Progress in retrieving canopy structure parameters from NASA multi-angle remote sensing

All remote sensing of the Earth's surface in the solar wavelengths is impacted by the Bidirectional Reflectance Distribution Function (BRDF): all Earth surfaces scatter light anisotropically. The BRDF describes the angular distribution of scattered light and is thus what most remote sensing instruments sample. The BRDF is dependent on the structural as well as the optical characteristics of surfaces and multiple samples of the BRDF may be exploited to retrieve structural parameters. It is less than seven years since instruments designed explicitly to exploit the directional signal obtained by viewing at multiple Sun and/or viewing angles have been placed in orbit. NASA's flagship multi-angle imager, the Multi-angle Imaging SpectroRadiometer (MISR) flown on the Terra satellite is the first of these to image the surface with a ground resolution of less than 1 km. This review will focus on recent progress in exploiting multi-angle data from MISR and the Moderate Resolution Imaging Spectroradiometer (MODIS) to retrieve parameters providing information on canopy structure.