On the measurement of microwave vegetation properties: some guidelines for a protocol

In support of algorithm development for the multiangle interferometric synthetic L-band radiometer on the Soil Moisture and Ocean Salinity sensor (SMOS) and for possible synergistic approaches with higher frequency microwave radiometers such as the Advanced Microwave Scanning Radiometer (C-band), an inventory has been made of polarization, angular, and frequency dependencies of vegetation optical depth and single-scattering albedo. Both parameters form the basis of a zero-order radiative transfer model, which is often used for inverse modeling of microwave observations from space. The inventory is based on experimental data published in the literature. Underlying models have been reviewed because data comparison is impossible without due consideration of the theoretical background. In general, it can be concluded that both single-scattering albedo and optical depth are angular, polarization, and frequency dependent. This dependence, however, depends on the canopy type and structure. Angular dependence implies that the cosine correction for the slant path through the canopy is no longer valid. Knowledge of these dependencies, therefore, is important for processing multiangle observations such as those anticipated for the planned SMOS and for possible synergistic approaches with C-band observations. Because of the existing variety of methods and procedures found in the literature, some guidelines for a protocol for field experiments are proposed in order to facilitate intercomparison of experimental results and proper incorporation of the parameters in zero-order transfer models.