Eyes in the sky: ESA images forests in 3-D to analyse biomass

Nowadays there is a lot of research into analysing the capacity of ecosystems to act as 'carbon sinks' as they offer a potential way to mitigate climate change by storing carbon dioxide from the atmosphere. Producing accurate forest biomass estimates, and how they are changing over time, are a critical challenge to environmental scientists to whom national governments are turning for help in meeting their international obligations to stabilise greenhouse-gas emissions under the Kyoto Protocol.

Recent research revealed that not all forest types and not all locations would be suitable for afforestation, reforestation or conservation aimed at soaking up greenhouse gases, as some of the forests are and would be net CO2-contributors. This should make people think more critically about fashionable carbon offset schemes ('plant a tree to reduce your carbon footprint'), as some of these initiatives may actually worsen climate change (earlier post). On the other hand, understanding the amount of carbon forests store allows environmental economists to estimate the true 'ecoservice value' of an ecosystem (earlier post).

The European Space Agency is contributing to this kind of research by developing tools that allow forests to be seen from space in 3-D. It is also training scientists to use the new visualisation technique. 140 scientists and researchers from 22 countries have attended ESA's weeklong POLinSAR 2007 workshop, “Science and Applications of SAR Polarimetry and Polarimetric Interferometry,” hosted at ESRIN, ESA’s Earth Observation centre in Frascati, Italy, to hear the first space borne results and to attend interactive training sessions.

The technique is called 'polarimetric interferometry' and is performed using two polarimetric SAR (synthetic aperture radar) images acquired from slightly different directions. The study of these data sets permits scientists to retrieve information related to the 3-D structure of forest or other natural volume scatterers, such as underlying topography, forest height (image, click to enlarge) and to estimate forest biomass – a quantitative estimate of the entire amount of organic material in a particular forest habitat.

"POLinSAR allows us to estimate key environmental parameters that are needed today. For instance, it allows us to make estimations of forest biomass on a global scale. The conclusions from these global estimates may also be important for climate change modellers and decision makers." -- Konstantinos Papathanassiou, a researcher with the Radar and Microwaves Institute at German Aerospace Centre.

The capability of radar to penetrate ground cover and 'see' the underlying terrain, coupled with POLinSAR techniques to detect forest canopies, make it possible to classify trees and estimate their height using SAR imagery. This may sound of interest only to a narrow band of scientists, until one realizes that determining the types and heights of trees in a forest are critical ingredients in determining its biomass:
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Workshop participants saw the first POLinSAR in-orbit results from the Japan Aerospace Exploration Agency’s (JAXA) Advanced Land Observing Satellite (ALOS). Launched on 24 January 2006, ALOS is supported as an ESA Third Party Mission.

Also at the workshop, ESA provided additional interactive training opportunities for the POLSARPRO tool developed by the University of Rennes 1, France, which includes a wide-ranging tutorial in Polarimetry and Polarimetric Interferometry. To date, some 700 registered users from 62 countries worldwide are using the tool.

Over the last 2 years, ESA has trained some 250 scientists to exploit Polarimetric airborne/spaceborne SAR data for science and applications development using in particular the POLSARPRO software and educational tool.

"POLinSAR is the starting point of future applications because we are at the point where new satellites will be launched, such as Germany’s TerraSAR-X and Canada’s Radarsat-2, that can provide polarimetry and interferometry," said Prof. Eric Pottier, head of the Radar Polarimetry Group at the University of Rennes. "Processing this kind of data will open many new application doors, so it is very important to train young students now to be able to handle this kind of data."


Image: ALOS PALSAR first forest height estimates by means of Single-Baseline Polarimetric Interferometry (POLinSAR) at L-Band obtained by German Aerospace Centre (DLR) on the Oberpfaffenhofen test site. The POLinSAR research group at DLR, supported by ESA, is studying the methodology for forest height measurement and validation. Credits: JAXA, ESA, DLR