Calculation of global radiation (SumSW) from DIF and DIR

Global radiation (SumSW) can be calculated in the following way:

SumSW = DIF + DIR * cos(z)

SumSW = calculated global radiation

DIF = diffuse sky radiation

DIR = direct normal radiation

z = solar zenith angle

The solar zenith angle can be calculated for any given time and location. Within the BSRN Toolbox (https://wiki.pangaea.de/wiki/BSRN_Toolbox) several options are offered. How to quickly compare SumSW with SWD is described here. Results can be found at: http://bsrn.awi.de/products/quality-code/comparisons/