Ground and Volume Decomposition as a Proxy for AGB from P-Band SAR Data
Abstract
:1. Introduction
- 2D SAR intensity is poorly correlated to AGB;
- TomoSAR intensity at 0 m is poorly and negatively correlated to AGB;
- TomoSAR intensity at 15 m is poorly correlated with AGB;
- TomoSAR intensity at 30 m is highly correlated to AGB and follows a log-linear relation for AGB values from 250 to 450 t/ha.
2. Paracou Data
3. Methodology
3.1. Model-Based Analysis of Tomographic Intensities
3.2. Ground/Volume Decomposition
- is the expected correlation between the SLC (single look complex) image in pass n and polarization and the SLC image in pass m and polarization ;
- and are the polarimetric correlations of ground and volume scattering in polarizations and polarization ;
- and are the interferometric coherences of ground and volume scattering in passes n and m.
- by casting the problem into an algebraic point of view, which leads to the algebraic synthesis technique [35].
- Outer ground solution: point-like ground structure and high entropy volume polarimetry;
- Inner ground solution: distributed ground structure and low/medium entropy volume;
- Outer volume solution: point-like volume structure and high entropy ground polarimetry;
- Inner volume solution: distributed volume structure and low/medium entropy ground polarimetry.
4. Results
4.1. Tomography
4.2. Ground/Volume Decomposition
5. Discussion and Conclusions
- It was shown that total volume backscatter, as extracted from the observed tomographic intensities, does not correlate with AGB, unless it is corrected by accounting for wave extinction and forest height. Most interestingly, very similar results in terms of correlation and sensitivity with respect to AGB were obtained by retrieving (uncorrected) total volume backscatter by tomography or by ground/volume decomposition when assuming a physically-based solution. Accordingly, we found actually no reason why the total volume backscatter should correlate with AGB;
- Removing ground scattering is not sufficient to ensure a strong correlation between Radar intensities and forest AGB: volume structure does matter. Admitting that ground rejection is a sufficient condition for AGB retrieval would contradict experimental observations, in that: any tomographic layer should provide the same sensitivity to AGB; the classical solution of the ground/volume decomposition problem should be the one that provides the best sensitivity to AGB;
- Tomography appears to bring the most complete information about AGB in dense tropical forests, not only because of rejection of ground scattering, but also by virtue of its capability to single out the returns from different layers within the vegetation. Characterizing ground scattering as a volumetric target and volume scattering as a point-like target results in scattering from the midcanopy to be mostly ascribed to ground scattering, whereas volume scattering mainly accounts for the main and upper canopy layer. Accordingly, although this solution is not physical, it approximates as much as possible the layering capabilities of SAR tomography, which improves correlation and sensitivity with respect to AGB. It is worth noting that this solution is not guaranteed in presence of temporal decorrelation or in case of multiple baselines as the range of permitted interferometric coherence of ground and volume scattering in the complex plane shrink from the outer boundaries toward the true ground and volume coherences [36].
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Parameter | Description | Unit | Correlation | Sensitivity |
---|---|---|---|---|
Volumetric backscattering | dB/m | 0.18 | 400 t/ha per dB/m | |
Forest height | m | 0.62 | 42 t/ha per m | |
Total volume backscatter (w/out extinction) | dB | 0.76 | 86 t/ha per dB | |
Total attenuation | dB | 0.1 | 714 t/ha per dB | |
Total volume backscatter (w extinction) | dB | 0.19 | 400 t/ha per dB | |
Approximated tomographic intensity at 30 m | dB | 0.5 | 120 t/ha per dB |
Solution (Ground/Volume) | Ground Vertical Structure | Volume Vertical Structure | Ground Polarimetry | Volume Polarimetry | Sensitivity | |
---|---|---|---|---|---|---|
Outer / Inner | Ground-locked pointlike target (coherence 1, phase center 0 m) | Distributed target (low coherence, phase center 13 m) | Low Entropy Multiple Scattering Events (60 < < 70 entropy 0.45) | High Entropy Vegetation Scattering (47 < < 52 entropy 0.95) | 0.20 | 370 t/ha per dB |
Outer / Outer | Ground-locked pointlike target (coherence 1, phase center 0 m) | Point-like target within forest canopy (coherence 1, phase center 24 m) | High Entropy Vegetation Scattering (50 < < 55 entropy 0.95) | High Entropy Vegetation Scattering (47 < < 52 entropy 0.95) | 0.26 | 238 t/ha per dB |
Inner / Outer | Distributed target (low coherence, phase center 6 m) | Point-like target within forest canopy (coherence 1, phase center 24 m) | High Entropy Vegetation Scattering (50 < < 55 entropy 0.95) | Medium Entropy Vegetation Scattering (40 < < 45 entropy 0.6) | 0.75 | 59 t/ha per dB |
Inner / Inner | Distributed target (low coherence, phase center 6 m) | Distributed target (low coherence, phase center 13 m) | Low Entropy Multiple Scattering Events (60 < < 70 entropy 0.45) | Medium Entropy Vegetation Scattering (40 < < 45 entropy 0.6) | 0.72 | 80 t/ha per dB |
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Banda, F.; Mariotti d’Alessandro, M.; Tebaldini, S. Ground and Volume Decomposition as a Proxy for AGB from P-Band SAR Data. Remote Sens. 2020, 12, 240. https://doi.org/10.3390/rs12020240
Banda F, Mariotti d’Alessandro M, Tebaldini S. Ground and Volume Decomposition as a Proxy for AGB from P-Band SAR Data. Remote Sensing. 2020; 12(2):240. https://doi.org/10.3390/rs12020240
Chicago/Turabian StyleBanda, Francesco, Mauro Mariotti d’Alessandro, and Stefano Tebaldini. 2020. "Ground and Volume Decomposition as a Proxy for AGB from P-Band SAR Data" Remote Sensing 12, no. 2: 240. https://doi.org/10.3390/rs12020240