the surface appear negatively correlated to those of the thermocline below, most important during the ENSO episodes. In summary, the major altimetric contributions to WOCE will remain the provision of global surface statistics, estimates of the large scale quasi-permanent circulation and detailed maps in regions ofparticular interest or where propagative patterns prevail, all required for the validation of realistic eddy-resolving GCMs and for the understanding of some features of the ocean circulation, like the anomalous equatorward transport in the southern Atlantic. In oversimplified terms, altimetry gives access to the time taken by a radar pulse to return to the emitting instrument after reflection at the sea surface and propagation through the ionosphere and troposphere. This time is then converted into a distance. Obviously the along-track slope of such measurements involves many contributions external to the signal induced by low-frequency ocean currents. First, the signal is affected by tropospheric moisture and temperature and by the electronic content in the ionospheric portion of the path. Corrections based on external pressure fields, microwave radiometer data, dual frequency measurements from the altimeter itself or from tracking system, and modelling must be applied. Second, the orbit must be reconstructed with the highest accuracy from tracking data and orbit modelling, so as to filter out errors on the orbit slope at all scales ofthe oceanic signal. Any altimetric system must thus be associated to a nadir viewing microwave radiometer and accurate tracking systems with global homogeneous coverage. Last but not least, the various signals that make up the ocean topography must be separated in order to isolate the lowfrequency current variability. Tides must be estimated from global models and interpolated at nadir, as well as complex effects of atmospheric pressure load, or interactions of ocean and electromagnetic waves. The static component of the signal, which would reflect the equilibrium of the motionless ocean under the Earth gravity field must be also substracted. Properly separating it is crucial to obtain the quasi-permanent and permanent parts ofthe oceanic signal, since all averaging methods assuming the decoupling of ocean and geoid signals in the frequency/time domain no longer apply. The merging and levelling of all available geodetic and tide gauge data with altimetry along with the use of inverse techniques are promising approaches to solve this issue, as long as a mission like the ESA’s ARISTOTELES has not provided an independant high resolution gravity field. The applicability of altimetry to the monitoring of the variability of the absolute sea level in response to global warming is critically dependent on the success ofsuch approaches.

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The TOPEX/POSEIDON mission. TOPEX/POSEIDON (Figure 2) is an altimetric space mission dedicated to WOCE and jointly conducted by NASA and CNES, since the signature of a Memorandum Of Understanding between the two agencies In March 1987. The project is under phase C~ and scheduled for launch in mid-1992. The design assumes a baseline three-year lifetime, but resources for at least 5years will be available. Gw~ HIGH

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Fig. 2. The TOPEX/POSEIDON satellite. The Topex/Poseidon Science Definition Team, renamed Science Working Team after the final confirmation of the selection ofthe P1’s, is composed of all the investigators selected by CNES and NASA, and associated to the definition of the mission. It is organised into six subcommittees in charge of submitting recommendations to the project in six important areas : system measurement accuracy, system performance verification, orbit selection, definition of the basic data files (the socalled Geophysical Data Records or GDR), intercomparison and merging of geodetic data and tide gauges, and tide models. The TOPEX/POSEIDON orbit will be a mission-dedicated, non sun-synchronous circular orbit with an altitude of about 1,336 km and an inclination close to 66 degrees, so as to minimize the effect of drag on precise orbit determination and to avoid aliasing of the lowfrequency oceanic signal by diurnal tides. The subcommittee on orbit selection, who already recommended to freeze the orbit cycle throughout the mission, embarked on a variety of sampling and impact simulation exercises, which resulted in late 1989 in the recommendation of a ten-day repeat cycle for the mission. Meanwhile, the subcommittee on intercomparison and merging of geodetic data and tide gauges has worked out a strategy to estimate absolute ocean topography. All subcommittees took advantage of the availability of GEOSAT data to prepare for the use of TOPEX/POSEIDON data. According to the NASAJCNES MOU, NASA supplies the spacecraft, the dual-frequency TOPEX altimeter, corner reflectors for laser tracking, a 3-channel microwave radiometer for wet tropospheric corrections, a GPS receiver for experimental tracking and is responsible for data acquisition via TDRSS. CNES is funding the launch by an Ariane 42P and supplies the POSEIDON solid state, low-power, low-weight, single-frequency altimeter sharing the TOPEX altimeter antenna, and the DORIS Doppler tracking system (Figure 3), now proven on SPOT-2. Both agencies will contribute to calibration and validation of both altimeters, produce or give access to environmental corrections (ionosphere, tides, troposphere, sea state, EM bias...), perform precise orbit determination, contract with their investigators, process the data under their respective responsibility and archive and distribute GDRS through AVISO (see below) and the

