ESA's CANDIDATE EARTH EXPLORER SAR MISSION CONCEPTS Klaus Scipal, Malcolm Davidson, Michael Kern, Alan Thompson, Florence Hélière, Chung-Chi Lin European Space Agency, 2200 AG Noordwijk, The Netherlands ABSTRACT In this paper we describe the scientific background, the objectives, the observation requirements and the system concept of the BIOMASS and the CoReH 2 O missions, two proposed candidate earth explorer Synthetic Aperture Radar (SAR) missions. 1. Introduction In the last call for ideas for the next Earth Explorer Core Mission in 2005, twenty-four proposals were received by the European Space Agency (ESA). Six proposals - PREMIER, TRAQ, A-SCOPE, FLEX, BIOMASS and CoReH 2 O - were selected for further study during Phase 0. In February 2009, the Earth Observation Program Board is expected to make a formal selection of up to three missions to go into feasibility Phase A, based on a proposal of the Director of Earth Observation Programs. One of the selected candidate missions will finally be implemented as 7 th Earth Explorer with a foreseen launch in 2016. Two of the six proposed candidate missions – BIOMASS and CoReH 2 O – use SAR technology. BIOMASS is based on a P-band, full polarimetric SAR payload which will provide first observations of the global distribution of forest biomass at a resolution and accuracy compatible with the needs of international reporting on carbon stocks and terrestrial carbon models. CoReH 2 O uses a high resolution dual frequency, dual polarisation SAR antenna system in Ku- band and X-band to measure key snow and ice parameters necessary for understanding the exchange processes between the land-surface, atmosphere and ocean. In the following we will describe the scientific background, the mission objectives, the observation requirements and the system concept of both missions. Full details can be found in the Earth Explorer Assessment Reports (ESA, 2008). The system description is mainly based on the results of the work performed during two parallel Phase 0 system studies by two industrial consortia (EADS Astrium GmbH, 2008; Thales Alenia Space Italy, 2008) 2. BIOMASS 2.1. Scientific Background The release of carbon dioxide (CO 2 ) into the atmosphere by human activities has been recognised as a major driver of climate change. Terrestrial ecosystems play an important role, both in the release of carbon through land use change and deforestation, and in the sequestration of carbon through vegetation growth processes. Over the last 25 years, there is strong evidence that the terrestrial biosphere has acted as a net carbon sink, removing from the atmosphere approximately one third of the CO 2 emitted from fossil fuel combustion. However, terrestrial ecosystems are the largest single source of uncertainty in the global carbon budget (Intergovernmental Panel on Climate Change (IPCC), 2007). The uncertainties lie in the spatial distribution of carbon stocks and carbon exchange, and in the estimates of carbon emissions due to human induced or natural disturbances. A central parameter in the terrestrial carbon budget is forest biomass, which represents a proxy for carbon. Despite its crucial role in the terrestrial carbon budget, forest biomass is poorly quantified across most parts of the planet due to great difficulties in measuring biomass on the ground and consistently aggregating measurements across scales. 2.2. Mission Objectives BIOMASS will be implemented as of a P-band Synthetic Aperture Radar (SAR) mission, which will provide first observations of the global distribution of forest biomass at a resolution and accuracy compatible with the needs of international reporting on carbon stocks and terrestrial carbon models. The mission will exploit the unique sensitivity of P-band SAR to forest biomass and employ advanced retrieval methods to map forest biomass globally across the whole biomass range encountered in tropical, temperate and boreal forests. BIOMASS will also provide the first opportunity of exploring the Earth's surface at the P-band wavelength. 2.3. Observation Requirements At present, the status, dynamics and evolution of the terrestrial biosphere are the least understood elements in the global carbon cycle, which is deeply embedded in the functioning of the Earth and climate systems. There are uncertainties in the distribution of carbon stocks and carbon exchange, in the estimates of carbon emissions due to land use change, and in the uptake of carbon due to forest regrowth. Forest biomass is the main repository of vegetation carbon and hence is a crucial quantity needed to reduce these uncertainties. Both its spatial _____________________________________________________ Proc. of ‘4th Int. Workshop on Science and Applications of SAR Polarimetry and Polarimetric Interferometry – PolInSAR 2009’, 26–30 January 2009, Frascati, Italy (ESA SP-668, April 2009)