Estimation of Vegetation Structure Parameters From SMAP Radar Intensity Observations
(2021) IEEE Transactions on Geoscience and Remote Sensing — Vol. 59, n° 1, p. 1-17 (2021)
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- Authors
Jagdhuber, Thomas 
Montzka, Carsten Lopez-Martinez, Carlos
- Abstract
- In this research study we present a multi-polarimetric estimation approach for two model-based vegetation structure parameters (shape A_P & orientation distribution ψ of main canopy elements). The approach is based on a reduced observation set of three incoherent (no phase information) polarimetric backscatter intensities (|S_HH |^2, |S_HV |^2, |S_VV |^2) combined with a two-parameter (A_P & ψ) discrete scatterer model of vegetation. The objective is to understand if this confined set of observations contains enough information to estimate the two vegetation structure parameters from L-band radar signals. In order to disentangle soil and vegetation scattering influences on these signals and ultimately perform a vegetation-only retrieval of vegetation shape A_P and orientation distribution ψ, we use the sub-pixel spatial heterogeneity expressed by the covariation of co- and cross-polarized backscatter Γ_(PP-PQ) of the neighboring cells and assume it is indicative for the amount of a vegetation-only co-to-cross-polarized backscatter ratio μ_(PP-PQ). The ratio-based retrieval approach enables a relative (no absolute backscatter) estimation of the vegetation structure parameters which is more robust compared to retrievals with absolute terms. The application of the developed algorithm on global L-band SMAP radar data acquired from April to July 2015 indicates the potential and limitations of estimating these two parameters when no fully polarimetric data is available. A focus study on six different regions of interest, spanning land cover from barren land to tropical rainforest, shows a steady increase of orientation distribution towards randomly oriented volumes and a continuous decrease in shape arriving at dipoles for tropical vegetation. A comparison with independent datasets of vegetation height and above ground biomass confirms this consistent and meaningful retrieval of A_P and ψ. The retrieved shapes and orientation distributions represent main vegetation elements matching literature results from model-based decompositions of fully polarimetric L-band data at the SMAP spatial resolution. Based on our findings, they can be directly applied for parameterizing the vegetation scattering component of model-based polarimetric decompositions. This should facilitate decomposition into ground and vegetation scattering components and improve retrieval of soil parameters (moisture & roughness) under vegetation.
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Citations
Jagdhuber, T., Montzka, C., Lopez-Martinez, C., Baur, M. J., Link, M., Piles, M., Das, N. N., & Jonard, F. (2021). Estimation of Vegetation Structure Parameters From SMAP Radar Intensity Observations. IEEE Transactions on Geoscience and Remote Sensing, 59(1), 1-17. https://doi.org/10.1109/tgrs.2020.2991252 (Original work published 2021)