Temporal changes of terrestrial vegetation have traditionally been monitored using empirical remote sensing tools, which are sensitive to perturbations as well as to the spectral properties of the sensor. Advances in the understanding of radiation transfer theory, and the availability of higher performance modern instruments, have led to the development of physically-based inverse methods to derive biogeophysical products. Jointly, these developments allow the retrieval of reliable, accurate information on the state and evolution of terrestrial environments. A series of optimized algorithms has been developed to document biogeophysical variables, and in particular to estimate the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) from a variety of optical instruments. As a result, monitoring managed (e.g., agriculture) or natural ecosystems will benefit from the availability of local, regional and global time series of remote sensing products such as FAPAR. This paper outlines the methodology and exhibits selected results in the form of temporal composites derived from the SeaWiFS sensor.
The availability of quasi-simultaneous multi-directional measurements from space, as provided by the Multiangle Imaging SpectroRadiometer (MISR) on board the Terra platform, offers new and unique opportunities to document the anisotropy of terrestrial surfaces at key solar wavelengths. This contribution outlines the physical reasoning underpinning a new quantitative interpretation of multi-angular reflectance measurements over terrestrial surfaces. The most innovative aspect of this approach concerns the characterization of the heterogeneity of these surfaces. Indeed, when appropriately parameterized, the shape of the reflectance anisotropy at specific optical wavelengths can be related to the structural characteristics of the observed target. This allows the detection of geophysical conditions for which surface heterogeneity is an essential ingredient to describe the measured reflectance pattern. This finding paves the way for the quantitative characterization of plant canopy structure on the basis of multi-angular data.
Vegetation structure significantly impacts the degree of anisotropy of the scattered radiation field. The proper analysis of multiangular data, such as those provided by the Multi-angle Imaging SpectroRadiometer (MISR) instrument on board Terra, could thus in principle yield statistical information on the structure of the observed environment. Preliminary investigation in this direction suggests that useful information on the heterogeneity of the vegetation can be retrieved at the subpixel scale.