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NASA Ocean Data System. Pursuant to a Science Team recommendation, there will be no exclusive right on the data. The performances of the system will ensure an unprecedented accuracy of less than 14 cm RSS on the topography, needed to explore the full potential ofaltimetry. The main objective of the mission is to improve our understanding of ocean circulation and its fluctuations, in accordance with WOCE requirements, but also to develop methods to interpret and use accurate altimetric data at best. Particular emphasis is puton: geostrophic currents and topography, deviations from geostrophy, oceanic variability (spec-

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measuring the variable and permanent Ocean Circulation, inference ofocean circulation at depth, • oceanic tides and gravity waves.

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Fig. 3. Status ofDORIS tracking beacons as of September 1990. Geophysical experiments are also part ofthe mission objectives and TOPEX/POSEIDON data will be used to document the oceanic bathymetry, the rigidity of the lithosphere, the convection in the mantle and some other geodetic issues. BEST and GEWEX GEWEX and its observational reQuirements. The investigation of the global energy and water cycle is the scientific objective ofGEWEX, the next major experiment considered for implementation by the World Climate Research Programme in the 1995-2000 timeframe, when WOCE and TOGA near completion. GEWEX is being designed by a joint WMO/ICSU Steering Committee and should meet the requirement for better observing, understanding and modelling the cycling ofwater and energy through the atmosphere, ocean and continental biosphere, prerequisite to the development of monthly, seasonal and other climate predictions. The water cycle not only affects the atmosphere but also determines the fresh water budget at the ocean surface, as the net result of evaporation and rainfall, which drives the deep ocean thermohaline circulation. The water cycle has also tremendous impact on human life. It regulates the biomass, and causes droughts and floods. It also influences tropospheric chemistry and the rate of acid deposition on land and forests through chemicals scavenging by rain. As a matter of fact, the troposphere being the most quickly responsive component of the Earth system to natural or anthropogenic climate anomalies, GEWEX has been recognized as a major contribution to IGBP in providing a comprehensIve and quantitative description of the wet troposphere. The water cycle is a prominent yet poorly documented element of the atmospheric engine. More than one third of the total energy input to the atmosphere (Figure 4) is supplied through latent heat flux at the interface. Moist convergence in the tropical boundary layer is also a major driving force ofthe general circulation, recognised to trigger deep moist convection, whereby latent and sensible heat are converted into potential energy, this potential energy being in turn exported to the subtropics by the upper branch of the Hadley cell. Similarly, the shift of convective areas from

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Indonesia to the Central Pacific and the resulting modification of the Walker cell are signatures of El Nino Southern Oscillation (ENSO) events. Thus improving the space observations of atmospheric water is a top priority, given the shortcomings of the current and planned space observing systems in this respect. Thresholding of infrared images in convective areas and microwave radiometry mainly over oceans provide only qualitative or semi-quantitative estimates ofcolumn-integrated rain rate, while infrared soundings in clear air give only access to moisture profiles in the free troposphere with poor vertical resolution. Whatever the interest of the all weather AMSU-B microwave moisture profilers, and despite the considerable potential of the planned polar platforms, there will be a lack of water-dedicated instrumental packages in the 1995-2000 timeframe.

