TanDEM-X Pol-InSAR Inversion for Mangrove Canopy Height Estimation

This paper presents mangrove canopy height estimations using single- and dual-pol TanDEM-X (TDX) data by means of Pol-InSAR techniques. Using the TDX data for forest applications, the penetration capability of X-band into the volume and the polarimetric diversity of interferometric coherence can create false effects that could sometimes lead to biased three-dimensional (3-D) forest parameter estimation. Moreover, in the case of single-pol TDX acquisition (i.e., one independent complex interferometric coherence), it is not possible to perform the inversion without external topographic information due to the underdetermined problem for Pol-InSAR inversion. To solve these problems, the ground phase in the Pol-InSAR model has been estimated directly from TDX interferograms with an assumption that the underlying topography (i.e., water surface level) over mangroves is flat and negligible. With the estimated ground phase that represents water-level elevation in mangroves, the Pol-InSAR inversion from the single- and dual-pol TDX data did not rely on an external DTM data set. The inversion results were validated against airborne lidar measurements in Campeche, Mexico, and in the Zambezi Delta, Mozambique. The single- and dual-pol inversion results showed a successful inversion performance with a high correlation coefficient (0.851-0.919) and with low RMSEs (1.069-1.727 m). The entire inversion performance quality over mangroves at X-band could reach to 10% height estimation accuracy. In addition to Pol-InSAR inversion results, the location of phase center was checked and assessed. The results showed a surprisingly deep location of phase centers from top mangrove canopy (5-7 m, up to 12.36 m) with a similarity at HH and VV polarization.