A €400-million (US$513 million) radar project that is designed to measure global forest biomass in unprecedented detail will very probably launch around the end of the decade as the next Earth-observation mission by the European Space Agency (ESA).

ESA’s Earth Science Advisory Committee (ESAC) says that it found BIOMASS the technically and scientifically most convincing of three candidate missions discussed last month at a user meeting in Graz, Austria (see Nature 495, 15; 2013). The two other missions — COReH2O and PREMIER — would have measured global snow cover and atmospheric composition, respectively.

The committee’s recommendation must still be ratified by ESA’s Earth Observation Programme Board, which comprises representatives of the agency’s 20 member states. The board, which meets 5–6 May in Svalbard, Norway, has never overturned an ESAC recommendation.

Estimated construction costs for BIOMASS amount to around €400 million. A spacecraft, scheduled for launch around 2020, will fly a 70-centimetre wavelength radar sensor capable of probing trees’ heights and structures, such as trunks and canopies, in minute detail. Multiple repeat orbits are to produce three-dimensional maps of most of the world’s forests.

Radar measurements of forest biomass are one indicator of changes in biodiversity and are particularly valuable across the tropical forest belt, where ground inventory data are scarce or do not exist. Accurate space observations of biomass will also help quantify carbon emissions resulting from deforestation and land-use changes.

But the BIOMASS mission might come with a caveat. The frequency band of its radar interferes with military applications such as the US space-object tracking radar. If planned discussions between ESA and the US department of defence over frequency allocation come to nothing, it might mean that the mission radar cannot operate over large swaths of North America and, possibly, Europe.

“We would certainly like to look at deforestation and forest re-growth in the United States and Europe,” says Shaun Quegan, a carbon-cycle researcher at the University of Sheffield, UK, who chairs the BIOMASS project. “But our main interest lies in the tropics — and it is there where the mission will make a real difference.”