  The GERmanium Detector Array, GERDA, searches for neutrinoless double beta
decay in Ge-76 using bare high-purity germanium detectors submerged in liquid
argon. For the calibration of these detectors gamma emitting sources have to be
lowered from their parking position on top of the cryostat over more than five
meters down to the germanium crystals. With the help of Monte Carlo
simulations, the relevant parameters of the calibration system were determined.
It was found that three Th-228 sources with an activity of 20 kBq each at two
different vertical positions will be necessary to reach sufficient statistics
in all detectors in less than four hours of calibration time. These sources
will contribute to the background of the experiment with a total of (1.07 +/-
0.04(stat) +0.13 -0.19(sys)) 10^{-4} cts/(keV kg yr) when shielded from below
with 6 cm of tantalum in the parking position.
