Sampling the surface bidirectional reflectance distribution function (BRDF): 1. Evaluation of current and future satellite sensors

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Abstract
The Bidirectional Reflectance Distribution Function (BRDF) of earth surface materials contains information relating to their physical structure and composition that cannot be inferred from their spectral properties alone. Knowledge of the BRDF is also critical to the accurate retrieval of earth surface albedo, since the BRDF describes the angular distribution of reflected radiation under given illumination conditions. Although the BRDF cannot be measured directly, it can be estimated using models of surface scattering in conjunction with reflectance data acquired at different viewing and illumination angles. The ability of a satellite sensor to characterise the BRDF of any point on the earth's surface is therefore dependent on (i) the range of view angles over which it is able to acquire data, (ii) the orbital characteristics of the satellite on which it is mounted, and (iii) the time period over which the data are recorded. This paper explores the BRDF sampling capabilities of several satellite sensors currently in operation (Landsat TM, SPOT HRV, NOAA AVHRR and ERS‐1 ATSR) or proposed for launch in the near future (MISR and MODIS). Sensors that are capable of off‐nadir viewing solely by virtue of having a wide field‐of‐view (e.g. NOAA‐AVHRR) or through across‐track pointing (e.g. SPOT‐HRV) provide a relatively sparse sample of the BRDF. On the other hand, future sensors with along‐track pointing, such as the MISR instrument of NASA's Earth Observing System (EOS), will provide a much more complete sample and are therefore better able to characterise the surface BRDF and albedo. Sensors such as these are also better equipped to obtain data at and around the ‘hot spot’ and, consequently, have the potential to extract detailed information on the biophysical properties of earth surface materials.