SAR tomography for the retrieval of forest biomass and height: Cross-validation at two tropical forest sites in French Guiana

Abstract

Developing and improving methods to monitor forest carbon in space and time is a timely challenge, especially for tropical forests. The next European Space Agency Earth Explorer Core Mission BIOMASS will collect synthetic aperture radar (SAR) data globally from employing a multiple baseline orbit during the initial phase of its lifetime. These data will be used for tomographic SAR (TomoSAR) processing, with a vertical resolution of about 20 m, a resolution sufficient to decompose the backscatter signal into two to three layers for most closed-canopy tropical forests. A recent study, conPreprint submitted to Remote Sensing of Environment December 17, 2015 ducted in the Paracou site, French Guiana, has already shown that TomoSAR significantly improves the retrieval of forest aboveground biomass (AGB) in a high biomass forest, with an error of only 10% at 1.5-ha resolution. However, the degree to which this TomoSAR approach can be transferred from one site to another has not been assessed. We test this approach at the Nouragues site in central French Guiana (ca 100 km away from Paracou), and develop a method to retrieve the top-of-canopy height from TomoSAR.We found a high correlation between the backscatter signal and AGB in the upper canopy layer (i.e. 20-40 m), while lower layers only showed poor correlations. The relationship between AGB and TomoSAR data was found to be highly similar for forests at Nouragues and Paracou. Cross validation using training plots from Nouragues and validation plots from Paracou, and vice versa, gave an error of 16 -18% of AGB using 1-ha plots. Finally, using a high-resolution LiDAR canopy model as a reference, we showed that TomoSAR has the potential to retrieve the top-of-canopy height with an error to within 2.5 m. Our analyses show that the TomoSAR-AGB retrieval method is accurate even in hilly and high-biomass forest areas and suggest that our approach may be generalizable to other study sites, having a canopy taller than 30 m. These results have strong implications for the tomographic phase of the BIOMASS spaceborne mission.

Forest Above-Ground Biomass Estimation Using Single-Baseline Polarization Coherence Tomography with P-Band PolInSAR Data

“…Compared with these methods, PCT technology can extract forest vertical structures from the vertical distribution of relative reflectivity and has the potential to improve biomass estimation. In addition, the potential of P-band TomoSAR to characterize forest structure was assessed in a number of studies relating forest vertical structure to forest biomass [50][51][52]. Compared with TomoSAR technology, the single baseline PCT reduces the amount of data, but it has the problem of inverting the instability of the vertical profile of relative reflectivity in some places ( Figure 5) and multi-baseline PCT technology may improve the phenomenon.…”

“…The third part (above h3) is the noise zone, where the backscatter power is mostly contributed by noise, unlikely to be associated with physically relevant components. A parameter related to the average height of the stand can be extracted by analyzing the power loss value in the first envelope [50][51][52], which is:…”

Trailblazing the Carbon Cycle of Tropical Forests from Puerto Rico

“…Recent field approaches for assessing carbon stocks of extensive tropical forest landscapes were also developed to assure the estimation of error of field carbon stock determinations (e.g., [157]) and expand the area covered by estimates (e.g., [158][159][160][161]). These inventory techniques using thousands of plots, coupled with high-resolution remote sensing images, LIDAR, and GIS analysis were used to conduct carbon accounting procedures for producing maps of carbon stocks and carbon emissions at various geographical scales [162,163].…”

Ground-Based 3D Radar Imaging of Trees Using a 2D Synthetic Aperture

“…Interferometric SAR uses an additional antenna that is vertically and/or horizontally displaced by a baseline to enable estimation of the surface vertical displacement [1]. Since the SAR signal penetrates vegetation canopies, multiple baselines can enable tomographic reconstruction of scattering within the vegetation canopy [5][6][7][8][9][10][11][12]. Multi-baseline SAR is a special case of a 2D synthetic aperture where the elevation dimension is sparsely sampled [10].…”