The Science and Measurement Concepts Underlying the Biomass Mission
Quegan, Shaun1; Chave, Jerome2; Dall, Jorgen3; Le Toan, Thuy4; Papathanassiou, Kostas5; Perrera, Andrea6; Rocca, Fabio7; Saatchi, Sassan8; Scipal, Klaus6; Shugart, Hank9; Ulander, Lars10; Williams, Mathew11
1Centre for Terrestrial Carbon Dynamics, University of Sheffield, UNITED KINGDOM; 2CNRS, Laboratoire EDB, University Paul Sabatier, Toulouse, FRANCE; 3National Space Institute, Technical University of Denmark, DENMARK; 4CESBIO, CNRS-CNES-Université Paul Sabatier-IRD, Toulouse, FRANCE; 5German Aerospace Center e. V. (DLR), Wessling, GERMANY; 6Mission Science Division, ESA-ESTEC, NETHERLANDS; 7Dipartimento di Elettronica ed Informazione, Politecnico di Milano, ITALY; 8Jet Propulsion Laboratory, Pasadena, UNITED STATES; 9University of Virginia, Charlottesville, UNITED STATES; 10Department of Radar Systems, FOI, Linkoping, SWEDEN; 11School of GeoSciences, University of Edinburgh, UNITED KINGDOM
Above-ground biomass is one of the fundamental quantities describing the terrestrial biosphere, but is poorly known; global estimates vary by a factor two, and there are equally large uncertainties at continental and regional scale. Forests hold 70-90% of the above-ground biomass, with the major part in the tropics. Uncertainties in the value, spatial distribution and changes in above-ground biomass are a major source of uncertainty in estimates of anthropogenic emissions of carbon dioxide due to Land Use Change (LUC), since about 50% of biomass is carbon. Even greater uncertainty attaches to the uptake of carbon by the land surface, since this is not independently calculated, but is estimated by balancing the carbon budget, i.e. as the residual when the atmospheric increase in CO2 and the estimated take-up of CO2 are subtracted from the source terms (fossil fuel emissions and LUC). Hence quantifying the changes in biomass due to deforestation, degradation and forest growth is essential to understand the role of the land surface in the carbon cycle. Accurate biomass information also has enormous social, environmental and political relevance. Forest biomass is a resource for energy and materials. It is strongly related to biodiversity and conservation issues. Lack of accurate, consistent, frequent and global measurements of changes in biomass is one of the major problems in formulating international agreements to conserve tropical forest under the UN Reduction of Emissions from Deforestation and Degradation (REDD+).
The Biomass mission is designed to tackle this crucial information gap, and takes advantage of the new opportunity that opened when the International Telecommunication Union gave a secondary allocation to P-band for Earth Observation in 2004. Previous airborne experiments had indicated that backscatter becomes increasingly sensitive to biomass as wavelength increases; the P-band wavelength (~70 cm) is effectively the longest wavelength usable for radar imaging from space. Longer wavelengths would run into very severe ionospheric problems, but these can be dealt with at P-band by use of a suitable (dawn-dusk) orbit and correction methods that require polarimetric data.
Polarimetric data also gives extra information on scattering mechanisms that can be exploited in estimating biomass. P-band also has the major advantage that the backscatter comes predominantly from large elements of the canopy (large trunks and branches) which tend to be stable. Hence interferometric coherence remains high over many days, allowing repeat-pass interferometry to be performed by a single satellite (hence keeping costs down) that is also capable of achieving global coverage annually or better. This combination of properties led us to the core instrument concept of the Biomass mission, as a polarimetric P-band SAR capable of carrying out polarimetric interferometry, and hence capable of measuring forest height.
This provides two avenues for estimating biomass, one through exploitation of the relation between polarimetric backscatter and biomass, and one through height estimation and conversion to biomass using allometric relationships. These two estimates can then be combined in a Bayesian framework to provide a biomass estimate with reduced uncertainty. Moreover, the Biomass mission makes a further radical step by implementing for the first time SAR tomography from space. This is currently envisaged as a short mission phase sampling only part of the world’s forests, but studies are assessing whether this phase could be extended to give global coverage, using suitable orbit manoeuvres. This will give unprecedented information on where in the canopy-soil complex the scattering is occurring as a function of polarisation, and fundamental insight into the physics underlying biomass recovery from backscatter and height estimates, allowing these individual estimates to be improved and adapted to different forest conditions before being combined.