EOQ N 66 July 2000 meteorology earthnet remote sensing solid earth future programmes Earth Observation Quarterly The Earth Explorer Missions - Current Status G. Mégie (1) and C.J. Readings (2) (1) Institut Pierre-Simon Laplace, Université Pierre & Marie Curie, Paris, France (2) ESA-ESTEC, Earth Sciences Division, Noordwijk, The Netherlands The ESA Living Planet Programme [ESA SP-1234, 1998] describes the plans for the Agency's new strategy for Earth Observation in the post 2000 time frame. It marks a new era for European Earth Observation, based on a programme that is user-driven, spanning the whole spectrum of interests ranging from scientific research-driven Earth Explorer missions through to application-driven Earth Watch missions. The user community is therefore now able to look forward to a programme of more frequent but very specific missions directed at the fundamental problems of Earth system sciences. In this Issue Earth Explorer Mission Special Issue The Earth Explorer Missions Current Status...1 Call for Ideas...4 The Gravity Field and Steady- State Ocean Circulation Explorer Mission GOCE...6 The Atmospheric Dynamics Mission ADM...12 The Soil Moisture and Ocean Salinity Mission SMOS...18 Envisat... 27 Conferences and Publications...28 Introduction The Earth's environment is a complex system which couples, at various temporal and spatial scales, the atmosphere, the oceans, the biosphere and the cryosphere. Unfortunately, despite its importance, many aspects of this Earth system are still not understood and so there remains a need for a world-wide scientific effort to better assess knowledge of the various global threats. These include climate change, Fig. 1: The Earth System
Earth Explorer Missions 2 Fig. 2: The Earth Explorer Missions stratospheric ozone depletion and tropospheric pollution, as well as more localised events such as the 1999 El Niño, fires in South East Asia, floods in southern Africa and the devastating earthquakes in Turkey. To further enhance our capacity to predict the evolution of the Earth's environment, under the influence of both natural variability and man's activities, the provision of data and their integration into appropriate models of the Earth system are of paramount importance. All elements of the system (atmosphere, land, cryosphere and oceans), as well as their couplings, have to be studied. So Earth observation from space has a crucial role to play as it can provide the globally coherent data sets which are an essential complement to ground-based, airborne and shipborne measurements. These considerations underlie the establishment of the European Space Agency's Living Planet programme, as described in a previous issue of the Earth Observation Quarterly (EOQ N 63, September 1999). An aim of this programme is to provide a series of regular flight opportunities, combining flexibility with certainty, to address the scientific issues highlighted by the Earth sciences community. It funds many of the Agency's Earth Observation activities including the Earth Explorer missions (see ESA-SP- 1227, Earth Explorers: the Science and Research Elements of ESA's Living Planet Programme). These are research/demonstration missions intended to advance understanding of the Earth's environment, which can also be used to demonstrate new observing techniques. There are two complementary types of Earth Explorer Missions, namely : Earth Explorer Core Missions - large research/demonstration missions led by ESA. Earth Explorer Opportunity Missions smaller research/demonstration missions not necessarily ESA led. The Living Planet programme is a userled programme which requires regular interactions between the Agency and the Earth sciences community. From the definition of several missions in both categories by scientists, a selection process based on extensive interaction with the community, assessment by the Earth Science Advisory Committee and further endorsement by the Programme Board for Earth Observation, has led over the past year to the selection of the first Earth Explorer Core and Earth Explorer Opportunity Missions. The purpose of this and the accompanying articles, is to describe these selected missions which are now in their implementation phase, setting them in the context of the Agency's overall Earth Observation programme. The Earth Explorer Core Missions In May 1996 a user consultation meeting was held in Granada, Spain, following which (and the subsequent peer review) four Earth Explorer Core Missions were selected for Phase-A study, namely: A Gravity Field and Steady-State Ocean Circulation Mission An Atmospheric Dynamics Mission An Earth Radiation Mission A Land Surface Processes and Interactions Mission The four Phase-A studies, together with accompanying scientific studies, were completed during the latter half of 1999. They were presented at a second Granada consultation workshop held in October 1999. Subsequently, following completion of a peer review by the Earth Sciences Advisory Committee (ESAC) and consideration by the Programme
