  High redshift quasars can be used to deduce the distribution of dark energy
in the Universe, as a complementary tool to SN Ia. The method is based on
determination of the size of the Broad Line Region from the emission line
delay, determination of the absolute monochromatic luminosity either from the
observed statistical relation or from a model of the formation of the Broad
Line Region, and determination of the observed monochromatic flux from
photometry. This allows to obtain the luminosity distance to a quasar
independently from its redshift. The accuracy of the measurements is however, a
key issue. We model the expected accuracy of the measurements by creating
artificial quasar monochromatic lightcurves and responses from the Broad Line
Region under various assumptions about the variability of a quasar, Broad Line
Region extension, distribution of the measurements in time, accuracy of the
measurements and the intrinsic line variability. We show that the five year
monitoring based on Mg II line should give the accuracy of 0.06 - 0.32
magnitude in the distance modulus which allows to put interesting constraints
on the cosmological models. Monitoring of higher redshift quasars based on CIV
lines is problematic due to much higher level of the intrinsic variability of
CIV in comparison with Mg II.
