Astrometric accuracy of complex modern VLBI arrays can not be calculated analytically .
We study the astrometric accuracy of phase-referenced VLBI observations for the VLBA , EVN and global VLBI array by simulating VLBI data for targets at declinations -25 \degr , 0 \degr , 25 \degr , 50 \degr , 75 \degr and 85 \degr .
The systematic error components considered in this study are calibrator position , station coordinate , Earth orientation and troposphere parameter uncertainties .
We provide complete tables of the astrometric accuracies of these arrays for a source separation of 1 \degr either along the right ascension axis or along the declination axis .
Astrometric accuracy is 50 ~ { } \mu as at mid declination and is 300 ~ { } \mu as at low ( -25 \degr ) and high ( 85 \degr ) declinations for the VLBA and EVN .
In extending our simulations to source separations of 0.5 \degr and 2 \degr , we establish the formula for the astrometric accuracy of the VLBA : \Delta _ { \alpha \cos \delta, \delta } = ( \Delta _ { \alpha \cos \delta, \delta } ^ { 1 ^ { \circ } } % -14 ) \times d + 14 ( \mu as ) where \Delta _ { \alpha \cos \delta, \delta } ^ { 1 ^ { \circ } } is the astrometric accuracy for a separation d = 1 \degr provided in our tables for various declinations and conditions of the wet troposphere .
We argue that this formula is also valid for the astrometric accuracy of the EVN and global VLBI array .