Studying rainforests from the skies – radar technology measures biomass

For climate action, it is important to understand in detail how forest biomass influences the carbon cycle. To do this, experts need very accurate data on how global forest biomass changes due to forest degradation, forest fires and deforestation. The daily measurement flights of GABONX covered a total of seven areas, each with different tropical rainforest characteristics.

The DLR Microwaves and Radar Institute created tomographic data sets of the forest areas, each approximately five kilometres wide and ten kilometres long. They used different radar frequencies to draw conclusions about the biomass stored in the forest. The DLR Dornier Do 228-212 research aircraft, operated by the Flight Experiments facility in Oberpfaffenhofen, was used for the measurement flights. The GABONX campaign was led by DLR and was carried out in cooperation with the Gabonese space agency (Agence Gabonaise d'Études et d'Observations Spatiales; AGEOS) as well as the US and European space organisations, NASA and ESA.

Rainforests as a carbon dioxide store

Trees store large amounts of carbon by absorbing carbon dioxide during photosynthesis and using it to form wood. Soils of intact and near-natural forests, bogs and wetlands also store a lot of carbon. Rainforests are among the largest carbon dioxide stores on Earth. If carbon dioxide is suddenly released in large quantities, for example through deforestation or fires, this has a correspondingly drastic effect on the climate.

Gabon is home to tropical rainforest in its most diverse forms – some regions are used for forestry and some remain untouched in national parks, but others experience illegal logging. Mangrove forests are widespread along the coasts – ideal test areas for DLR researchers to develop their radar sensors and processing techniques for future Earth remote sensing missions.

One of the important applications used during the campaign is radar tomography. With wavelengths in the decimetre range (L- and P-band), DLR's F-SAR radar system 'scanned' the forest and generated information on tree heights and the structure of the rainforest. The different 'levels' of the forest were mapped, from the dense treetops to the forest floor. Similar to medical diagnostics, the researchers were able to obtain three-dimensional information from the tomography. This gave them unique insights into areas that are otherwise difficult to access and also hidden carbon dioxide repositories.

The project team had already flown over most of the GABONX measurement areas in 2016 as part of the AfriSAR campaign. In order to make the temporal changes in the rainforest visible, they created up-to-date comparative images. These data sets are valuable because, among other things, they provide information about how much biomass has been lost or regrown.

As was the case seven years ago, NASA also participated in the most recent measurement campaign. At the same time as DLR, personnel from the NASA Goddard Space Flight Center conducted flights over the same test areas with their 'Land, Vegetation, and Ice Sensor' (LVIS) laser altimeter system. In good visibility conditions without cloud cover, LVIS can provide very precise information about the height of the trees, their height distribution and the underlying surface. This information served as a reference for DLR's radar measurements.

Measurement data for satellite missions

For effective climate action, it is important to record biomass on a global scale and with high accuracy. Only the next generations of satellites will be able to do this. Radar systems have the particular advantage that they work in the same way when deployed on an aircraft as on a satellite in orbit. DLR's airborne F-SAR is therefore widely used to prepare for space missions.

Two European Earth observation missions in particular will benefit from the GABONX measurement flights. The Biomass radar satellite is scheduled for launch in 2024 and will work with long wavelengths in the P-band to determine information about the biomass stored in forests and its dynamics. In addition, ESA is expected to launch the Radar Observing System for Europe in L-band (ROSE-L) mission in 2028. This radar satellite will address a wide range of applications across the Arctic regions and the tropics, including forest monitoring.