SAR Remote Sensing of Forests, from Microwaves to VHF

Doctoral thesis, 1998

The number of SAR systems available for civilian remote sensing purposes has grown in the nineties. A Synthetic Aperture Radar produces high resolution images of the earth independent on daylight and atmospheric conditions. This thesis mainly examines the applications of SAR remote sensing to monitoring forested areas. In order to explain and to improve the understanding of the backscattered radar signal from vegetation electromagnetic models are developed. The models are applied to predict SAR data from diverse types of vegetation of various biomass classes. Several SAR systems operating at radar frequencies that range from 20 MHz to 10 GHz have been used in this thesis work. The variation of backscattering mechanisms among radar frequencies naturally affects the information content of the SAR images. Generally lower frequencies contain more biomass related information. There is a saturation point where the backscattering is not affected by increasing forest biomass. This point moves towards higher biomass values as the radar frequency decreases. At C-band it is in general not possible to discriminate any forest types using the backscattering coefficient only. At L-band the sparse forest stands exhibiting low biomass values can be identified. The use of interferometric SAR however increases the image information content. At P-band a few various biomass classes can be identified. Still forests with very high biomass values are not possible to classify. VHF improves the dynamic range of the backscattering coefficient compared to P-band and shows good potential. The objective of the thesis has been to evaluate different techniques and to develop models to understand and explain the experimental results. Scattering at the microwave frequencies have been modelled using the radiative transfer approach. The branches are modelled as dielectric cylinders, and the scattered fields from the cylinders can be expressed analytically. Using the model the polarimetric extinction and phase matrices of each layer can be determined and the radiative transfer equation has been solved to the first order. At VHF frequencies the incoherent radiative transfer approach is not accurate enough. The coherent multi-path scattering is thus added to the solution. Various configurations of dielectric cylinders above a dielectric ground layer are evaluated and compared to numerical solutions using the Finite Difference Time Domain (FDTD) method.