SRS Sensors 3 THEORY
measurements acquired under different sun-sensor-surface configu-
rations, it is necessary to first calculate a bidirectional reflectance
distribution function (BRDF). Once you have empirically derived
a BRDF model from the measurements and canopy-specific param-
eters, you can use it to reduce variations that arise from changes
in sun-sensor-surface geometry across diurnal time series. For addi-
tional details on BRDF normalization of vegetation index time series,
see Hilker et al. (2008).
Sometimes NDVI values will exhibit erratic behavior due to envi-
ronmental conditions (see Day 178 on both graphs). Data filtering
(e.g., visual inspection for short time series or automated despiking
algorithms for longer time series) may be required to remove spurious
data points.
Figure 6: Daily variation in NDVI measurements 1 m above a
sub-alpine meadow
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