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Fig. 4. Proportion of the various forms of energy absorbed on the average by the troposphere (after /4/). The development ofnew active sensors like rain radars and differential absorption lidars, giving respectively access to profiles of rain rate and boundary layer moisture, are therefore crucial to the GEWEX effort towards the monitoring ofwater fluxes, sources, sinks and conversions. This development would be in itself a challenge if the sampling and scale interaction issue is to be properiy addressed, but the coupling ofwater to the other components of atmospheric energetics is an additional complicating factor, imposing other observational requirements. Clouds produced through the water cycle are interacting with short and long wave radiation. Improved paraineterizations of their optical properties in the atmospheric general circulation models are needed, to represent their radiative forcing, and ultimately reliably simulate the possible feedback of cloudiness on the enhanced greenhouse effect. Tropical moist convection is also intimately coupled with the wind field. Convergence in the tropical boundary layer triggers deep moist convection, whose patterns are in turn suspected to excite easterly Madden-Julian waves affecting the wind field over the entire tropical belt and modulating the monsoon circulation. As a consequence, global observations of the three-dimensional wind field are equally crucial to GEWEX, especially in the tropics where the wind field cannot be reconstructed from the mass field since geostrophy no longer applies. Cloud imaging and radiation budget measurements are needed as well. The overall GEWEX observational requirements were reviewed and reported by the GEWEX Study Group /5/, who emphasized the need for a low-inclination, low-orbiting platform carrying visible and infrared radiometers, a wind measurement package including a Doppler lidar, a precipitation measurement package including a microwave radiometer and a rain radar and an Earth radiation budget instrument, in addition to the planned polar orbiting missions. The rationale is the recognition of the importance of the tropics in the global cycle, as well as its specific observational requirements, driven by the paramount importance ofthe wind, the necessity to resolve the scales of convective rain cells and to sample the diurnal variations by an adequately precessing orbit. The BEST urolect. Proposed to CNES by the French scientific community at its prospective seminar in 1985, the BEST project (Bilan Energétique du Système Tropical Tropical System Energy Budget /6/) attempts to meet this fundamental requirement of GEWEX. CNES initiated a three-year long phase A study for BEST in late 1988, including studies of instrumental design, payload and platform accommodation, managed by a project team and carried out internally or under industrial contracts. The BEST project is proposed to international cooperation in view of a launch around the year 2000. -

The baseline instrumental payload is composed of a rain radar, a Doppler lidar and a multifrequency microwave radiometer. A differential absorption lidar, a SCARAB type sensor (see

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below), a high resolution visible/infrared radiometer, are also studied as options within the phaseA, in order to tentatively provide simultaneous measurements of moisture, Earth radiation budget and cloud distribution. The reference precessing orbit for BEST has an inclination of 28 degrees, an altitude close to 430 km and samples the tropical diurnal cycle in 24 days. This orbit reflects tradeoffs between sampling requirements, and constraints on budget link for the active instruments and orbit maintenance. The concepts ofthe innovative active sensors are outlined below. __V~a~ b:Four fixed

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Fig. 5. Pointing configurations : baseline Doppler lidar of BEST (right) and LAWS-type scanning sensor (left). The Doupler lidar. The baseline instrument considered in the phase A study is a configuration with four fixed orthogonal telescopes pointing at 45 degrees off nadir and at 45 and 135 degrees from the spacecraft velocity vector in horizontal projection (Figure 5). Though concentrating measurements along two parallel narrow tracks 600 km apart (foran orbit altitude of 430 kin), this configuration is preferred to a more challenging conical scanning sensor similar to NASA’s LAWS, given the climate objective and early target launch date of BEST. A first major simplification is the easy extraction of the wind-induced Doppler effect as compared to the complex dynamic lag angle compensation required to control and separate the time varying scan-induced Doppler signal for any scanning sensor. Moreover and most importantly, a series of successive laser shots from a given telescope is sensing the same absolute component of the wind, which allows straightforward alongtrack averaging over resolution cells ofprogrammable length. This is most useful to increase the signal-to-noise ratio while producing wind estimates representative of the typical GCM grid mesh. The resulting flexibifity on the along-track sampling and resolution strategy allows a greater variety of mission tradeoffs in terms of lifetime, grid spacing, accuracy and three-dimensional resolution, given the energy and affordable number of shots of a CO2 laser source. In other words the specifications on the laser source can be relaxed to 3 J, 2 Hz and about 3.108 shots for a nominal three-year mission, 50 km resolution cells and 400 km along-track sampling, all consistent with the BEST objective of mapping the large scale curl and divergence ofthe tropical wind. The rain radar. The radar is designed to sound rain within a 20 km-deep atmospheric layer extending down to the ground, at a tranverse resolution of 1.6 km, so as to minimize biases on rain estimates averaged over one month and five degree boxes, and to enable detailed case studies as well. Uniform layers as deep as 5 km of rain up to 60 mm/h must be recovered. A 250 m radial resolution is required to resolve the bright band and rain profile, whose divergence yields the latent heat release. A simulator developed by Marzoug /7/ was used to derive realistic specifications of the baseline single frequency instrument. Frequency agility is adopted to increase signal-to-noise ratio while adaptive pointing enables to select a 100 km swath within a 300 km wide band centered on nadir, in order to properly sample rainy areas detected in advance within the swathwidth of the for-