3 Current Status Fig. 3: The Gravity Field and Steady-State Ocean Circulation Mission (satellite configuration and accommodation of the 3-axis gradiometer) Board for Earth Observation (PB-EO), it was agreed that the first two Earth Explorer Core Missions to be implemented (in order of implementation) should be the Gravity Field and Steady-State Ocean Circulation Mission (GOCE) and the Atmospheric Dynamics Mission (ADM). The Gravity and Steady-State Ocean Circulation Earth Explorer Core Mission (ESA/SP-1233(1)) is intended to address the needs of the Solid Earth physics, geodetic and oceanographic communities by measuring the Earth's gravity field and a derived geoid to unprecedented accuracy and resolution, using a three-axis gradiometer. This will help to advance knowledge of the Earth's interior structure and provide a much better reference for oceanographic and climate studies. Specifically it will focus on the use of better knowledge of the Earth's gravity field for studies in: For this it is necessary to determine the Earth s gravity field and the geoid (i.e. the equipotential surface for a hypothetical ocean at rest) to high accuracy (1 mgal and 1 cm) and high spatial resolution (~ 100 km). This will be achieved with the aid of a threedimensional gradiometer. The mission is scheduled for launch in 2004/5. More information can be found in another article in this issue of the EOQ. The Atmospheric Dynamics Earth Explorer Core Mission (ESA/SP- 1233(4)) will, for the first time, provide, with the aid of a Doppler wind lidar, direct observations on global scale of atmospheric wind profiles over the depth of the atmosphere, a notable deficiency of the current observing systems. With these data it will be possible to increase understanding of atmospheric processes for climate studies, particularly in tropical regions, as well as advancing the performance of numerical models used in weather forecasting. It is intended to improve understanding of atmospheric dynamics and global atmospheric transport as well as the global transfer of energy, water, aerosols and chemicals. This will lead to improved analyses of atmospheric state, providing the more complete threedimensional picture of the atmosphere which is required to: improve the parameterisations of atmospheric processes in models; advance climate and atmospheric flow modelling; provide better initial conditions for weather forecasting. A Doppler wind lidar, flying in a sunsynchronous orbit at about 400 km, will be used to observe global winds in clear air in the troposphere. The instrument will operate in the ultraviolet (355 nm) and will be able to detect backscattered signals from both aerosols and molecules. The mission is scheduled for launch in 2006/7. Further details will be found in an accompanying article in this issue of EOQ. Solid Earth physics anomalous density structure of lithosphere and upper mantle; oceanography dynamic ocean topography and absolute ocean circulation; ice sheet dynamics ice sheet mass balance; geodesy unified height systems, "levelling by GPS"; sea level change. Fig. 4: The Atmospheric Dynamics Mission Satellite
Earth Explorer Missions 4 Although the Land Surface Processes and Interactions Mission (ESA/SP- 1233(2)) and the Earth Radiation Mission (ESA SP-1233(3)) were not selected for implementation, both were highly commended, being seen as of crucial importance to the Agency s Living Planet Programme. Work should continue in support of both missions. divergence of radiative energy in the atmosphere, aerosol-cloud-radiation interactions, the vertical distribution of water and ice, and their transport by clouds, and cloud-precipitation interactions. It would have determined the vertical profiles of the radiative properties of clouds and As result, it was agreed that the first Earth Explorer Opportunity Mission to be launched should be Cryosat followed by SMOS. Progress would be maintained on a third mission, namely ACE so that it could serve as a "hot" spare in the event of unforeseen The main research objectives of the Land Surface Processes and Interactions Mission were to increase knowledge of land surface processes, including the carbon cycle. It would have supported the investigation of geo-biospheric processes for environmental research and climate modelling, to advance work on surface/atmospheric exchange processes. An imaging spectrometer, supported by a large number of well equipped experimental sites and field stations, would have been used to obtain the required data. The Earth Radiation Mission was intended to advance knowledge of the A Call for Ideas for the Next Earth Explorer Core Missions This issue of the Earth Observation Quarterly provides an overview of the status of the Earth Explorer missions and it can be seen that, during the course of last year, two Earth Explorer Opportunity Missions and Two Earth Explorer Core Missions have been selected for launch. Reference is also made to the future, notably the issue of a Call for Ideas for the Next Earth Explorer Core Missions. This was discussed during the meeting of the Programme Board for Earth Observation on 24-25 May and it was agreed that the