  The primary purpose of the collective Thomson scattering (CTS) diagnostic at
ITER is to measure the properties of fast-ion populations, in particular those
of fusion-born $\alpha$-particles. Based on the present design of the
diagnostic, we compute and fit synthetic CTS spectra for the ITER baseline
plasma scenario, including the effects of noise, refraction, multiple fast-ion
populations, and uncertainties on nuisance parameters. As part of this, we
developed a model for CTS that incorporates spatial effects of
frequency-dependent refraction. While such effects will distort the measured
ITER CTS spectra, we demonstrate that the true $\alpha$-particle densities can
nevertheless be recovered to within ~10% from noisy synthetic spectra, using
existing fitting methods that do not take these spatial effects into account.
Under realistic operating conditions, we thus find the predicted performance of
the ITER CTS system to be consistent with the ITER measurement requirements of
a 20% accuracy on inferred $\alpha$-particle density profiles at 100 ms time
resolution.
