Comparison of monthly means of DLR-ISIS irradiance data to surface
measurements shows good agreement, illustrated by the two examples given
below. DNI is mostly underestimated during summer, but overestimated
during winter. This effect is mostly caused by scaling effects due to
the comparison of a 280 km x 280 km grid box average to measurements of
a single station within this box.
Validation of
DLR-ISIS DNI with data from 78 stations shows an average
underestimation of 3% for monthly means. For DLR-ISIS GHI, validation
with data from 89 stations indicates an overestimation of monthly means
by 3%.