Capabilities of BIOMASS Tomography for Investigating Tropical Forests

Abstract

The objective of this paper is to provide a better understanding of the capabilities of the BIOMASS tomography concerning the retrieval of forest biomass and height in tropical areas. The analysis presented in this paper is carried out on airborne data acquired by Office National d'Etudes et de Recherches Aérospatiales (ONERA) over the site of Paracou, French Guiana, during the European Space Agency campaign TropiSAR. This high-resolution data set (125-MHz bandwidth) was reprocessed in order to generate a new data stack consistent with BIOMASS as for the bandwidth (6 MHz) and the azimuth resolution (about 12 m). To do this, two different processing approaches have been considered. One approach consisted of degrading the resolution of the airborne data through the linear filtering of raw data, followed by standard SAR processing. The other approach consisted of recovering the 3-D distribution of the scatterers at a high resolution, which was then reprojected onto the BIOMASS geometry. The latter procedure allows us to obtain a data stack that is the most realistic emulation of BIOMASS imaging capabilities. In both approaches, neither ionospheric disturbances nor temporal decorrelation has been considered. The connection to the forest biomass has been examined in both cases by investigating the correlation between the backscatter at different forest heights and the above-ground biomass (AGB) values from in situ data. As expected, the reduction of the system bandwidth to 6 MHz resulted in significant vertical resolution losses compared with the original airborne data. Nevertheless, it was possible to retrieve the forest height to within an accuracy of better than 4 m, whereas the backscattered power at the volume height (30 m above the ground) exhibited a correlation higher than 0.8 with the in situ data and no bias phenomena over the AGB values ranging from 250 to 450 t/ha. Index Terms-BIOMASS mission, forest height, forest vertical structure, P-band, spaceborne geometry simulation, synthetic aperture radar (SAR) tomography, tomographic phase, tropical forest biomass, 6-MHz bandwidth, 30-m layer.

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:…”

“…The power loss value K ranges from 0 to 1 with the step of 0.1. The optimal K is determined by the forest height that is closest to the true value obtained by LiDAR [51]. We name the corresponding forest height as the average canopy height H ac .…”

“…Using the top-of-canopy height LiDAR model [51], we can get the parameter's location corresponding to a power loss value in each stand, with respect to the stand canopy phase center, ranging from −10 dB to 0 dB. Figures 6 and 7 present the average bias of the stand height and the root mean square error (RMSE) with respect to the LiDAR measurements in different flight tracks, respectively.…”

The Impact of Temporal Decorrelation on BIOMASS Tomography of Tropical Forests

“…The interferometric revisit time during this phase is planned to be about 3-4 days, after which it will be increased to approximately 17 days to allow polarimetric interferometric synthetic aperture radar (SAR) (PolInSAR) measurements, depending on the final mission design. The BIOMASS Tomographic Phase is aimed at providing, for the first time from space, the possibility to see the vertical structure of the illuminated forests through the SAR tomography (TomoSAR) technique [2], leading new insights about the interaction of electromagnetic waves with the vegetation layers and providing useful inputs and recommendations for Manuscript improving PolInSAR inversion during the Operational Phase [1], [3], [4]. Still, one potential show-stopping factor is represented by temporal decorrelation, i.e., the instantaneous (quick), shortterm and long-term decorrelation mechanisms [5], [6].…”