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ward-looking microwave radiometer. In parallel with this baseline configuration, a number of options are studied, including dual-frequency configuration with pulse compression and dual-beam concept with along-track stereo-viewing capability. Significance of BEST beyond GEWEX. Short and medium range weather numerical prediction models await better water and wind observations for their initialisation in order to reduce their spinuptime and increase the value of short-term predictions. Such measurements should be provided by active sensors similar to those developed for BEST, but which cannot be flownyet on polar orbiting satellites, mainly due to the more stringent link budget constraints probably incompatible with the technology available by the end of the century. Thus BEST, as one of the very first potential opportunities of flying such sensors, will provide guidance for the development offuture operational meteorological sensors. The colocation of BEST measurements with geostationnary observations is also expected to benefit the interpretation ofoperational imagery and soundings. GLOBSAT The GLOBSAT concept. The concept of a space mission called GLOBSAT was defined by the French scientific community at the last CNES Prospective Seminar in September 1989. It derives from a rather thorough analysis ofthe scientific priorities associated with the IGBP and the related needs of space observations, outlined in a report to the CNES Science Programme Committee on the Evolution of the Global Environment /8/. Taking into account the observational needs to be covered in the period 1992-96 by ERS-l and -2, UARS, TOPEX/POSEIDON, ADEOS, it appears that an additional effort should address two classes of scientific objectives, namely: understanding the main biogeochemical cycles and their impact on tropospheric and stratospheric chemistry and on continental and marine ecosystems ; study the evolution ofthe concentration of radiatively-active trace gases (e.g. CO2, CH4, CO, 03, ...) and its climatic impact through the regulation of the greenhouse effect. These themes are closely linked through the relationships between the water and carbon cycles and an enhanced greenhouse effect, or the radiative properties of tropospheric species and their chemistry, and this is in favour of a synergetic observational strategy. Four major requirements are put forward for this strategy: document key processes ; give access to accurate budgets on the global scale ; provide time series of data representatives of low-frequency variability (whether natural or man-made) and climatic trends ; allow an identification ofthe role of human activities. In order to cover the full space-time scales ofinteracting phenomena it should rely on the combination of space data and other observations, and on numerical models capable of data assimilation. To pin-point the weak signals associated with climate perturbations, the space systems involved should be seen in a perspective of continuit~’and optimized with respect to mission objectives (in other words mission tradeoffs that couldjeopardize observationalpriorities should be rejected). The GLOBSAT payload. In conformance with the above requirements GLOBSAT is defined as a medium-size mission in polar sun-synchronous orbit. The following instrumental payload was proposed to achieve the stated scientific objectives: an advanced infrared spectrometer, for the accurate retrieval of temperature and moisture profiles, the determination of surface radiative properties, and the measurement of integrated contents ofminor constituents (C02, CH4, CO, 03), a UV/vlsible/near infrared spectrometer using limb stellar occultation for the determination of ozone, temperature and aerosols profiles in the stratosphere, and nadir SBUV-mode observations for total ozone mapping, a visible-infrared radiometer for cloud imagery with a specific sampling (resolution 100 m, sampling every 1 km), abroadband radiometer (ScaRaB-type) for radiation budget study, an ocean colour imager, for chlorophyll mapping in ocean and coastal waters, and the study of the ocean role in the global carbon cycle, as an option, a visible-near infrared imaging radiometer and polarimeter (POLDER-type), for the study ofdirectional effects, and the determination ofaerosols and surface properties. -

