I. Introduction
The exchange of carbon between forests and atmosphere is a vital component of the global carbon cycle. Accurate estimates of terrestrial carbon storage are required to determine its role in the global carbon cycle, to estimate the effect of anthropogenic disturbances (i.e., land use/land cover changes) on the cycle, and to monitor mitigation efforts that rely on carbon sequestration through reforestation [1]. The estimation of forest biomass from SAR data is limited by the lack of forest structure information. Interferometric synthetic aperture radar (InSAR) is a potential tool for the extraction of forest structure because it is sensitive to the vertical distribution of forest components over forested areas. The digital elevation model derived from InSAR is typically the elevation of the scattering phase center (SPC) rather than that of the bare ground surface. The SPC is located at a certain position between the forest canopy top and ground surface. It will change along with the SAR system parameters, understory conditions, and forest structures. There is consensus that the SPC of short-wavelength data (such as X- or C-band) should be located within the forest canopy while that of long-wavelength data should be lower due to its deeper penetration. Some researchers have successfully estimated the forest height using the dependence of the SPC on wavelength. Neeff et al. [2] used the difference between the digital terrain model from the X-band and the P-band InSAR data as a measure of vegetation height in the estimation of forest biomass. Balzter et al. [3] used the X-band and L-band InSAR data acquired by Experimental Synthetic Aperture Radar (E-SAR) airborne sensors to estimate the top heights of forest stands. Praks et al. [4] presented results from the Finnish Synthetic Aperture Radar (FinSAR) project, where the E-SAR (operating at the L- and X-bands) and the Helsinki University of Technology Scatterometer (operating at the X- and C-bands) instruments were operated together to validate tree-height retrieval algorithms for boreal forest. Aside from the dependence of the SPC on wavelength, its dependence on polarization [i.e., polarimetric SAR interferometry (PolInSAR)] is currently a hot topic in the retrieval of forest vertical structures. PolInSAR technology allows the separation of ground/forest phase centers through the selection of proper polarimetric basis and polarimetric combinations. Although PolInSAR is a new and advanced technology, it is still very important to explore algorithms for the estimation of forest biomass using single- or dual-polarization InSAR for heterogeneous forest because there has been and will be near global coverage of such kind of data acquired, for example, by the Phased Array type L-band Synthetic Aperture Radar (PALSAR) onboard Advanced Land Observing Satellite (ALOS) (under fine mode single polarization or fine beam double polarization) and ALOS/PALSAR-2 (under fine beam dual polarization).