Digital Terrain Model Retrieval in Tropical Forests Through P-Band SAR Tomography

Abstract

This paper focuses on the retrieval of terrain topography below dense tropical forests by means of synthetic aperture radar (SAR) systems. Low-frequency signals are needed to penetrate such a thick vegetation layer; however, this expedient alone does not guarantee proper retrieval. It is, here, demonstrated that the phase center of P-band backscatter may lie several meters above the ground, depending on the slope and incidence angle. SAR tomography is shown to overcome this problem and retrieves the actual topography even in the presence of dense trees up to 50 m tall. Digital terrain models returned by SAR tomography are, here, put in comparison with light detection and ranging (LiDAR) terrain models: the accuracy of radarderived maps is found to be at least comparable with the one offered by LiDAR systems. Moreover, the discrepancy between tomography and LiDAR is larger if large-footprint LiDAR is considered thus suggesting that, in this case, tomographic maps should be considered the reference height. Analyses are carried out by processing three data sets gathered over different tropical forests in western Africa. The robustness of the radar estimates is assessed with respect to both ground slope and treetop height.

The BIOMASS Level 2 Prototype Processor: Design and Experimental Results of Above-Ground Biomass Estimation

“…Since commonly available global Digital Elevation Models (DEM) are estimated from sensors operating at higher frequencies than P-band (e.g., Tandem-X or SRTM [42]), the elevation model may not correspond to the real terrain level, due to the limited penetration capabilities of higher frequencies into natural media. Thus, a suitable DTM will be produced from BIOMASS data themselves by using the full 3D information achievable from multi-baseline tomographic data [43] in the TOM phase.…”

“…Another critical point is that an accurate DTM is required for the height estimation algorithm to be unbiased. To minimize the impact of this uncertainty the algorithm is expected to use the terrain elevation of the DTM derived during the tomographic phase [43], similarly to the ground cancellation described in Section 3.1.…”

Cross Sensor Simulation of Tomographic SAR Stacks

“…Tomographic profiles returned by the back-projection algorithms when applied to (a) real airborne data and (b) simulated spaceborne stack. The two axes are analogous to the ones shown in Figure 5; the black line shows the ground topography below the forest layer estimated through SAR tomography [25].…”

Ground and Volume Decomposition as a Proxy for AGB from P-Band SAR Data

“…The knowledge of precise terrain topography allows focusing the signal at a specific height with respect to the terrain rather than in an absolute reference, resulting in the generation of intensity maps associated with a specific height inside the vegetation layer [16]. Precise DTM can be estimated from multi-baseline data themselves thanks to the long wavelength [17]. An example of tomographic focusing is shown in Figure 1.…”

“…The BIOMASS tomographic phase [18] will provide the first global coverage during the first 14 months of operations, by collecting seven ascending and seven descending passes with 3-days repeat pass time, designed to maintain coherency at P-band [19]. Thanks to the long wavelength of about 70 cm it will be possible not only to characterize electromagnetic interactions of radar waves with the forest structure and generate AGB maps over tropical biomes, but also to generate a global map of terrain topography below the vegetation layer (DTM) [17] and subsurface features [20,21]. Previous studies reported that a TomoSAR intensity of 30 m canopy layer in dense tropical forests is strongly correlated to AGB [22][23][24], whereas the ground scattering alone is poorly correlated to biomass.…”