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Simultaneous data acquired by several instruments on-board GLOBSAT could be used synergistically to study cloud-radiation interactions, or tropospheric and stratospheric chemistry, or biosphere-atmosphere interactions. Other instruments are considered essential for the GLOBSAT research programme, such as a dual-swath scatterometer, a multichannel microwave radiometer, a radar altimeter, as well as the operational meteorological instruments currently flown on NOAA satellites. It has been assumed that these sensors would fly on other platforms simultaneously with GLOBSAT. In its preliminary defmition GLOBSAT is seen as a mission capable to provide early responses to WCRP and IGBP data needs presently uncovered. It will make use of innovative instrument concepts based on available technology. As such GLOBSAT is proposed as a precursor to the European Polar Platform planned for 1997-98. GLOBSAT current status. A phase A study is planned to start in the industry in the fall of 1990, with two mission scenarios based respectively on: a baseline GLOBSAT one-shot research mission, with a target launch date in 1996, and oriented toward a possible combination with the German ATMOS project, a series of three missions combining GLOBSAT sensors and operational or pre-operational meteorological instruments, with a first launch in 1997 and a programme duration of 12 to 15 years. A contribution ofthe european consortium Eumetsat to this study is expected. -

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DATA MANAGEMENT Data management appears as a real challenge of the future years. Pluridisciplinary space programmes will convey terabytes of information. This is the reason why data centres have to be implemented as soon as possible for the currently approved missions like ERS, UARS, TOPEX/POSEIDON to prepare for future, more complex missions. In addition to the SPOT-related effort, CNES participates with national and European partners to two data management projects, which are an integral part of a consistent national strategy aimed at developing space oceanography. The CERSAT Project The CERSAT is one of the Precision Processing and Archiving Facilities (PAF’s) that make up the off-line ground segment of ESA’s ERS-1, together with the Earthnet central user facility located in Italy. In accordance with Earthnet harmonisation scheme, CERSAT is responsible, on behalf of ESA, for archiving, off-line processing and distributing low bit rate data collected over the ocean by the ERS-1 scatterometer, altimeter, microwave radiometer and SAR in wave mode. The centre is jointly funded by ESA, IFREMER, CNES and Meteo France. It is developed by the three French institutes under the leadership of IFREMER, and will be operational for ERS-1 launch (may 1991). The AVISO Project The AVISO project is aimed at delivering to the research community high level products elaborated from basic data produced by GEOSAT, ERS- 1, TOPEX/POSEIDON and other oceanographic missions. Several components are considered, the main focus being at the moment on the altimetric component. A pilot system using GEOSAT data is already distributing altimetry research products to the French investigators, while the future operational system is under phase A study at CNES. This system will also distribute the basic TOPEX/POSEIDON data to the European and African investigators selected by CNES. SPACE INSTRUMENTS OF OPPORTUNITY On-going Prolects CNES is also developing various sensors of opportunity for flight on third platforms, in cooperation with national and international partners. French-Soviet cooperation. French-Soviet space cooperation in environmental studies is currently focussedon Earth radiation and clouds, with the SCARAB and ALISSA projects. JASR 11:3-0

