Bidirectional effects in Landsat reflectance estimates: Is there a problem to solve?

Since the 2008 opening of the USGS Landsat archive, research has focused on standardization and compositing of dense time series of Landsat images to increase measurement precision sufficiently for long-term and/or global land cover mapping. Most of these efforts rely on atmospherically corrected estimates of surface reflectance, some of which also incorporate corrections for effects of sun-target-sensor geometry. However, these effects have not yet been well-characterized in Landsat data. Using reflectance and Bidirectional Reflectance Distribution Function (BRDF) parameters derived from daily Moderate-resolution Imaging Spectroradiometer (MODIS) reflectance estimates and the sun-target-sensor geometry of the Landsat-5 orbit, we simulated the BRDF effects within the Landsat-5 data archive at three sites representing low, mid, and high latitudes. We found that bidirectional effects are prevalent in Landsat measurements, but their effect varies by latitude. Seasonal variations in illumination geometry affect reflectance as well as vegetation indices, adding spurious seasonality in otherwise stable evergreen phenology at a lower-latitude tropical forest site. The ±7.5° variation in View Zenith Angle (VZA), usually considered negligible, resulted in as much as 20% along-scan variation in reflectance at the lower latitude site. Although variation in Solar Zenith Angle (SZA) is less pronounced in the tropics than in the upper latitudes, its impact is most significant in the tropics due to the relative proximity of Landsat scans to the principle plane. For this reason, even small variations in viewing and illumination geometry have large impacts on reflectance and vegetation indices near the equator. In contrast, the amplitude of SZA and phenological variations have the greatest impact on reflectance estimates in mid-and upper latitudes, where interaction between phenological variation and Landsat's 16-day orbital cycle can be greater than the combined effect of BRDF and orbital drift. Our analysis suggests that accounting or correction for BRDF effects will increase precision of land cover mapping and monitoring based on Landsat data alone or in combination with other sensors.