Improving Forest Height Retrieval by Reducing the Ambiguity of Volume-Only Coherence Using Multi-Baseline PolInSAR Data

Abstract

Nonvolume decorrelation (<inline-formula> <tex-math notation= LaTeX >$\\gamma _{\\mathrm {Nonvol}}$ </tex-math></inline-formula>) united with the unknown ground contribution will bring a 2-D ambiguity to volume-only coherence, making the inversion underdetermined even when multiple baselines are available. In the context of random volume over ground (RVoG) model and three-stage algorithm, this paper theoretically presented the varied response of different baselines to both <inline-formula> <tex-math notation= LaTeX >$\\gamma _{\\mathrm {Nonvol}}$ </tex-math></inline-formula> and ground contribution, and then proposed a new multi-baseline inversion method to reduce the 2-D ambiguity. The proposed method includes two steps, calculating the common overlapped ambiguity from different baselines and fixing the extinction coefficient, to more accurately retrieve the volume-only coherence and forest height. It makes no assumptions on <inline-formula> <tex-math notation= LaTeX >$\\gamma _{\\mathrm {Nonvol}}$ </tex-math></inline-formula> and ground contribution. The method was validated and compared with three single-baseline inversions and two published multi-baseline inversions by using the airborne P-band polarimetric SAR interferometry (PolInSAR) data and the reference data of LiDAR canopy height model (CHM) over a dense rainforest site. Results showed that the developed multi-baseline method successfully reduced the combined influence of both <inline-formula> <tex-math notation= LaTeX >$\\gamma _{\\mathrm {Nonvol}}$ </tex-math></inline-formula> and ground contribution, and performed better than any single baseline, improving the <inline-formula> <tex-math notation= LaTeX >${R}^{2}$ </tex-math></inline-formula> from 0.60 to 0.77 and unbiased root-mean-square error (RMSE) from 1.32 to 1.04 m at the scale of ca. <inline-formula> <tex-math notation= LaTeX >$100 \\times 140\\,\\,\\text{m}^{2}$ </tex-math></inline-formula>. Moreover, the multi-baseline scheme is relatively robust among different baseline combinations.