  ArielRad, the Ariel radiometric model, is a simulator developed to address
the challenges in optimising the space mission science payload and to
demonstrate its compliance with the performance requirements. Ariel, the
Atmospheric Remote-Sensing Infrared Exoplanet Large-survey, has been selected
by ESA as the M4 mission in the Cosmic Vision programme and, during its 4 years
primary operation, will provide the first unbiased spectroscopic survey of a
large and diverse sample of transiting exoplanet atmospheres. To allow for an
accurate study of the mission, ArielRad uses a physically motivated noise model
to estimate contributions arising from stationary processes, and includes
margins for correlated and time-dependent noise sources. We show that the
measurement uncertainties are dominated by the photon statistic,and that an
observing programme with about 1000 exoplanetary targets can be completed
during the primary mission lifetime.
