BRDF Normalization

The view angle effect on surface reflectance is noticeable even for narrow field-of-view sensors like Landsat and Sentinel-2, especially where forward scattering and backward scattering are concerned. HLS normalizes the view angle effect in the Landsat/Sentinel-2 common bands and the Sentinel-2 red-edge bands using the c-factor technique and the global coefficients provided by Roy et al. (2016) and Roy et al. (2017).

In HLS v2.0 the L30 and S30 reflectance products are normalized for a nadir view. The solar zenith angle is largely intact and only slightly adjusted to be the mean of the predicted solar zenith angle at the respective moments the Landsat and Sentinel-2 overpass the tile center's latitude. This setting of the solar zenith angle is substantially different from the use of a constant solar zenith angle for a given latitude throughout the year in HLS v1.4.

BRDF illustrations

Figure: View angle effect is present even for narrow field-of-view sensors: Forward scatter is slightly weaker (left) than the backward scatter (right) for this vegetated area.