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SCARAB is a broadband, low resolution (60 km), 4-channel visible and infrared scanning radiometerfor accurate Earth Radiation Budget (ERB) measurements. Germany is also associated to the project. Two flight models will be flown on-board two successively launched METEOR-3 Soviet satellites in 1991 and 1992 to ensure proper sampling. The main objective is to monitor Earth radiation budget in the short and long wave parts of the spectrum and improve our understanding of the radiative forcing of clouds. This programme is taking over the US ERBE programme, thereby ensuring continuity of ERB data and starting to fill the data gap between ERBE and the polar platforms. SCARAB is also considered for inclusion in the BEST payload, in order to take advantage of the diurnal sampling capability of the precessing orbit of BEST, in the same spirit as ERBS. ALISSA is a backscatter lidar to be flown in 1991 on-board the MIR manned station. Short duration experiments are planned to investigate the potential of the synergism of lidar cloud altimetry and co-located geostationary or polar imagery. Middle atmosphere sensors. CNES and French laboratories are associated to the development of a few middle atmosphere experiments. The Grille spectrometer, flown in 1983 as part of the European First Spacelab experiment on the NASA Shuttle, will be flown again in 1992 on the ATLAS-i mission, together with NASA’s ATMOS infrared spectrometer. It is a French-Belgian experiment using a thermal infrared spectrometer operated in solar occultation mode, which gives access to stratospheric and mesospheric profiles ofvarious trace species. WINDII, an interferometer jointly developed by Canada and France, is part of the Upper Atmosphere Research Satellite (UARS) payload. Estimates of the horizontal wind in the upper mesosphere (70-100km) on a global scale, with a vertical resolution of 3 kin, should be inferred from Doppler shifts of 0, OH and 02 emission lines. Thus WIND!! observations will supplement solar forcing and trace species measurements by companion sensors. The Solar Spectrum Experiment to be flown on-board the European EURECA platform and on the ATLAS-i mission is a high resolution spectrometer aimed at investigating the fluctuations of the most rapidly varying components of the UV and visible solar spectrum. ATSR/M. The ATSR/M is the microwave sounder component of the Along Track Scanning Radiometer of ERS- 1. It is a nadir viewing, dual frequency (22 and 36 0hz), microwave radiometer sensing column-integrated precipitable water and cloud liquid water. Its main objective is to produce accurate wet tropospheric corrections to ocean topography measurements performed by the ERS-1 altimeter. Proiects under Phase A Study GOMOS. After the discovery of the Antarctic ozone hole in 1985 and the subsequent confirmation of a global decrease in column-integrated ozone at a rate of the order of 0.1% a year, the threat of human activities on global environment is given stronger attention from the public and governmental authorities throughout the world. Whatever the promises of the Montreal protocol, the catalytic destruction of life-protecting stratospheric ozone by long-lived chlorofluorocarbons (CFC’s) already present in the atmosphere will persist, while ozone production starts decreasing with the weakening of solar forcing around 1992. The monitoring of stratospheric chemistry and ozone budget will therefore remain a top priority for the forthcoming decade. The cross analysis of available space data and reference ground-based ozone observations revealed significant drifts ofmany optical sensors and stressed the need for reliable or self-calibrated sensors based on robust measurement techniques, adapted to climate trend monitoring. Among the existing instruments, UV/visible or near infrared limb viewing spectrometers operating In occultation mode proved long-lived and capable of delivering data of stable, reliable quality, as a result of the differential nature of the occultation technique less sensitive to instrument de-

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gradations. The GOMOS (Global Ozone Monitoring by Occultation of Stars) instrument proposed by a group of European investigators for flight on-board the first ESA polar platform /9/ combines the advantages of occultation and those of using stars as reference occulted light sources, which enables global homogeneous coverage unaffordable by solar occultation techniques applicable only at sunrise and sunset. Stratospheric profiles of 03, H20, NO2, NO3, aerosols, temperature and possibly other minor species will be measured by the dual grating spectrometer operating in the spectral range 250-950 nanometers. The phase A study has jointly been conducted by French and Fmnish laboratories, institutes and industrial companies and funded by CNES and the TEKES Finnish technologyagency. POLDER. POLDER (POLarisation and Directionality of Reflectances) is a seven-band imaging polarimeter selected for flight on-board the japanese ADEOS satellite to be launched in 1995. The phase A study of POLDER hasjust been completed. The originality of the instrument stems from its ability to observe a given pixel under various geometrical, spectral and polarization conditions. A 400 x 300 CCD matrix corresponds to the instantaneous square scene composed of 5 km pixels and centred on nadir. This scene migrates along track with the satellite motion, which enables the observation of a given target under different viewing angles. The various polarisations and spectral bands are selected during this migration using in particular a filter wheel. The mission objectives are vegetation monitoring (including the investigation of polarization and directional signatures), biosphere processes, ocean colour, detailed studies of aerosol loads and properties, and observation and modelling of directional effects on Earth radiation budget due to clouds, vegetation and other targets. Given these objectives, this instrument would advantageously be coupled on the same platform with a radiation budget sensor, an ocean colour scanner or any visible/near infrared spectrometer, subject to a consistent adjustment of all the narrow spectral bands ofthese synergetic sensors. AIRBORNE SYSTEMS The role of In sItu, ground-based and airborne measurements in environmental research programmes is unanimously recognised. They are needed to document local processes which necessitate high resolution, high frequency sampling and accurate measurements of a comprehensive set of local parameters, all features unaccessible from space. The WOCE observational strategy described previously, the way the Antarctic ozone hole was discovered from ground-based measurements or the recent understanding of the role of Polar Stratospheric Clouds in polar ozone chemistry from balloon-borne experiments are illustrations of this potential. Regarding the development ofremote sensing systems, airborne instruments often appear as useful precursors to space sensors, most useful to validate technological concepts and assess scientific and observational potential, and to calibrate and validate spaceborne sensors during ad hoc campaigns. For all these reasons, CNES supports the development of airborne sensors and operates a variety of balloons made available to the scientific community. Some airborne instruments developed with a contribution of CNES are preparing for BEST. This includes the French LEANDRE project ofbackacatter and differential absorption (DIAL) lidars to be flown on the French Atmospheric Research Aircraft in 89 and 90 ; the WIND scanning Doppler lidar under phase A study in cooperation with the German DLR; the ASTERIX airborne stereo rain radar expected to enter its phase B in 1990. Other airborne instruments already available or being built are precursors ofSCARAB, POLDER, GOMOS and ATSR/M. CONCLUSION The discoveryof the so-called Antarctic ozone hole and the discussion of a possible global warming due to increasing emissions of greenhouse gases have alerted the international public opinion and governmental authorities. After in depth analysis of the scientific issues raised, the international research community has concluded to the need for a challenging global approach of the Earth as a system, which materialised with the definition of the International Geosphere Biosphere Programme. A project like GLOBSAT would clearly provide an early response to the increased need of global multidisciplinary data. This new approach does not deny the relevance of relatively

