SAR for Ecologists
SAR Data for Terrestrial Ecologists From ASF
by Keith Cunningham and Jeremy Nicoll, ASF
NASA recently supported collaboration among ASF, the National Snow and Ice Data Center (NSIDC), and the Oak Ridge National Laboratory (ORNL). This collaboration emphasized the awareness, understanding, and use of synthetic aperture radar (SAR) satellite imagery to support the missions of ORNL terrestrial ecologists and NSIDC climate-change scientists. ASF specializes in SAR, which it typically collects from polar-orbiting satellites.
SAR requires knowledge and experience, in part because its microwave ranging measurements must be manipulated in several highly technical processing steps before an image can be formed. The satellite imagery can be processed to a GeoTIFF format, a universal input for geographic information systems (GIS), but interpretation of the SAR imagery requires an understanding of how radar illuminates the earth and interacts with the features that reflect and scatter the microwave signals.
The physics of SAR microwave interactions determine the various geospatial applications for which it is applicable. Every SAR sensor operates at one of many specific microwave frequencies, allowing various features to be resolved. Some frequencies are optimized for mapping the elevations of urban and vegetative surfaces, useful for digital surface modeling. Other frequencies work well for discerning snow from ice, and are well adapted for monitoring glaciers, icebergs, and global climate change. Another SAR frequency is well adapted for penetrating the leaves of trees. In this case, the microwaves scatter among the woody branches and trunks of trees, allowing estimations of forest biomass. The frequency and scatter of the microwaves determines the application of SAR technology for geospatial applications.
ASF recently developed products to expand the familiarity and use of SAR products, generating approximately 100 SAR images for about 50 research locations of interest to ORNL and NSIDC. Figure 3 provides two examples of the images created. The research locations were selected based on the presence of micrometeorological sensors known as flux towers, which measure changes in atmospheric, riverine, and soil chemistry (specifically carbon dioxide and other isotopes important to understanding climate change). Importantly, these flux tower sites have accompanying geospatial data, primarily from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS). By creating SAR images for locations where flux tower data and MODIS imagery exist, terrestrial ecologists and climatic researchers can validate the use of SAR against existing data. These images are available from ASF at http://www.asf.alaska.edu/program/sdc/project/terrec.
ASF chose the ALOS-PALSAR instrument for the Project, because the L-band sensor is well adapted for terrestrial-ecology applications such as land cover and biomass mapping. ALOS was launched in 2006 and ASF serves as an archive site for the data this satellite acquired over the Americas through 31 March 2011.
ALOS-PALSAR data are archived in a format known as CEOS (Committee on Earth Observation Satellites). In contrast, NASA’s Earth Observing System (EOS), the primary data repository for understanding global climate change, uses the selfdescribing hierarchical data format (HDF). HDF is well-suited to specific needs of the climate change archive, with its vast variety of communities, datasets, and acquisition formats. It is scalable, accommodating of variability in data object size, and efficient.
Because SAR is a key remote-sensing technology for monitoring global climate change, ASF and NASA were interested in exploring the feasibility and utility of storing SAR data in the HDF file format, specifically the version of the format called HDF5. ASF has developed software tools to ensure that the existing CEOS data and future SAR missions could be archived and distributed in HDF. These tools have been prototyped for a future release of ASF’s MapReady software, a robust SAR data processing application. The modified MapReady software performs several data processing tasks to make SAR imagery accessible for GIS use and is integral to expanding the use of SAR data products from ASF. Sample products in HDF5 are available to approved users. If interested in working with the HDF5 products, please contact ASF’s User Support office at firstname.lastname@example.org.