  Many aspects of the design trade-off of a space-based instrument and its
performance can best be tackled through simulations of the expected
observations. The complex interplay of various noise sources in the course of
the observations make such simulations an indispensable part of the assessment
and design study of any space-based mission. We present a formalism to model
and simulate photometric time series of CCD images by including models of the
CCD and its electronics, the telescope optics, the stellar field, the jitter
movements of the spacecraft, and all important natural noise sources. This
formalism has been implemented in a versatile end-to-end simulation software
tool, called PLATO Simulator, specifically designed for the PLATO space mission
to be operated from L2, but easily adaptable to similar types of missions. We
provide a detailed description of several noise sources and discuss their
properties, in connection with the optical design, the allowable level of
jitter, the quantum efficiency of the detectors, etc. The expected overall
noise budget of generated light curves is computed as a function of the stellar
magnitude, for different sets of input parameters describing the instrument
properties. The simulator is offered to the scientific community for future
use.