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focussedinitiatives in the spirit of the famous First Global GARP Experiment, which paved the way for years of atmospheric research. On the contrary, experiments like TOGA and WOCE are an integral part ofany attempt to monitor Global Change, on the same footing as other well-delineated initiatives proposed within the !GBP. Consequently, space projects like ERS, TOPEX/POSEIDON and UARS, designed more than five years ago to meet the requirements of such experiments and due for launch around 1990, remain major contributors to the IGBP. They will in particular help to establish pluriannual and global reference data sets and to define some of the observing components of a future monitoring system. In a slightly more distant future, GEWEX is expected to relate the global energy and water cycle to the greenhouse effect and establish the basis fore more reliable climate predictions. However new requirements are brought up and call for appropriate answers. As stressed by the IGBP, the continental and marine biosphere deserves more attention as well as the chemistry ofthe troposphere and polar stratosphere, unfortunately poorly documented by UARS. The complexity of the processes involved, the variety of non-linearly interacting time and space scales and the need for a global view can only be embraced through a combination of in situ, ground-based, airborne and spaceborne observing systems and numerical models with data assimilation capabilities. Clearly, closer interactions between the communities involved in IGBP and WCR.P is needed to define the pluridisciplinary cost-effective space programmes, in support of an optimised commonly agreed scientific strategy, rather than in response to a conjunction ofmore or less conflicting requirements from independent lobbies. Such programmes will strongly rely on the current operational satellites and the plannedpolar platforms, assuming the missions ofthe latter are specified accordingly. They will also certainlyand critically rely on more sharply focussed, dedicated space missions similar to TOPEX/POSEIDON, the Tropical Rain Measurement Mission (TRMM) and BEST, driven by specific sampling and observational requirements uncovered by existing or planned missions. REFERENCES 1. First implementation plan for the World Climate Research Programme, WCRP publication n°5, WMOII’D n°80,124 pp. (1985). 2. L.L. Fu, On the wave number spectrum of ocean mesoscale variability observed by the SEASAT altimeter, .1. Geophys. Res. 88, 4331-4342 (1983). 3. P. Dc Mey and A.R. Robinson, Assimilation of altimeter eddy fields in a limited area quasi geostrophic model, .1. Phys. Oceanography, 2280-2293 (1987). 4. K.N. Llou, An Introduction to atmospheric radiation, Academic Press (1980). 5. Concept of the global energy and water cycle experiment, WMOITD N°215. Report of the JSC Study Group on GEWEX, Montreal, Canada, 8-12 June 1987 and Pasadena, 5-9 January 1988 (1988). 6. BEST (Tropical System Energy Budget) : Scientific objectives and preliminary definition of a satellite mission dedicated to GEWEX and Global Change, CNES, 58 pp. (1988). 7. M. Marzoug, Etude d’un radar spatial pour la mesure des précipitations : application au projet BEST. PhD. Thesis, 274 pp. (1989). 8. The Evolution of the Global Environment, Report to the Scientific Programs Committee of the Centre National d’Etudes Spatiales, CNES, 90 pp. (1989). 9. J.L. Bertaux, G. Mégie, T. Widemann, E. Chassefière, R. Pellinen, E. Kyrola, S. Korpela and P. Simon, Monitoring ofozone trend by stellar occultation : the GOMOS instrument, this issue.