Call should be issued on 1 June 2000 with a deadline of 1 September 2000 for the receipt of responses. Full details, including instructions of how to submit a response, will be found on <http://www.estec.esa.nl/explorer/> Fig. 5: Possible satellite concept for Cryosat aerosol on a global scale, providing data for numerical modelling and atmospheric process studies. In particular it would have exploited the capabilities of a backscatter lidar and a cloud radar. Iterations continue on both missions, notably for the radiation mission with Japanese agencies and scientists who have shown a clear interest in an advanced Earth Radiation Mission. The next milestone in the long term implementation of the Living Planet Programme will be the issue of a Call for Ideas for the next Earth Explorer Core Missions which start a new phase of implementation (see announcement on this page). The Earth Explorer Opportunity Missions In parallel with the work on the Earth Explorer Core Missions there has also been considerable activity on the Earth Explorer Opportunity Missions front. A call for proposals was issued in July 1998 and no less than 27 full proposals were received in response. These were subject to peer review by the ESAC and to consideration by the PB-EO. problems on Cryosat or SMOS. Two other missions, SWARM and SWIFT, were placed on a reserve list. Cryosat is being designed to measure variations in the thickness of the polar ice sheets and the thickness of floating sea ice. Its data are intended to be used for the study of the mass balances of the Antarctic and Greenland ice sheets, to investigate the influence of the cryosphere on global sea level rise and to provide important observations of sea ice thickness for use in Arctic and global climate studies. It will fly in a high inclination orbit at around 600 km, exploiting a Ku-band radar altimeter. It is scheduled for launch in 2003. The mission was described in EOQ N 63 (September 1999). SMOS is intended to demonstrate the observation of two key variables of the Earth system, namely soil moisture over land and salinity over oceans, to advance the development of climatological, meteorological and hydrological models. In addition, the mission should provide new insights into snow and ice structure, so helping to advance understanding of
5 Current Status the cryosphere. It is proposed to use a passive microwave radiometer, operating at L-band (1.4 GHz). The mission is scheduled for launch 2005. It is more fully described in an accompanying article in this issue of the EOQ. The implementation of ACE involves the provision of two different instruments, namely GRAS and COALA. GRAS would exploit the atmospheric refraction of signals from GNSS satellites, to provide observations of temperature and humidity in the upper atmosphere. These data would be used in atmospheric analysis and modelling as well as for energy balance and transport studies. An array of 6 satellites would be deployed. Only this option is being retained as the "hot" spare. The other instrument associated with the original ACE proposal (i.e. COALA) is being considered separately and is retained on the reserve list. COALA seeks to measure ozone and other chemical species by exploiting stellar occultation. The aim of SWARM is to measure the Earth's magnetic field to high precision and resolution, enabling the various sources to be separated both temporally and spatially. This will provide new insights into the structure of the Earth. SWIFT will employ a Doppler interferometer for the simultaneous observation of wind and ozone concentration profiles in the stratosphere. These data will be used to advance understanding of transport processes in the stratosphere. The Agency is still looking into the possibility of implementing those opportunity missions, which are on the reserve list having not been selected for immediate implementation, in a cooperative framework with other space agencies. The possibility of a second call for Earth Explorer Opportunity Missions is being considered for 2001. Fig. 6: SMOS (courtesy Alcatel) Concluding Remarks This article has provided an overview of the current status of the Earth Explorer Missions, at the end of the first phase of the implementation process which led to the selection of two Earth Explorer Core Missions and two Earth Explorer Opportunity Missions. Together these provide a well balanced scientific programme addressing the concerns of scientists interested in Solid Earth, oceanography, atmospheric physics, the land surface and the cryosphere. Further, the aim of providing regular flight opportunities at reasonably frequent intervals, is well on the way to being achieved. The continuity of the programme will be further ensured by the issue of new calls for ideas for both Earth Explorer Core and Earth Explorer Opportunity Missions, which are planned for 2000-2001, and by the continuous involvement of the research community via the various advisory groups and consultative workshops. Further information on the Earth Explorer Missions will also be found on <http//:www.estec.esa.nl/explorer>.